------------------------------------------------------------------------ wijnands_monitoring.tex A&A, submitted Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Virus-Scanned: by amavisd-new X-MailScanner-Information: Please contact postmaster@aoc.nrao.edu for more information X-MailScanner: Found to be clean X-MailScanner-SpamCheck: not spam, SpamAssassin (score=0.029, required 5, autolearn=disabled, LOTS_OF_STUFF 0.03) X-MailScanner-From: rudy@science.uva.nl %astro-ph/0508648 \documentclass[]{aa} \usepackage{graphicx} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \usepackage{txfonts} \usepackage[figuresright]{rotating} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % \begin{document} % \title{The {\itshape XMM-Newton}/{\itshape Chandra} monitoring campaign on the Galactic center region} \subtitle{Description of the program and preliminary results} \author{R. Wijnands, \inst{1} J.J.M. in 't Zand, \inst{2} M. Rupen, \inst{3, 4} T. Maccarone, \inst{5} J. Homan, \inst{6} R. Cornelisse, \inst{5} R. Fender, \inst{5, 1} J. Grindlay, \inst{7} M. van der Klis, \inst{1} E. Kuulkers, \inst{8} C.B. Markwardt, \inst{4, 9} J.C.A. Miller-Jones, \inst{1} Q.D. Wang, \inst{10} } \offprints{R. Wijnands} \titlerunning{{\itshape XMM-Newton/Chandra} monitoring campaign} \authorrunning{Wijnands et al.} \institute{Astronomical Institute ``Anton Pannekoek'', University of Amsterdam, Kruislaan 403, 1098 SJ, The Netherlands\\ \email{rudy@science.uva.nl; michiel@science.uva.nl; jmiller@science.uva.nl} \and SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands\\ \email{jeanz@sron.nl} \and National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA \and NASA, Goddard Space Flight Center, Greenbelt, MD 20711, USA\\ \email{mrupen@milkyway.gsfc.nasa.gov; craigm@milkyway.gsfc.nasa.gov} \and School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK\\ \email{tjm@astro.soton.ac.uk, cornelis@astro.soton.ac.uk, rpf@phys.soton.ac.uk} \and Center for Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA \\ \email{jeroen@space.mit.edu} \and Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA\\ \email{josh@cfa.harvard.edu} \and ISOC, ESA/ESAC, Urb. Villafranca del Castillo, P.O. Box 50727, 28080 Madrid, Spain\\ \email{Erik.Kuulkers@esa.int} \and Department of Astronomy, University of Maryland, College Park, MD 20742, USA \and Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA\\ \email{wqd@astro.umass.edu} } % \date{Received September 15, 1996; accepted March 16, 1997} \abstract{We present the first results of our X-ray monitoring campaign on a 1.7 square degree region centered on Sgr A* using the X-ray satellites {\it XMM-Newton} and {\it Chandra}. The purpose of this campaign is to monitor the X-ray behavior (below 10 keV) of X-ray sources (both persistent and transient) which are too faint to be detected by monitoring instruments aboard satellites currently in orbit (e.g., {\it Rossi X-ray Timing Explorer}; {\it INTEGRAL}). Our first monitoring observations (using the HRC-I aboard {\it Chandra}) were obtained on June 5, 2005. Most of the sources detected could be identified with foreground sources, such as X-ray active stars. In addition we detected two persistent X-ray binaries (1E 1743.1--2843; 1A 1742--294), two faint X-ray transients (GRS 1741.9--2843; XMM J174457--2850.3), as well as a possible new transient source at a luminosity of a few times $10^{34}$ erg s$^{-1}$. We report on the X-ray results on these systems and on the non detection of the transients in follow-up radio data using the Very Large Array. We discuss how our monitoring campaign can help to improve our understanding of the different types of X-ray transients (i.e., the very faint ones). \keywords{Accretion, accretion disks -- Binaries:close -- X-rays:binaries } } \maketitle % %________________________________________________________________ \section{Introduction} Many X-ray sources (the so-called X-ray transients) exhibit orders of magnitude variability in their X-ray luminosities. Normally they are too dim to be detected and they are only discovered when they experience one of their bright outbursts. The brightest transients can be identified with Galactic neutron stars and black holes accreting matter from a companion star (the ``X-ray binaries''). These outbursts are ascribed to a huge increase in the accretion rate onto the compact object. Several other types of sources can also manifest themselves as X-ray transients (e.g. accreting white dwarfs, magnetars, $\gamma$-ray bursts, flare stars, young stellar objects, active binaries), however, their peak luminosities are many orders of magnitude lower than those of the transient X-ray binaries (except for $\gamma$-ray bursts and bursts from magnetars). One usually refers only to transient X-ray binaries when talking about X-ray transients. Phenomenologically, the different X-ray transients can be classified based on their peak X-ray luminosity as seen during outburst. However, the high-mass X-ray binary systems among the transients can in principle be rather bright or arbitrarily faint depending on the binary parameters and the matter density and velocity in either the stellar wind or the circumstellar decretion disk (e.g., in Be/X-ray binaries). For this reason we will not discuss the high-mass X-ray binary systems in detail, instead we will concentrate on the X-ray transients in which a neutron star or black hole is accreting from a low-mass companion star (the low-mass X-ray binaries) or for which the type of companion star is not yet established. These transients can be classified in different groups based on their maximum observed peak X-ray luminosities (from here on we will quote 2--10 keV luminosities unless otherwise noted): \paragraph{{\bf Bright to very bright X-ray transients:}} These transients have peak X-ray luminosities of $10^{37-39}$ erg s$^{-1}$ (e.g., \cite{1997ApJ...491..312C}). Monitoring campaigns with several satellites (e.g., {\it BeppoSAX}, {\it RXTE}, {\it INTEGRAL}; see, e.g., \cite{2001egru.conf..463I, 1996ApJ...469L..33L, 2001ASPC..251...94S, 2004AstL...30..382R}; see \cite{2001egru.conf..463I} for an overview of past and current monitoring campaigns focusing on the Galactic center region) have been very successful in discovering many such bright X-ray transients and monitoring their X-ray properties. Intensive studies of the large amount of available data have yielded a good understanding of their behavior and it has been found that a large fraction of them harbor accreting black holes. \paragraph{{\bf Faint X-ray transients:}} These transients have peak X-ray luminosities of $10^{36-37}$ erg s$^{-1}$. Several detections of such systems have been made in the past (e.g., \cite{1990IAUC.5104....1S, 1994ApJ...425..110P}), but only recently it was realized that they might form a distinct class of X-ray transients different from the brighter systems. This realization came when many such faint systems were detected by the Wide Field Cameras (WFC) aboard {\it BeppoSAX} (\cite{1999ApL&C..38..297H, 2001egru.conf..463I}). Several global characteristics of the faint transients sets them apart from the brighter systems. First, contrary to what is observed for the brighter systems, a very large fraction of the faint systems contain neutron-star accretors as determined by the detection of type-I X-ray bursts or millisecond X-ray pulsations\footnote{It is interesting to note that all but one of the seven currently known accreting millisecond X-ray pulsars (see \cite{2004AIPC..714..209W} for a review) are faint X-ray transients. The one which is not a faint transient (XTE J1751--305; peak X-ray luminosities of a few times $10^{37}$ erg s$^{-1}$) had a very short outburst duration (e-folding time of only $\sim$7 days) resulting in a similar low time-averaged accretion rate as the other accreting millisecond pulsars.} from them. Second, their Galactic distribution is different from that of the brighter systems (\cite{2002A&A...392..885C}), with the faint transients more concentrated toward the Galactic center. \cite{2000MNRAS.315L..33K} argued that the faint transients are indeed different from the bright systems and that they are mainly neutron star X-ray binaries in very compact binaries (orbital periods $<$80 min.). However, clearly not all systems have such short orbital period (e.g., the faint transients and millisecond X-ray pulsars SAX J1808.4--3658, XTE J1814--338, and IGR J00291+5934 have periods $>$80 min.) and not all systems are neuron stars (e.g., XTE J1118+480 is a strong black hole candidate; \cite{2001ApJ...556...42W}). \paragraph{{\bf Very faint X-ray transients (VFXTs):}} These transients have peak X-ray luminosities of $10^{34-36}$ erg s$^{-1}$. Until recently, only limited evidence was available for the existence of this class because detecting such faint transients is challenging due to sensitivity and/or angular resolution limits of many X-ray instruments. Despite the difficulties in finding VFXTs, pointed observations with relatively sensitive X-ray satellites (e.g., {\it Granat}; {\it ASCA}) resulted in the detection of several VFXTs near the Galactic center (e.g., \cite{1994ApJ...425..110P, 1998ApJ...508..854T, 1996PASJ...48..417M}). More recently, a significant number of systems have been found (\cite{1999ApJ...525..215S, 2004MNRAS.351...31H, 2003A&A...406..299P, 2005MNRAS.357.1211S, 2003ApJ...598..474M}), thanks to the sensitive {\it Chandra} and {\it XMM-Newton} X-ray satellites, supporting the claim (e.g., \cite{2005MNRAS.357.1211S}) that a class of VFXTs exists in our Galaxy. It is very likely that these VFXTs are accreting neutron stars and black holes since only one accreting white dwarf has exhibited outbursts above $10^{34}$ erg s$^{-1}$ (\cite{1985MNRAS.212..917W}). For this reason we use $10^{34}$ erg s$^{-1}$ as the lower luminosity border for the VFXTs class (see also \cite{1984MNRAS.210..899V}). In section~\ref{subsection:VFXTs} we discuss the VFXTs further.\\ Our classification of {\it bright to very bright}, {\it faint}, and {\it very faint} X-ray transients is somewhat arbitrary and it is clear that hybrid systems do exist. For example, the neutron star X-ray transient in the globular cluster NGC 6440 was classified as a faint X-ray transient (\cite{1999A&A...345..100I}) but later it was found to also exhibit bright X-ray outbursts (\cite{2003ApJ...598..481K}). Furthermore, the bright neutron star X-ray transient SAX J1747.0--2853 (e.g., \cite{2004A&A...416..311W}) was seen on several occasions at luminosities of only a few times $10^{35}$ erg s$^{-1}$ (e.g., \cite{2002ApJ...579..422W}). If for some reason the bright outbursts of this source had been missed (e.g., because no X-ray satellite was pointed in the source direction), then it would have been misclassified as a VFXT. Nevertheless, in this paper such a classification will prove quite useful in talking about the different types of transients. Recently, another type of accreting neutron star has been identified in the Galaxy: the so-called 'burst-only sources' (\cite{2002A&A...392..885C}). The burst-only sources are a group of nine objects detected by the {\it BeppoSAX}/WFC when they exhibited a type-I X-ray burst. No detectable accretion emission around the bursts could be detected with that instrument, with typical upper limits on the accretion luminosities of the order of $10^{36}$ erg s$^{-1}$. Subsequent X-ray observations of these systems (using a variety of satellites) revealed that one source is a persistent X-ray source (albeit at very faint luminosities; \cite{intzandetal2005}), two exhibited faint outbursts (e.g., \cite{2002A&A...392..931C}; they are faint X-ray transients), and one exhibited a very faint outburst (e.g., \cite{2004MNRAS.351...31H}; it is a VFXT). The other five sources could not yet be classified, although they are all X-ray transients since follow-up observations with {\it Chandra} could not detect any persistent accretion luminosities from them (\cite{2002A&A...392..931C}). Presumably, they experienced their X-ray bursts during brief accretion outbursts which had peak luminosities below $\sim$$10^{36}$ erg s$^{-1}$. Therefore, these systems are good candidates to be classified as VFXTs, although definitive proof has to come from detecting these systems during such very faint outbursts (see also \cite{2004NuPhS.132..518C} for a discussion on the classification of burst-only sources). \subsection{VFXTs in more detail \label{subsection:VFXTs}} Currently, little is known about the properties of the VFXTs due to the low number of systems known and the scarcity of observations during their outburst episodes. Some of them might be intrinsically bright transients at large distances; however, many are observed near the Galactic center, indicating source distances of $\sim$8 kpc and therefore very low peak intrinsic luminosities. In addition, some VFXTs might be intrinsically brighter than observed due to inclination effects (e.g., \cite{munoetal2005}), but this can be argued for only a small fraction of the VFXTs (see the appendix; see also \cite{kingwijnands2005}). Therefore, we consider it likely that most VFXTs indeed have very faint intrinsic X-ray luminosities. The characteristics of the VFXTs (e.g., their spectra, their outburst light curves, timing properties; e.g., \cite{2005MNRAS.357.1211S, 2005ApJ...622L.113M, 1998ApJ...508..854T}) indicate that they are not a homogeneous class of sources but that different types of accreting neutron stars and black holes show themselves as VFXTs. The detection of slow pulsations in some VFXTs (e.g., \cite{1998ApJ...508..854T}) indicates that at least some VFXTs are likely to have a high mass donor star since a slow X-ray pulsar is usually associated with high-mass X-ray binaries. No significant pulsations have so far been detected for the other VFXTs and they have so far not been detected at optical or infrared wavelengths. For the VFXTs in the Galactic center region it was found the they harbor companions fainter than B2 IV stars (\cite{2005ApJ...622L.113M}). Therefore, it is likely that a significant fraction of the VFXTs are neutron stars and black holes accreting matter at a very low rate from a low-mass companion star. Moreover, at least one VFXT has exhibited type-I X-ray bursts (SAX J1828.5--1037; \cite{2002A&A...392..885C, 2004MNRAS.351...31H}) which are usually identified with low-mass X-ray binaries. Such low-mass X-ray binaries have very low time-averaged accretion rates, which could become a challenge for our understanding of their evolution. If their time averaged accretion rates do not drop significantly below $10^{-13} M_\odot$ yr$^{-1}$ then these systems can be explained as low-mass X-ray binaries which have spent most of the age of the Galaxy reducing their companion stars to values $\sim 0.01 M_\odot$ (\cite{kingwijnands2005}). If, however, it is proven that some systems have time averaged accretion rates significantly lower than this value, then more exotic explanations may be needed such as accretion from planetary companions or accretion onto intermediate mass black holes. To improve on the limited amount of knowledge about the observational properties of VFXTs, monitoring campaigns are required with instruments which: \begin{itemize} \item are sensitive enough to detect the very faint X-ray luminosities of these systems \item have a large field-of-view (FOV) to monitor a large region allowing for the discovery and monitoring of many systems \item have rapid data turn-around time to allow fast follow-up observations at all wavelengths to study the VFXTs when they are still active \item have (sub-)arcsecond resolution to limit source confusion and to allow for unique determination of the counterparts at other wavelengths which will help to establish the nature of the systems (e.g., IR observations might help to determine the type of companion star). \end{itemize} Such a monitoring instrument does not exist and in practice might be difficult to achieve since, for example, a large FOV and very good sensitivity are usually mutually exclusive. Fortunately, {\it XMM-Newton} and {\it Chandra} are excellent to perform such a monitoring program but only within a limited FOV. Therefore, we have secured such a program using both satellites for a limited region close to Sgr A*. We will first describe the details of the program and then present some initial preliminary results. \section{Description of our program} In proposal cycle 4 of {\it XMM-Newton}, we have secured a program using this satellite in combination with the {\it Chandra} X-ray Observatory to monitor a 1.7 square degree region centered around Sgr A*. The FOV of our program is shown in Figure~\ref{fig:fov}. We have secured 4 X-ray epochs (two with {\it XMM-Newton} and two with {\it Chandra}) with each epoch consisting of seven observations (their pointing directions are indicated in Figure~\ref{fig:fov}). Each pointing lasts $\sim$5 ksec and will reach an overall sensitivity of $\sim$$5\times 10^{33}$ erg s$^{-1}$ (at 8 kpc; from here on we assume a distance of 8 kpc when quoting X-ray luminosities) for the {\it Chandra} observations to an order of magnitude better for the {\it XMM-Newton} observations. The FOVs of the different pointings overlap for several arcminutes (Fig.~\ref{fig:fov}) to compensate for the loss in sensitivity toward the edge of the FOV of the instruments. For the {\it Chandra} pointings we have chosen to use the HRC-I detector because it has the largest FOV of any instrument aboard {\it Chandra} (the FOV of the HRC-I is approximately equal to the FOV of the instruments aboard {\it XMM-Newton}), although this comes at a loss of sensitivity for hard sources and the loss of any spectral information. All detectors will be on during the {\it XMM-Newton} observations but we will mostly focus on the data obtained with the EPIC detectors (the two MOS and the pn chips). The RGS will only be useful when a single bright source is in the FOV. The purpose of our monitoring campaign is to study the variability behavior of the transient and persistent sources in the survey region at levels which are not possible by other monitoring instruments in orbit. Although our interests focus on the accreting neutron stars and black holes in the FOV, our campaign will also be used to study the X-ray properties of X-ray active stars, dense star clusters (e.g., the Arches cluster), flares from Sgr A*, accreting white dwarfs, and any other object in the FOV which might exhibit persistent or transient X-ray emission at levels detectable during our observations. The different observation epochs are separated in time from each other by at least one month so that we can monitor the X-ray behavior of the detected sources on timescales of about 1 month to almost a year. The first epoch data were gathered on 5 June 2005 ({\it Chandra}) and the remaining three epochs are currently scheduled for 14--15 September 2005 ({\it XMM-Newton}), between 17--24 October 2005 ({\it Chandra}), and around 28 February 2006 ({\it XMM-Newton}). Here we report on the initial results obtained during the first epoch {\it Chandra} data. \section{Data analysis} The first data of our monitoring campaign were obtained on 5 June, 2005, using the {\it Chandra} satellite (see Tab.~\ref{table:observations} for a log of the observations) and the HRC-I detector. We processed the data using the {\it Chandra} CIAO tools (version 3.2.1) and the standard {\it Chandra} analysis threads\footnote{Available from http://cxc.harvard.edu/ciao/}. We checked for background flares during our observations, but none were found, allowing us to use all available data. We merged the 7 different HRC-I observations into one image which we show in Figure~\ref{fig:images}. Clearly, several bright sources are visible by eye. We used the tool {\it wavdetect} to search for point sources in our data and to obtain the coordinates of each source that was detected. We ran the tool on the combined image as well as on each individual observation. Due to variations in the size and shape of the point-spread-function as a function of offset angle from the pointing directions, we ran wavdetect on images with different binning factors. We are still exploring ways to optimize our detection method so it is likely that we will find more sources than the ones we report on in this paper. However, we expect that we are complete for sources with inferred X-ray luminosities $>$$10^{34}$ erg s$^{-1}$. The errors on the source positions are difficult to estimate for the sources found at relatively large offset angles. The asymmetries in the point-spread-function at large off-axis angles can result in large systematic uncertainties when using wavdetect (e.g., \cite{hong2005}). We are pursuing extensive simulations to investigate the effects on the positional errors for a large range of offset angles as well as different source luminosities. A similar investigation has already been performed for the {\it Chandra}/ACIS-I combination by \cite{hong2005}. Although they investigated the ACIS-I (and not the HRC-I), used only offset angles up to $10'$ (instead of $>$20$'$ as we sometimes encounter), and focused mainly on the faint sources, we will use their results as a first order approximation on the accuracy of the positions we obtain using wavdetect. We use equation 5 in \cite{hong2005} to estimate the uncertainties in our positions. If the sources were detected in multiple observations, we only give the positions and their errors obtained from the data set in which the sources had the smallest offset in order to minimize systematic uncertainties. However, we urge caution when using our positional uncertainties. For each detected source we extracted the background corrected count rate from the images using the standard CIAO tools for the full energy range of the HRC-I (0.08--10 keV) since no spectral information can be currently extracted from HRC-I data. The count rates obtained can be converted into fluxes using PIMMS\footnote{Available from http://cxc.harvard.edu/toolkit/pimms.jsp}, assuming particular spectral models and values for the interstellar absorption. Again, the analysis is complicated because many sources are detected at large off-axis angles and vignetting becomes a serious issue. It is currently difficult to correct the count rates for vignetting because the effects depend strongly on both the off-axis angle and the assumed source spectra (which are unknown since the HRC-I does not currently allow to extract energy information). Therefore, the count rates we quote are the uncorrected count rates and are therefore lower limits to the actual count rates of the sources. Depending on the off-axis angles and the source spectra, the true count rates could be larger by a factor of a few. Again, if the sources were detected in multiple observations, we only give the count rates from the data set in which the sources had the smallest offset to minimize the systematic errors on the count rates. \section{Results} In total we have detected 21 sources so far. Two sources (the Sgr A* complex and the Arches cluster) are known to embody a complex of point sources in combination with strong diffuse emission (\cite{2003ApJ...589..225M, 2002ApJ...570..665Y}). The analysis of these complex regions is still in progress. Ten of the remaining sources can be identified with known stars (e.g., HD 316314, HD 316224, HD 161274, TYC 06840-38-1, ALS 4400; Fig.~\ref{fig:images} left) or have clear counterparts in the Digital Sky Survey images indicating that they are foreground objects (and hence have relatively low X-ray luminosities). We will not discuss the detections of the foreground objects further in this paper but we will focus on the detected X-ray binaries. \subsection{The persistent sources} We detected the two persistent X-ray binaries known to be present in the surveying region: 1E 1743.1--2843 and 1A 1742--294. \subsubsection{1E 1743.1--2843} 1E 1743.1--2843 is a persistent X-ray binary for which the type of accreting object is not yet known. The source was in the FOV of two of the seven HRC-I pointings and was detected during both observations but we detected no bursts from the source. The position obtained from our {\it Chandra}/HRC-I data is consistent with that derived from a previous {\it XMM-Newton} observation (\cite{2003A&A...406..299P}) although, due to the systematic uncertainties in our positional errors, it cannot currently be determined if our position is better than the {\it XMM-Newton} one. We used PIMMS to convert the obtained count rate (see Tab.~\ref{table:binaries}). We assumed an absorbed power-law model similar to what was found by \cite{2003A&A...406..299P} when fitting the {\it XMM-Newton} observation of the source (they obtained an equivalent hydrogen column density $N_{\rm H}$ of $2\times10^{23}$ cm$^{-2}$ and a photon index of 1.8). This results in unabsorbed fluxes of $1.8\times 10^{-10}$ (2--10 keV) and $3\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$ (0.5--10 keV) and X-ray luminosities of 1.4 and $2.3\times 10^{36}$ erg s$^{-1}$, respectively. These X-ray luminosities are very similar to what has been seen before for this source (e.g., \cite{2003A&A...406..299P}). \subsubsection{1A 1742--294} 1A 1742--294 is a persistent X-ray binary harboring a neutron-star accretor as evidenced by the type-I X-ray bursts observed from this system (see, e.g., \cite{1994ApJ...425..110P}). We detected this source during two of our HRC-I pointings. During the GC-10 pointing we detected an X-ray burst. Our {\it Chandra} position is fully consistent with the best position so far reported on this source (using {\it ROSAT}; \cite{2001A&A...368..835S}) and despite the possible unknown systematic uncertainty in our errors, our position is better. We again used PIMMS to convert the obtained count rate (see Tab.~\ref{table:binaries}) and used the absorbed power-law model ($N_{\rm H} \sim$$6\times10^{22}$ cm$^{-2}$ ; photon index $\sim$1.8) found when fitting the {\it BeppoSAX} and {\it ASCA} data of the source (\cite{1999ApJ...525..215S,2002ApJS..138...19S}). This results in unabsorbed fluxes of $2.3\times 10^{-10}$ (2--10 keV) and $3.7\times 10^{-10}$ erg cm$^{-2}$ (0.5--10 keV) s$^{-1}$. The corresponding X-ray luminosities are 1.8 and $2.8\times 10^{36}$ erg s$^{-1}$, consistent with what has been observed before for this source (e.g., \cite{1999ApJ...525..215S}). \subsection{The transient sources} Two transients were clearly visible during the HRC-I observations: GRS 1741.9--2853 and XMM J174457--2850.3. We made a preliminary announcement of the detection of these new outbursts on 6 June 2005 (\cite{2005ATel..512....1W}). Following these detections, we obtained an additional {\it Chandra} observation of both sources (using the ACIS-I detector) on 1 July 2005 (see Tab.~\ref{table:observations} for details). Because the two transients were only $\sim$4.6$'$ away from each other, we could observe both sources with only one ACIS-I pointing. We placed both sources at an off-axis angle of 7$'$ in order to limit pile-up in case the sources were as bright as seen during the HRC-I observations. The ACIS-I data were also analyzed using CIAO and the standard threads. Again, all data could be used since no episodes of high background emission occurred during our observation. \subsubsection{GRS 1741.9--2843} GRS 1741.9--2853 is a neutron star X-ray transient (it exhibits type-I bursts; e.g., \cite{1999A&A...346L..45C}) which has been detected several times in outburst since its original discovery in 1990 (\cite{1990IAUC.5104....1S}). Its peak luminosity is typically a few times $10^{36}$ erg s$^{-1}$ making it a faint X-ray transient (see \cite{2003ApJ...598..474M} for more details). This source was detected during two of our pointings (Tab.~\ref{table:binaries}) but we detected no bursts. The position of the source was consistent with, but not better than the one obtained by \cite{2003ApJ...598..474M}. The observed count rate was converted into fluxes using PIMMS and assuming an absorbed power-law with $N_{\rm H} = 9.7\times10^{22}$ cm$^{-2}$ and a photon index of 1.88 (\cite{2003ApJ...598..474M}). This results in unabsorbed fluxes of $1.1\times 10^{-10}$ (2--10 keV) and $1.8\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$ (0.5--10 keV), yielding X-ray luminosities of 0.8 and $1.4\times 10^{36}$ erg s$^{-1}$, respectively (for comparison with previous {\it Chandra} data on this source reported by \cite{2003ApJ...598..474M}, we also list the 2--8 keV luminosity of $7.0\times10^{35}$ erg s$^{-1}$). GRS 1741.9--2853 was also detected during the additional {\it Chandra}/ACIS-I observation (Fig.~\ref{fig:extra_image}). We extracted the source spectrum using a source extraction region of 10$''$ and a background extraction circle of 50$''$ from a source-free region close to GRS 1741.9--2853. The spectrum was rebinned to have at least 15 counts per bin to allow the $\chi^2$ fitting method. The resulting spectrum is shown in Figure~\ref{fig:spectra}. We used XSPEC to fit the spectrum and the fit results obtained are listed in Table~\ref{table:spectral_fits}. Clearly, the source flux had decreased by almost an order of magnitude since 5 June 2005. The long-term light curve of the source is plotted in Figure~\ref{fig:lc} showing the multiple outbursts of the source in the last 15 years. \subsubsection{XMM J174457--2850.3} XMM J174457--2850.3 is also clearly detected during our HRC-I observations (Fig.~\ref{fig:images}). This source has been detected only once before in outburst in 2001 (using {\it XMM-Newton}; \cite{2005MNRAS.357.1211S}). During that outburst the source was seen at a peak luminosity of $5 \times 10^{34}$ erg s$^{-1}$, justifying a classification as a VFXT. We detected it during two of our pointings (Tab.~\ref{table:binaries}) but saw no bursts. Our source position is consistent with that obtained by \cite{2005MNRAS.357.1211S}, although the exact uncertainty on our HRC-I position is currently unclear. However, the source was also detected during our additional ACIS-I observation yielding a more reliable position even though the source was relatively weak (see Tab.~\ref{table:binaries}). This position is significantly better than the {\it XMM-Newton} one. The observed HRC-I count rate was converted into fluxes using PIMMS and assuming an absorbed power-law with $N_{\rm H} = 6\times10^{22}$ cm$^{-2}$ and a photon index of 1.0 (\cite{2005MNRAS.357.1211S}). This resulted in unabsorbed fluxes of $1.1\times 10^{-10}$ (2--10 keV) and $1.3\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$ (0.5--10 keV) and in X-ray luminosities of 0.8 and $1.0\times 10^{36}$ erg s$^{-1}$, respectively. This is significantly brighter than what was previously found for the source and makes it a borderline case as a VFXT. As stated above, XMM J174457--2850.3 was also detected during the additional {\it Chandra}/ACIS-I observation (Fig.~\ref{fig:extra_image}). We extracted the source spectrum using a source extraction region of 5$''$. Due to the rather low number of source photons (26 counts in the 0.3--7.0 keV energy range) we did not rebin the spectrum or subtract the background (which was $<$0.3 photon in the source region and therefore negligible) so that we could use the Cash statistics (\cite{1979ApJ...228..939C}) when fitting the spectrum in XSPEC. The fit results obtained for this observation are also listed in Table~\ref{table:spectral_fits} and the resulting spectrum is shown in Figure~\ref{fig:spectra}. Clearly, the source flux had decreased by nearly three orders of magnitude since 5 June 2005. The long-term light curve of the source is plotted in Figure~\ref{fig:lc}. \subsubsection{A possible new VFXT} None of the other known transients in the FOV of our observations (see Tab.~\ref{table:sources_in_FOV}) were conclusively detected in our HRC-I data. The upper limits on their luminosities depend highly on their spectral shape and their off-axis positions, with a rough estimate of $\sim$$10^{34}$ erg s$^{-1}$. Several additional weak sources were detected during our observations which could not be identified with a star in the Digital Sky Survey database. Only one of these had a large enough count rate (see Tab.~\ref{table:binaries}) that its X-ray luminosity exceeded $10^{34}$ erg s$^{-1}$ if it had a 'prototypical X-ray binary' spectrum (power-law model with photon index of 1.8 and a typical $N_{H}$ of $6\times10^{22}$ cm$^{-2}$). Using such a spectral shape, the source had unabsorbed X-ray fluxes of 1.9 and $3.1\times10^{-12}$ erg s$^{-1}$ for 2--10 keV and 0.5--10 keV, respectively, and thus luminosities of $1.5\times 10^{34}$ erg s$^{-1}$ (2--10 keV) and $2.4\times10^{34}$ erg s$^{-1}$ (0.5--10 keV). We note that we do not know the intrinsic source spectrum and therefore these fluxes and luminosities could be significantly off if the real source spectrum is considerably different. We investigated the archival data sets of {\it Chandra} and {\it XMM-Newton} and found that the source was in the FOV of one previous {\it XMM-Newton} observation. The source was not detected during this {\it XMM-Newton} observation, but it was at the edge of its FOV making it difficult to obtain a reliable upper limit on the flux especially because we do not know the spectral shape of the source. We estimate that the luminosity of the source was at least a factor of a few fainter during the {\it XMM-Newton} compared with our HRC-I data. Although this is suggestive of a transient nature for this source, it could also be a highly variable persistent source. Our monitoring observations in September 2005 using {\it XMM-Newton} will resolve this ambiguity. Currently, we will refer to this source as a possible new VFXT. \subsubsection{Observations at other wavelengths} We obtained VLA observations at 4 and 6 cm on 8--9 June, 2005 of GRS 1741.9--2853, XMM J174457--2850.3, and the possible new VFXT. The analysis of these radio data is complicated by the strong side-lobes of Sgr A* and we are still in the process of fully analyzing these data. A preliminary analysis of the 4 cm data shows that none of the sources were conclusively detected with radio fluxes of $0.003\pm0.060$, $-0.002\pm0.046$, and $0.032\pm0.043$ mJy/beam, respectively. On 8 June 2005, \cite{2005ATel..522....1L} obtained I-band images of GRS 1741.9--2853 and XMM J174457--2850.3 using the Magellan-Baade telescope but could not detect the I-band counterparts of the sources. This is not surprising when considering the high absorption column in front of both sources. \section{Discussion} We have presented our initial results of the first observations taken as part of our {\it XMM-Newton}/{\it Chandra} monitoring campaign of the inner region of our Galaxy. Using our {\it Chandra}/HRC-I observations we detected mostly foreground objects (like X-ray active stars), but we also detected two persistent X-ray binaries, two X-ray transients, and one possible very faint X-ray transient (but its transient nature requires further confirmation). Clearly, our monitoring {\it XMM-Newton}/{\it Chandra} campaign has detected transients in outburst which were missed by the other monitoring instruments in orbit. Our campaign therefore complements other programs which find mainly the brighter transients or the faint transients far away from the crowded fields near Sgr A*. The faint X-ray transient GRS 1741.9--2853 was detected at a level of $\sim$$10^{36}$ erg s$^{-1}$ very similar to what has been observed previously for this source. A month after our initial HRC-I observations this source could still be detected at $\sim$$10^{35}$ erg s$^{-1}$ with the ACIS-I. The parameters obtained for the source spectrum during this observation were consistent with those found by \cite{2003ApJ...598..474M} when the source was an order of magnitude brighter, indicating that the source spectrum is not very dependent on source luminosity. Although we did not detect X-ray bursts during our observations, this source is known to exhibit such phenomena making it very likely to be a neutron star accreting from a low-mass companion star. Even though no optical/infrared counterpart has so far been found for this source, type-I X-ray bursts have only been seen for low-mass X-ray binaries making it very likely that GRS 1741.9--2853 is also such a system. The fact that GRS 1741.9--2853 harbors a neutron star also is consistent with the non-detection of the source in our radio data since neutron-star low-mass X-ray binaries are known to exhibit very low radio luminosities (e.g., \cite{2001MNRAS.324..923F, 2005ApJ...626.1020M} ) Figure~\ref{fig:lc} shows that the source has been seen to be in outburst at least 5 times with X-ray luminosities above $10^{34}$ erg s$^{-1}$. Its recurrence time can be estimated to be between 2 and 5 years, making GRS 1741.9--2853 one of the most active transients in our FOV, with a duty cycle of about 50\% (as estimated from Fig.~\ref{fig:lc})\footnote{We note that this is likely an upper limit on the duty cycle since actual source detections are more frequently reported in the literature than non-detections which will skew the data toward detections. For example, the data presented by \cite{2003ApJ...598..474M} of GRS 1741.9--2853 (as used in Fig.~\ref{fig:lc}) does not report the non-detections of the source as seen with {\it ROSAT} (\cite{2001A&A...368..835S}) or {\it BeppoSAX} (\cite{1999ApJ...525..215S}; using the Narrow Field Instruments). Since no upper limits on the source flux are given in these papers, we also do not include these non-detections in Fig.~\ref{fig:lc}. \label{footnote:duty}} . Its peak luminosity is very similar to the accreting millisecond X-ray pulsar SAX J1808.4--3658. For that system and the other accreting millisecond X-ray pulsars, it has been suggested that their pulsating nature is related to their rather low time averaged accretion rates (e.g., \cite{2001ApJ...557..958C}). Although the time averaged accretion rate of GRS 1741.9--2853 seems to be higher than for the accreting millisecond pulsars due to its higher duty cycle, GRS 1741.9--2853 could still be a millisecond X-ray pulsar as well (see also \cite{2003ApJ...598..474M}), especially if its duty cycle has been overestimated (see footnote~\ref{footnote:duty}). However, its faintness and its location in the Sgr A* region make it very difficult to detect these pulsation using {\it RXTE} because of significant contribution to the detected count rate from other sources in the FOV. With {\it Chandra} or {\it XMM-Newton} very long exposure times would be needed to detect any pulsations at amplitudes similar to those seen for the accreting millisecond pulsars. The VFXT XMM J174457--2850.3 was also detected during our HRC-I observations at an X-ray luminosity close to $10^{36}$ erg s$^{-1}$. This is about a factor of 20 higher than what was previously seen for this source (\cite{2005MNRAS.357.1211S}). This demonstrates that VFXTs can exhibit a large range of X-ray luminosities (similar to what has been observed for the brighter systems) and XMM J174457--2850.3 is at the border between faint and very faint X-ray transients (this clearly demonstrates that our luminosity boundaries are somewhat arbitrary as discussed in the introduction). It is possible that the previous detection of this source was made either during the rise or decay of a full outburst and that the maximum luminosity reached at the time was closer to what we have observed for the source during our HRC-I observations. Within a month the source luminosity has decreased by nearly 3 orders of magnitude. Its X-ray spectrum at this low X-ray luminosity was consistent with that found by \cite{2005MNRAS.357.1211S} demonstrating that for this source the shape of its spectrum is not strongly dependent on luminosity for luminosities below $5\times 10^{34}$ erg s$^{-1}$. Since we cannot extract any spectral information from the HRC-I data we cannot determine if the spectrum was significantly different at times when the source had X-ray luminosities close to $10^{36}$ erg s$^{-1}$. Since only very few observations have been performed of this source (see Fig.~\ref{fig:lc}), it is difficult to estimate its recurrence time (at most of order 3 years according to Fig.~\ref{fig:lc}) and its time-averaged accretion rate. The non-detection at radio wavelengths might indicate that the source harbors a neutron-star accretor since according to the radio-X-ray correlation found for low luminosity black hole binaries (\cite{2003MNRAS.344...60G}) the source should have had (if it harbors a black-hole accretor which was accreting at $\sim$$10^{36}$ erg s$^{-1}$) a radio flux of $\sim$1 mJy, significantly higher than our radio upper limit. Alternatively, XMM J174457--2850.3 could still harbor a black hole, but one which does not follow this correlation. We did not detect any unambiguous new VFXTs during our observations although we detected a possible new VFXT but its transient nature must be confirmed. This will be possible with our next set of monitoring observations currently scheduled for 14--15 September 2005. Our three additional epochs will also be very important to find further VFXTs, either previously unknown transients or recurrent ones. Our observations will allow us to set tighter constraints on the time averaged accretion rates of these systems than is currently possible with the available data. Such constraints are especially important for the low-mass X-ray binaries among the VFXTs because if their time-averaged accretion rates drop significantly below $\sim$$10^{-13}$ $M_\odot$ yr$^{-1}$ then our theoretical understanding of the evolution of such systems will have a very hard time explaining their existence without invoking exotic scenarios such as accretion from a planet or intermediate mass black hole accretors (\cite{kingwijnands2005}). The latter option cannot be invoked if type-I X-ray bursts have been observed for these systems since this establishes the existence of a neutron star accretor. Potential candidates for such systems are the burst-only sources mentioned in the introduction. Monitoring observations of these sources with sensitive X-ray telescopes would be very useful to constrain their time-averaged accretion rates to determine if indeed these rates are very low for these systems. Finding new VFXTs and determining their time averaged accretion rate using our monitoring campaign is only one way forward to increase our understanding of these enigmatic transients. We now discuss other avenues that can be explored as well to achieve that goal. First, a search in the data archives for previously unnoticed VFXTs (e.g., by comparing different exposures of the same fields) might lead to the detection of several more systems. Second, larger regions of our Galaxy need to be monitored at the desired sensitivity to detect the low fluxes observed from VFXTs. It is especially important to determine if a large number of VFXTs also exist outside the inner region of our Galaxy. \cite{2005ApJ...622L.113M} found that the excess of VFXTs within 10 arcminutes of Sgr A* is significant and might point to an unusual formation history of these systems. However, if a large number of VFXTs are also found further away from Sgr A* (e.g., systems like XMM J174716--2810.7 or SAX J1828.5--1037; \cite{2003ATel..147....1S, 2004MNRAS.351...31H}), then any production mechanism which requires the high stellar density near Sgr A* cannot be invoked for these VFXTs. There is currently no monitoring satellite in orbit which can perform that task mainly due to a lack of sensitivity and angular resolution of the instruments. However, it is possible to derive a first approximation to the number density of VFXTs at large distances from the center of a spiral galaxy by performing several deep pointings of the core of other spiral galaxies. The most obvious choice is the nearest large spiral galaxy to our own, M31. Within a $\sim$250 ksec exposure it is possible to observe a 3.7 kpc $\times$ 3.7 kpc region of M31 using the {\it Chandra}/ACIS-I detector with a limit sensitivity of $1-4 \times 10^{34}$ erg s$^{-1}$ (depending on the spectral properties of the sources). Several such deep pointings would detect all but the faintest X-ray transients in a large region of M31. Alternatively, such programs can also be performed for the smaller spiral galaxy M33 or for galaxies further away. In the latter case, the limit sensitivity will be of course less. \begin{acknowledgements} RW thanks Michael Muno and Andrew King for useful discussions about very faint X-ray transients. 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06:37 & 5.14 & ''\\ GC-7 & 6194 & 06:37 -- 08:09 & 5.14 & ''\\ GC-8 & 6196 & 08:09 -- 09:41 & 5.14 & ''\\ GC-9 & 6198 & 09:41 -- 11:13 & 5.15 & ''\\ GC-10 & 6200 & 11:13 -- 13:08 & 5.15 & ''\\ \hline GRS 1741.9--2854 \& & 6311 & 02:04 -- 03:40 July 1 & 4.01 & ACIS-I\\ XMM J174457--2850.3 & & & & '' \\ \hline \end{tabular} \end{table*} \begin{sidewaystable*} \begin{minipage}[t]{\columnwidth} \caption{The X-ray binaries detected during our observations} \label{table:binaries} \centering \renewcommand{\footnoterule}{} % to avoid a line before footnotes \begin{tabular}{c c c c c c c l } \hline\hline Source name & In FOV & Offset\footnote{Offset between the source position and the pointing position} & \multicolumn{3}{c}{Coordinates} & Count rate\footnote{Count rates are for the full {\itshape Chandra}/HRC-I energy range (0.08--10 keV) or the 0.3--7 keV energy range for the {\itshape Chandra}/ACIS-I; they are background corrected, but are not corrected for offset.} & Comment \\ & of frame & ($'$) & RA & Dec & Error ($''$)\footnote{The errors on the coordinates are calculated using equation 5 in \cite{hong2005} with the addition of a 0.7$''$ pointing uncertainty, corresponding to 95\% confidence levels.} & (counts s$^{-1}$)& \\ \hline 1E 1743.1--2843 & GC-1 & 6.6 & 17 46 21.094 & -28 43 42.3 & 1.1 & 0.247$\pm$0.007 & Persistent X-ray binary \\ & GC-2 & 18.5 & & & & & \\ 1A 1742--294 & GC-9 & 11.1 & 17 46 05.201 & -29 30 53.3 & 1.3 & 0.84$\pm$0.01 & Persistent neutron star low-mass X-ray binary \\ & GC-10 & 20.6 & & & & & Burst detected during GC-10 \\ GRS 1741.9--2853 & GC-2 & 9.9 & 17 45 02.385 & -28 54 50.2 & 1.5 & 0.267$\pm$0.008 & Neutron star faint X-ray transient \\ & GC-7 & 15.9 & & & & & \\ & ACIS-I & 6.9 & 17 45 02.350 & -28 54 49.9 & 1.2 & 0.269$\pm$0.008 & \\ XMM J174457--2850.3 & GC-7 & 11.4 & 17 44 57.440 & -28 50 20.3 & 1.5 & 0.337$\pm$0.009 & Very faint X-ray transient \\ & GC-2 & 13.6 & & & & & \\ & ACIS-I & 7.0 & 17 44 57.451 & -28 50 21.1 & 2.1 & 0.0064$\pm$0.0015& \\ \hline Possible VFXT & GC-3 & 8.7 & 17 47 37.671 & -29 08 09.6 & 2.5 & 0.007$\pm$0.002 & Transient nature needs confirmation \\ \hline \end{tabular} \end{minipage} \end{sidewaystable*} \begin{table*} \begin{minipage}[t]{1.1\columnwidth} \caption{Spectral results for GRS 1741.9--2854 and XMM J174457--2850.3} \label{table:spectral_fits} \centering \renewcommand{\footnoterule}{} % to avoid a line before footnotes \begin{tabular}{l c c} \hline\hline Parameter & GRS 1741.9--2854 & XMM J174457--2850.3 \\ \hline $N_{\rm H}$ ($10^{22}$ cm$^{-2}$) & $10.5^{+4.9}_{-3.7}$ & 6\footnote{Parameter fixed to the value found by \cite{2005MNRAS.357.1211S}.} \\ Photon index & $1.8^{+1.0}_{-0.8}$ & $1.3\pm1.1$ \\ Flux ($10^{-12}$ erg cm$^{-2}$ s$^{-1}$, unabsorbed) & & \\ ~~~~ 0.5--10.0 keV & $37^{+93}_{-14}$ & $0.5^{+0.4}_{-0.5}$ \\ ~~~~ 2.0--10.0 keV & $22^{+10}_{-3}$ & $0.4^{+0.2}_{-0.4}$ \\ ~~~~ 2.0--8.0 keV & $17^{+12}_{-3}$ & $0.3^{+0.1}_{-0.3}$ \\ \hline \end{tabular} \end{minipage} \end{table*} \newpage\clearpage \begin{figure} \centering \includegraphics[width=\textwidth]{rwijnands_f1.ps} \caption{Field-of-view of our monitoring campaign in Galactic coordinates. The circles indicate the {\it XMM-Newton}/MOS FOV which is similar to the {\it Chandra}/HRC-I FOV. The FOV is over-plotted on the {\itshape Chandra} Galactic center survey data as presented by \cite{2002Natur.415..148W}, which is centered around Sgr A* and covers a region of approximately 1\degr $\times$ 2\degr. The 'field name' (see Tab.~\ref{table:observations}) of each pointing is indicated.} \label{fig:fov} \end{figure} \begin{figure} \centering \includegraphics[width=0.5\textwidth]{rwijnands_f2a.ps}\includegraphics[width=0.5\textwidth]{rwijnands_f2b.ps} \caption{The merged image of the 7 {\itshape Chandra}/HRC-I observations. {\bf Left panel:} a sub-set of the detected foreground objects are indicated (only those stars which are known in Simbad are labeled) and also the Arches cluster. {\bf Right panel:} the detected X-ray binaries as well as the possible VFXT. Also indicated is the complex X-ray emission around Sgr A*. } \label{fig:images} \end{figure} \newpage\clearpage \begin{figure} \centering \includegraphics[width=.48\textwidth]{rwijnands_f3.ps} \caption{{\bf Left panel:} the {\it Chandra}/HRC-I image of XMM J174457--2850.3 and GRS 1741.9--2853 as obtained on June 5, 2005 (field GC-2), {\bf Right panel:} the {\it Chandra}/ACIS-I image of both sources obtained on July 1, 2005. To show the two sources most clearly we have rebinned the images to a resolution of $\sim$2$''$. The apparent extended nature of the sources is due to their relatively large offset positions with respect to the pointing direction of the satellite.} \label{fig:extra_image} \end{figure} \begin{figure} \centering \includegraphics[angle=-90,width=0.48\textwidth]{rwijnands_f4.ps} \caption{The spectra of GRS 1741.9--2853 (top graph) and XMM J174457--2850.3 (bottom graph). The solid lines trough the data points indicate the best absorbed power-law fit to the data. Before fitting, the data points of GRS 1741.9--2853 were rebinned so that each bin has 15 counts. The data of XMM J174457--2850.3 were fitted without any rebinning but for display purposes that data in the figure are rebinned to 3 counts per bin.} \label{fig:spectra} \end{figure} \begin{figure} \centering \includegraphics[width=0.48\textwidth]{rwijnands_f5.ps} \caption{The light curves of GRS 1741.9--2853 (top panel) and XMM J174457--2850.3 (bottom panel). In both panels, the triangles indicate the new data reported in this paper. In the top panel, the squares and the upper limits are taken from \cite{2005ApJ...622L.113M} (see this paper for the exact energy ranges for each point; our {\it Chandra} luminosities are for 2--8 keV). In the bottom panel, the squares and the upper limit are from \cite{2005MNRAS.357.1211S} and all luminosities are for 2--10 keV.} \label{fig:lc} \end{figure} \newpage \clearpage \appendix \section{Distribution of transients in the FOV \label{section:appendix}} In Table~\ref{table:sources_in_FOV} we have listed the known X-ray transients and persistent X-ray binaries which are in the FOV of our {\it XMM-Newton} and {\it Chandra} monitoring observations. This table also shows the distance of the sources with respect to Sgr A* and the maximum reported X-ray luminosity for these sources as found in the literature. We have converted the luminosities to the 2--10 keV energy range using the reported spectral parameters of the sources. From this table and from Figure~\ref{fig:Lx_v_distance} it can clearly be seen that only one bright to very bright transient is present (1A 1742--289) within a distance of $<$$15'$ from Sgr A*, but eight VFXTs\footnote{The VFXT CXOGC J174535.5--290124 is located in the error circle of the faint transient and eclipsing source AX J1745.6--2901. It is possible that both sources are the same one, which would reduce the number of VFXTs to seven although this would not affect the conclusions in this appendix. However, no eclipses were found for CXOGC J174535.5--290124 in the {\it Chandra} data available for this source (M. Muno 2005, private communication), making it less likely that both sources are the same one.} and two faint X-ray transients (although the two faint transients are just barely brighter than $10^{36}$ erg s$^{-1}$ and they could just be the brightest VFXTs in the FOV). Clearly, the number of VFXTs within 15$'$ of Sgr A* is significantly larger than that of the brighter transients. When extending the region out to 25$'$, we see eight VFXTs, three faint transients and only 3 bright to very bright transients. Note, however, that the region $>$10$'$ away from Sgr A* has been less sampled with sensitive X-ray instruments than closer to Sgr A*, meaning that the number of VFXTs within $25'$ of Sgr A* is likely to grow thanks to our monitoring program. This strongly suggests that the number density of VFXTs is indeed significantly higher than that of the brighter transients. The fact that the brighter systems were until recently much easier to detect than the fainter systems means that this discrepancy in number densities will only become larger in the future. However, one VFXT (CXOGC J174540.0--290031) and one faint transient (AX J1745.6--2901) have exhibited eclipses and there is strong evidence (\cite{munoetal2005}) that at least CXOGC J174540.0--290031 was intrinsically much brighter ($>$10$^{36}$ erg s$^{-1}$) than what we observe because the inner part of the system is blocked from our line of sight and we only observe the scattered (e.g., in a corona) X-ray emission from the system making it artificially seem very faint. One can argue that this holds true for the other eclipsing source and possibly for all VFXTs for which we have not yet observed the eclipses or the X-ray dips associated with high inclination. If we allow a large inclination range of 60$^\circ$--90$^\circ$, then a random distribution of orbital inclinations should give roughly equal numbers of bright transients and VFXTs (see also \cite{kingwijnands2005}; the solid angle is proportional to the cosine of the inclination). Clearly, the distribution of observed transients in the FOV of our monitoring campaign (see Tab.~\ref{table:sources_in_FOV}) shows a lack of brighter systems, even when also considering the two persistent X-ray binaries. Moreover, strong evidence exists that for the known dippers and eclipsing X-ray binaries in other parts of the Galaxy we {\it do} directly look at the inner part of the systems and not just indirectly via scattering (e.g., due to the detection of kHz QPOs or nearly coherent oscillations during type-I X-ray bursts in several high inclination sources; e.g., \cite{2000A&A...361..121B, 2000ApJ...539..847H, 2001ApJ...549L..71W, 2001ApJ...549L..85G}. Therefore, the observed X-ray luminosity is indeed the intrinsic luminosity of these systems. Only for the so-called accretion-disk-corona sources (which have the highest inclination of all systems) do we have evidence that only the scattered emission is seen making these systems appear fainter than they are intrinsically. The inclination range for such sources is significantly smaller than what we used above, thus making the problem even worse. Clearly, it is very likely that most of the VFXTs do not appear very faint as a result of inclination effects but rather that they are intrinsically very faint. The fact that VFXTs seem to be overabundant close to the Galactic center compared to brighter systems might also have consequences for certain types of models for the VFXTs. For example, \cite{kingwijnands2005} discussed briefly the possibility that the VFXTs harbor neutron stars and black holes which accrete from the weak wind of low-mass companion stars resulting in very faint outbursts. However, they argued that these systems must eventually evolve into brighter states because the companion stars will fill their Roche lobes at a certain time in the future. If indeed, as they suggested, these brighter states have longer durations than the wind accreting states, then the lack of brighter systems present among the very faint ones would suggest that this model cannot explain the nature of the VFXTs. \begin{table*} \begin{minipage}[t]{2.0\columnwidth} \caption{Persistent and transients X-ray binaries in the FOV of our observations} \label{table:sources_in_FOV} \centering \begin{tabular}{l c l l c c} \hline\hline Source name & Distance from Sgr A* & Maximum luminosity\footnote{These maximum observed luminosities are taken from the references and converted to a 2--10 keV luminosity for a distance of 8 kpc} & Classification & Comment & Reference\footnote{References: 1: \cite{2005ApJ...622L.113M}; 2: \cite{1976MNRAS.175P..47B}; 3: \cite{1996PASJ...48..417M}; 4: \cite{2003ApJ...598..474M}; 5: \cite{2005MNRAS.357.1211S}; 6: this paper; 7: \cite{2004A&A...416..311W}; 8: \cite{1996ApJ...469L..25G}; 9: \cite{1991AdSpR..11..187I}; 10: \cite{1999A&A...345..826C}; 11: \cite{2002ApJS..138...19S} } \\ & ($'$) & (erg s$^{-1}$) & & & \\ \hline \multicolumn{5}{c}{{\bf Transient sources}}\\ CXOGC J174540.0-290031 & 0.05 & $1\times 10^{35}$ & very faint & eclipser, radio counterpart & 1 \\ CXOGC J174541.0-290014 & 0.31 & $6\times 10^{33}$ & very faint & & 1\\ CXOGC J174540.0-290005 & 0.37 & $4\times 10^{34}$ & very faint & & 1\\ CXOGC J174538.0-290022 & 0.44 & $3\times 10^{34}$ & very faint & & 1 \\ 1A 1742-289 & 0.92 & $7\times 10^{38}$ & very bright & radio counterpart & 2\\ CXOGC J174535.5-290124\footnote{This source is located in the error circle of AX J1745.6--2901 and it is possible that both are the same source. However, no eclipses were found for CXOGC J174535.5--290124 in the {\it Chandra} data available for this source (M. Muno 2005, private communication) making this identification less likely.} & 1.35 & $4\times 10^{34}$ & very faint & & 1\\ AX J1745.6-2901 & 1.37 & $2\times 10^{36}$ & faint & burster, eclipser & 3\\ CXOGC J174554.3-285454 & 6.38 & $8\times 10^{34}$ & very faint & & 1 \\ GRS 1741.9-2853 & 10.00 & $2\times 10^{36}$ & faint & burster & 4\\ XMM J174544-2913.0 & 12.56 & $5\times 10^{34}$ & very faint & & 5\\ XMM J174457-2850.3 & 13.78 & $9\times 10^{35}$ & very faint & & 6 \\ SAX J1747.0-2853 & 19.55 & $4\times 10^{37}$ & bright & burster & 7\\ GRO J1744-28 & 21.71 & $3\times 10^{38}$ & bright & X-ray pulsar & 8\\ KS 1741-293 & 22.09 & $5\times 10^{36}$ & faint & & 9\\ \hline \multicolumn{5}{c}{{\bf Persistent sources}}\\ 1E 1743.1-2843 &18.99 & $3\times 10^{36}$ & faint & & 10\\ 1A 1742-294 &30.95 & $1\times 10^{37}$ & bright & burster & 11\\ \hline \end{tabular} \end{minipage} \end{table*} \begin{figure} \centering \includegraphics[width=0.48\textwidth]{rwijnands_fa1.ps} \caption{The observed maximum X-ray luminosities (2--10 keV) of the X-ray transients in our FOV as a function of the distance of the transients to Sgr A*. \label{fig:Lx_v_distance} } \end{figure} \end{document}