------------------------------------------------------------------------ gctransient.tex ApJ, submitted MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 7bit User-Agent: Internet Messaging Program (IMP) H3 (4.0.3) 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, required 5, USER_AGENT 0.00) %astro-ph/0508264 \documentclass[12pt,preprint]{aastex} % % \received{2005} %\revised{} %\accepted{} %\articleid{START PAGE}{END PAGE} %\paperid{} %\ccc{} %\cpright{}{} \shorttitle{GCRT~J1745$-$3009} \shortauthors{Hyman et al.} \newcommand\src{\protect\objectname[]{GCRT~J1745$-$3009}} \newcommand{\mjybm}{\mbox{mJy~beam${}^{-1}$}} \newcommand{\kms}{\mbox{km~s${}^{-1}$}} \begin{document} \title{A New Radio Detection of the Bursting Source \src} \author{Scott D.~Hyman} \affil{Department of Physics and Engineering, Sweet Briar College, Sweet Briar, VA 24595} \email{shyman@sbc.edu} \author{T.~Joseph~W.~Lazio} \affil{Remote Sensing Division, Naval Research Laboratory, Washington, DC 20375-5351} \email{Joseph.Lazio@nrl.navy.mil} \author{Subhashis Roy} \affil{ASTRON, P.O. Box 2, 7990 AA Dwingeloo, The Netherlands.} \email{roy@astron.nl} \author{Paul S.~Ray} \affil{E.~O.~Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375-5352} \email{Paul.Ray@nrl.navy.mil} \and \author{Namir E.~Kassim} \affil{Remote Sensing Division, Naval Research Laboratory, Washington, DC 20375-5351} \email{Namir.Kassim@nrl.navy.mil} \begin{abstract} \src\ is a transient bursting radio source located in the direction of the Galactic center, discovered in 330~MHz VLA observations from 2002 September 30--October 1 by \citeauthor{hlkrmy-z05}. We have searched for bursting activity from \src\ in nearly all of the available 330~MHz VLA observations of the Galactic center since 1989 as well as in 2003 GMRT observations. We report a new radio detection of the source in 330~MHz GMRT data taken on 2003 September 28. A single $\sim$0.5 Jy burst was detected, approximately 3$\times$ weaker than the five bursts detected in 2002. Due to the sparse sampling of the 2003 observation, only the decay portion of a single burst was detected. We present additional evidence indicating that this burst is an isolated one, but we cannot completely rule out additional undetected bursts that may have occured with the same $\sim$77 min. periodicity observed in 2002 or with a different periodicity. Assuming the peak emission was detected, the decay time of the burst, $\sim$2 min, is consistent with that determined for the 2002 bursts. Based on the total time for which we have observations, we estimate that the source has a duty cycle of roughly~10\%. \end{abstract} \keywords{Galaxy: center --- radio continuum --- stars: variable: other} \section{Introduction}\label{sec:intro} Transient radio emission has been detected from many astronomical sources including flare stars, brown dwarfs, masers, gamma ray bursts, pulsars, supernovae, neutron star and black-hole X-ray binaries, and active galactic nuclei. Efficient \emph{blind} searching for radio transients requires a telescope for which the product of the field of view~$\Omega$, the sensitivity or collecting area~$A$, and the ratio of the total observing time to the time resolution~$T/\delta t$ is ``large.'' Generally, radio telescopes have been able to maximize only two of these three quantities. Thus, the majority of radio transients have been found by either monitoring objects thought to be potential radio emitters (e.g., flare stars and brown dwarfs) or by followup observations of objects detected at higher energies (e.g., X-ray binaries and gamma-ray bursts). Recent developments in low frequency imaging techniques have produced wide-field images ($\approx 3\arcdeg$~FWHM) with uniform and high resolution across the field \citep{lklh00,nlkhlbd04} thereby enabling efficient searches for radio transients \citep{hlkb02,hlknn03} \src\ is a novel bursting radio source \citep{hlkrmy-z05}, whose notable properties include ``flares'' approximately 1~Jy in magnitude lasting approximately 10~min.\ each and occurring at apparently regular 77~min.\ intervals. This object is located about~1.25\arcdeg\ south of the Galactic center (GC, Figure~\ref{fig:chart}) and was identified from~330~MHz (90~cm) observations with the Very Large Array (VLA) on~2002 September~30. The source \src\ is notable because it is one of a small number of \emph{radio-selected} transients. Moreover, with only a few exceptions \citep{m02} such as electron cyclotron masers from flare stars and the planets, plasma emission from solar radio flares, pulsar radio emission, and molecular-line masers, most radio transients are incoherent synchrotron emitters. For an incoherent synchrotron emitter, the energy density within the source is limited to an effective brightness temperature of roughly $10^{12}$~K by the inverse Compton catastrophe \citep{readhead94}. The properties of \src\ suggest strongly that its brightness temperature exceeds $10^{12}$~K by a large factor and that it is a member of a new class of coherent emitters. The discovery observations of \src\ were based on VLA 330~MHz observations at a single epoch, from which only a limited amount of information about the source could be gleaned. This paper reports on a second detection of \src, made with the Giant Metrewave Radio Telescope (GMRT) in 2003, as well as on a series of 330~MHz nondetections resulting from archival observations and our Galactic center radio transient monitoring program. The observations are summarized in \S\ref{sec:observe} and the results in \S\ref{sec:discuss}. We discuss briefly the environment of the source in \S\ref{sec:environment}, and we present our conclusions in \S\ref{sec:conclude}. \section{Observations}\label{sec:observe} Table~\ref{tab:log} summarizes the 330~MHz observations with the two telescopes. Most observations consist of a few, long scans with occasional interruptions for phase calibration (see below). At a few epochs, however, the duration of the observations was not obtained in a single observation, but in multiple, short and widely spaced scans. About half of the observations had durations shorter than the 77~min.\ burst period observed in 2002. At both telescopes, both right- and left-circular polarization were recorded. The flux density of \src, even at its peak, is far less than the total flux density contributed by other sources in the field of view. Thus, the source can be detected only in images. In turn, because of the relatively large fields of view and the number of sources within the field of view, the entire field of view must be imaged. Production of the images was conducted in a consistent manner from epoch to epoch. Calibration of the flux density was by reference either to~\objectname[3C]{3C~48} or~\objectname[3C]{3C~286}. Initial calibration of the visibility phases was obtained by observations of a nearby VLA or GMRT calibrator, typically \objectname[]{J1714$-$252}. At~330~MHz, radio frequency interference (RFI) can be a substantial problem, and, if not excised from the visibility data, it would limit the dynamic range of the final image. We examined the visibility data for \hbox{RFI} and excised it. At~330~MHz, neither the VLA nor the GMRT can be assumed to be coplanar; in order to image the entire field of view, we used a polyhedral imaging algorithm to compensate for the non-coplanarity of the arrays \citep{cp92}. In order to approach thermal noise limits in the images, several iterations of imaging, deconvolution (\textsc{clean}ing), and self-calibration were used. In order to search for bursts from \src, the \textsc{clean} components of all other sources in the field were subtracted from the $u$-$v$ data, and the residual data were then imaged in 10~min.~subsets. Noise levels of the 10~min.\ images range from approximately 10~\mjybm\ for the GMRT and 20~\mjybm\ for the most extended VLA configurations (A and B) to approximately 250~\mjybm\ for the more compact configurations (C and D), which have both a lower angular resolution and are more susceptible to \hbox{RFI} and sidelobe confusion. If a burst was detected, the residual data then were imaged with a higher time resolution (from~5 to~30~s) in order to search for structure within the burst. All but four of the VLA observations listed in Table~\ref{tab:log} are pointed in the direction of \objectname[]{Sgr~A*}, approximately 1.25\arcdeg\ north of \src. (Coincidentally, the discovery observations were pointed nearly directly at the source.) The GMRT observations are pointed approximately 0.5\arcdeg\ west. The primary beam attenuation of the VLA and the GMRT reduces the apparent flux density of the source by a factor of approximately 2 and 1.5, respectively. While significant, this level of primary beam attenuation would not be sufficient to prevent the recovery of the source, provided that the amplitude of the bursts is approximately 1~Jy. However, if the bursts have a range of amplitudes, significantly weaker bursts ($\lesssim 150$~mJy) could have gone undetected in the vast majority of our observations. \section{Results}\label{sec:discuss} We detect \src\ at two epochs, 2002 September~30--October~1 and 2003 September~28. The latter epoch is a new recovery of the source, while the former epoch is that of the discovery by \cite{hlkrmy-z05}. Figure~\ref{fig:contour2002} shows contour images before, during, and after the fourth burst detected on~2002 September~30. Figure~\ref{fig:lightcurve} shows the light curves for the five 2002 September~30 bursts, with 30-s sampling, and the 2003 September~28 burst with 17-s sampling. Unfortunately, the recovery observation on~2003 September~28 consisted of approximately 10~min.\ scans spaced approximately an hour apart for several hours. Only a single burst, already in its decay phase, is detected at the beginning of a scan (2003 September~28 11:44:53, IAT). % Figure~\ref{fig:contour2003} shows the image of the 2003 burst As shown in Figure~\ref{fig:lightcurve}, the shape of the decay profile for the 2003 September~28 burst is consistent with that seen for the 2002 September~30 bursts. Assuming that this burst is consistent in duration with those from 2002 September~30, the 2003 September~28 burst had a peak of approximately 0.5~Jy, compared to~1--1.5~Jy for those on~2002 September~30. Clearly, a longer burst duration implies a higher peak flux density. Figure~\ref{fig:5seclightcurve} shows the fourth burst from~2002 September~30 with the full 5-s sampling. The steep decay of the bursts is depicted much more clearly with higher time resolution. We have fitted both the rising and decay portions of the bursts with an exponential function. None of the apparent structure in the light curves (Figure~\ref{fig:lightcurve}) is significant above the $2\sigma$ level, and no significant structure is evident in the 5-s light curves that is not also present in the 30-s light curves. The source is unresolved in both epochs. The angular resolution ($20\arcsec\times10\arcsec$) and sensitivity (50~\mjybm\ for 17-s integrations) of the 2003 September~28 recovery observation is significantly improved over the 2002 September~30 discovery observation. Fitting a Gaussian to the source yields a position of (J2000) right ascension $17^{\mathrm{h}}$ $45^{\mathrm{m}}$ 5\fs23 ($\pm 0\fs38$), declination $-30\arcdeg$ 09\arcmin\ 53\arcsec\ ($\pm 5\arcsec$), which is approximately a factor of two more more accurate in each dimension than determined in the 2002 September~30 observation. Observations at~330~MHz are affected strongly by ionospheric phase fluctuations. Their impact includes refractive position shifts. We used eight nearby small-diameter sources from the NRAO VLA Sky Survey (NVSS) \citep{ccgyptb98} to register our images. The NVSS was conducted at~1400~MHz and has a substantially better astrometric accuracy of 0\farcs5 in both right ascension and declination for bright sources. We found an average ionospheric-induced refraction of $0\fs23 \pm 0\fs24$ in right ascension and $-4\farcs6 \pm 3\farcs4$ in declination. The source position and uncertainty cited above include a correction for this refraction. No frequency dependence was detected in the 2002 September~30 bursts, and none is detected across the 60-channel, 15-MHz bandpass for the 2003 September~28 burst. %A wavelength dependence of $\lambda^{6}$ is predicted for a coherent %source A power-law fit across the 15-MHz bandpass of the 2003 September~28 observation yields a wavelength dependence of $S \propto \lambda^{4 \pm 5}$. No circular polarization is detected in the bursts with an upper limit of 15\% obtained for both epochs. Linear polarization measurements are not available for either the discovery or recovery observations. No emission is detected from \src\ when imaging the 2003 September~28 observation at times when the burst is not occurring. We are able to improve the ($5\sigma$) upper limit for~330~MHz interburst emission from~75~mJy, for the discovery observations, to~25~mJy, for the recovery observation. The upper limit on quiescent emission during periods of no burst activity is 15~mJy at~330~MHz \citep{hlkrmy-z05}. Nondetections on~2005 March~25 at both 330 and~1400~MHz also yield an upper limit of~15~mJy at~330~MHz, but a significantly reduced upper limit of~0.4~mJy at~1400~MHz, as compared to a 35~mJy upper limit obtained from a 2003 January observation at that frequency. We have also learned that \src\ has been observed in early and mid-2005 with the Westerbork Synthesis Radio Telescope (WSRT) at both 330 and~1400~MHz. Upper limits on any emission are approximately a few milliJanskys (R.~Braun~2005, private communication). As \cite{hlkrmy-z05} reported, during the discovery epoch (2002 September~30--October~1) the bursts from the source had an approximate 77~min.\ periodicity. As noted above, the recovery on 2003 September~28 occurs in the midst of a set of~10-min.\ scans, spread over several hours. Assuming that the source was emitting periodic bursts at this epoch, each of 10~min.\ duration, we have determined the periods at which bursts could occur while being consistent with the gaps and non-detections during the 2003 September~28 epoch. There are 212~min.\ between the detected burst and the end of the observation. Thus, we can place no constraints on periods longer than 212~min. For shorter periods, only the following ranges of periods are allowed: 36--37~min., 71--78~min., 107--112~min., 131--160~min., and 179--195~min. We estimate that the uncertainty in making these determinations is perhaps 1~min.\ and results from slightly varying noise levels within the scans and the assumption that the duration of the bursts remains fixed at~10~min. Thus, while consistent with the gaps and non-detections, the possible 36--37~min.\ periodicity is perhaps only marginally so. A 77~min.\ periodicity remains consistent with the 2003 September 28 observations. We cannot use the interval between~2002 September~30 and~2003 September~28 to constrain the burst activity because the uncertainty on the 77~min.\ period determined from the 2002 September~30 observations is sufficiently large ($\sim 15$~s) that we cannot connect the phase between the two observations. Indeed, the nearest observation to the 2002 September~30 observation precedes it by~70~days while the nearest to the 2003 September~28 observation, other than that on 2003 September~29, follows it by~16~days. The current uncertainty is large enough that a single burst could not be connected in phase over these intervals, even if the source had been detected. In addition, the sparse sampling of the observation made one day after the 2003 September~28 detection does not include any scan at multiples of 77 min.\ later, nor do the scans, when taken together with those on September~28, significantly alter the allowed ranges of other periods given above. Thus, we cannot use the nondetection on 2003 September~29 to place limits on the duration of the active period of the source. The 2003 September~28 and~29 observations were part of a series of GMRT observations designed to survey the entire Galactic center region. These include a number of other pointings, not included in Table~\ref{tab:log}, that potentially could be used to detect \src, albeit with much larger primary beam attenuation. In particular, there are scans on \objectname[]{Sgr~A}, some 1\fdg3 away from both \src\ and the recovery observation's pointing center, that end approximately 10~min.\ before, 77~min.\ before, and~154~min.\ after the decay portion of the burst detected in the recovery observation. We estimated the amount of primary beam attenuation by measuring the peak flux density of \emph{\objectname[]{Sgr~A}} from the recovery observation of \src. The primary beam attenuation is approximately a factor of~10. Thus, in the scans on \objectname[]{Sgr~A}, if the transient were bursting at the level of about~1~Jy, it would appear as a 100~mJy source. Such a source would still be well above the noise level in the images for each scan, but \src\ is not detected in any of them. Thus, the upper limit for the duration of the detected burst is approximately 13~min.\ and consistent with the 10~min.\ duration observed for the 2002 September~30 bursts. Furthermore, the 77~min.\ period and the range of other periods allowed by considering only the recovery observation scans (see above) are largely ruled out by nondetections in the \objectname[]{Sgr~A} scans from 2003 September 28. Thus, this burst appears to be an isolated one, in contrast to the 2002 September~30 bursts. Finally, we note that our northern GC-pointed GMRT scans, centered $\sim$2\fdg5 to the northeast of \src\ and the bright source, Sgr~E~46 (see Figure~\ref{fig:contour2002}), detect Sgr~E~46, but at a very low level due to severe primary beam attenuation ($\sim$200$\times$). One of these northern pointings lasted from 11:36 to 11:44 on 2003 September 28, just before the burst's decay phase was detected at 11:45 at the beginning of the southern pointing that followed. Since Sgr~E~46 is located only 0\fdg1 closer to the pointing center than \src, we have corrected the 5~min.\ noise level at the position of \src\ by the $\sim$200$\times$ primary beam attenuation factor to crudely estimate the upper limit of the peak of the 2003 detected burst. An upper limit of $\sim$5~Jy is obtained, consistent with the 1.5~Jy peak values observed in the 2002 bursts. However, we consider this result and the evidence that the burst is an isolated one to be very tentative, since the location of \src\ is very far out on the primary beam for these pointings, and since the beam shape could be asymmetric. As an initial crude estimate for the duty cycle of the bursting behavior of \src, we compare the time during which the source is observed to be active to the total amount of observing time. The total observing time is almost exactly 70~hr. The 2002 September~30 bursts lasted for at least 6~hr; because only a single burst was detected on~2003 September~28, we assume that the source was active for~1~hr. Thus, the apparent duty cycle of \src\ is approximately 10\%. Finally, we note that both the original discovery and the recovery observation occur in late September and are separated by~$\sim$1~yr. However, given that the discovery and recovery observations occurred with different telescopes and that there was no detection in our 6-hr 1998 September 25 observation, we can identify no seasonal nor instrumental explanation that would indicate that the source is not a celestial object. By analogy with the model proposed by \cite{tpt05}, it might be the case that the activity of \src\ results from orbital motion with an approximate 1-yr period. However, any such model would also have to explain the non-detections in the 1998 September~25 and~26 epochs. \section{Environment of \src}\label{sec:environment} If \src\ is located at the Galactic center, prevailing models explain it as a compact object, most likely a neutron star \citep[e.g.,][]{tpt05}. Motivated by a prediction in \cite{tpt05}, we have examined images that contain the field around \src\ in an effort to detect any faint nebulosity, such as might result from a supernova remnant. We have examined images at~330~MHz \citep{lklh00,nlkhlbd04}, 1400~MHz \citep{y-zhc04}, and~2~\micron\ (2MASS); the number of images that we can search is small because the location of the source is outside of the field of view of many images of the Galactic center region. As seen in Figure~\ref{fig:chart}, \src\ is located approximately 10\arcmin\ from the center, and just outside, of the shell-type supernova remnant \objectname[]{SNR~G359.1$-$0.5} \citep{rf84}. At a distance of~$8D_8$~kpc, this angular distance corresponds to a transverse distance of approximately $25D_8$~pc. The SNR itself is old, as evidenced by its size and the extent to which the shell appears ``broken up.'' Assuming that its age is $10^5T_5$~yr, if \src\ and the SNR are related, then \src\ would have to have a velocity of about~$225D_8/T_5$~\kms\ to have reached its current location. This velocity is well within those observed for neutron stars detected as pulsars \citep{acc02}. In general, there is no diffuse emission surrounding the location of \src. One possible exception is some faint emission from the shell of SNR~G359.1$-$0.5, which lies about~1\arcmin\ north of the position of the transient. While this close proximity could be indicative of a connection between the SNR and \src, there is otherwise no distortion in the shell of the \hbox{SNR}, akin to that seen for \objectname[]{G5.4$-$1.2} and \objectname[PSR]{PSR~B1757$-$24} \citep{fk91}, nor does \src\ have a cometary appearance similar to a pulsar wind nebula (PWN) like the \objectname[]{Mouse} \citep{gaensleretal04}. \section{Conclusions}\label{sec:conclude} We have summarized a series of Very Large Array and Giant Metrewave Radio Telescope observations of \src\ (Table~\ref{tab:log}). We detect \src\ at two epochs, 2002 September~30--October~1 and 2003 September~28. The latter epoch is a new recovery of the source, while the former epoch is that of the discovery by \cite{hlkrmy-z05}. The two sets of detections of \src\ are consistent with the source producing approximately 1~Jy bursts; we cannot exclude the possibility that the source produces significantly weaker bursts ($\lesssim 150$~mJy) more frequently. \cite{hlkrmy-z05} reported that the bursts appear to have a $77.1 \pm 0.3$~min.\ periodicity; we have provided tentative evidence indicating that the 2003 September~28 burst is an isolated one. Given the epochs of observations, we estimate crudely that the source is active for approximately 7~hr, and the apparent duty cycle of its activity is roughly 10\%. We have examined the field around \src\ at radio and infrared wavelengths. We find possible nebulosity at 1.4~GHz in the shell of \objectname[]{SNR~G359.1$-$0.5} near the location of the source, but otherwise no connection between the SNR and the transient. The velocity required for \src\ to have originated at the center of \objectname[]{SNR~G359.1$-$0.5} and reached its current transverse separation is only roughly 225~\kms. While well within the range of velocities observed for various neutron stars, there is also no compelling reason to think that \src\ and the SNR are related. Additional observations are required to determine more about the nature of \src. As well as additional searches such as those that we report here, infrared observations to search for a counterpart, a periodicity search for weaker pulsed emission, and X-ray observations to search for quiescent X-ray emission would all be useful. \acknowledgements We thank W.~Cotton for providing us with the 20~cm image from \cite{y-zhc04} to search for possible nebulosity near \src. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. S.D.H.\ is supported by funding from the Jeffress Memorial Trust and Research Corporation. Basic research in radio astronomy at the NRL is supported by the Office of Naval Research. \begin{thebibliography}{} \bibitem[\protect\citeauthoryear{Arzoumanian, Chernoff, \& Cordes}{Arzoumanian et al.}{2002}]{acc02} Arzoumanian, Z., Chernoff, D.~F., \& Cordes, J.~M. 2002, \apj, 568, 289 %The Velocity Distribution of Isolated Radio Pulsars \bibitem[\protect\citeauthoryear{Condon et al.}{1998}]{ccgyptb98} Condon, J.~J., Cotton, W.~D., Greisen, E.~W., Yin, Q.~F., Perley, R.~A., Taylor, G.~B., \& Broderick, J.~J. 1998, AJ, 115, 1693 \bibitem[\protect\citeauthoryear{Cornwell \& Perley}{1992}]{cp92} Cornwell, T.~J.\ \& Perley, R.~A. 1992, \aap, 261, 353 \bibitem[\protect\citeauthoryear{Frail \& Kulkarni}{1991}]{fk91} Frail, D.~A.\ \& Kulkarni, S.~R. 1991, \nat, 352, 785 \bibitem[\protect\citeauthoryear{Gaensler et al.}{2004}]{gaensleretal04} Gaensler, B.~M., van der Swaluw, E., Camilo, F., Kaspi, V.~M., Baganoff, F.~K., Yusef-Zadeh, F., \& Manchester, R.~N. 2004, \apj, 616, 383 \bibitem[\protect\citeauthoryear{Hyman et al.}{2005}]{hlkrmy-z05} Hyman, S.~D., Lazio, T.~J.~W., Kassim, N.~E., Ray, P.~S., Markwardt, C.~B., \& Yusef-Zadeh, F. 2005, \nat, 434, 50 \bibitem[\protect\citeauthoryear{Hyman et al.}{2003}]{hlknn03} Hyman, S.~D., Lazio, T.~J.~W., Kassim, N.~E., Nord, M.~E., \& Neureuther, J.~L. 2003, %A search for radio transients at 0.33 GHz in the GC {Astron. Nachr.}, 324, 79 \bibitem[\protect\citeauthoryear{Hyman et al.}{2002}]{hlkb02} Hyman, S.~D., Lazio, T.~J.~W., Kassim, N.~E., \& Bartleson, A.~L. 2002, %Low-frequency radio transients in the galactic center \aj, 123, 1497 \bibitem[\protect\citeauthoryear{LaRosa et al.}{2000}]{lklh00} LaRosa, T.~N., Kassim, N.~E., Lazio, T.~J.~W., \& Hyman, S.~D. 2000, %A wide-field 90 centimeter VLA image of the galactic center %region \aj, 119, 207 \bibitem[\protect\citeauthoryear{Melrose}{2002}]{m02} Melrose, D.~B. 2002, %Coherent emission in AGN: a critique Publ.\ Astron.\ Soc.\ Aust., 19, 34 \bibitem[\protect\citeauthoryear{Nord et al.}{2004}]{nlkhlbd04} Nord, M.~E., Lazio, T.~J.~W., Kassim, N.~E., Hyman, S.~D., LaRosa, T.~N., Brogan, C.~L., \& Duric, N. 2004, %High-resolution, wide-field imaging of the galactic center %region at 330 MHz \aj, 128, 1646 \bibitem[\protect\citeauthoryear{Readhead}{1994}]{readhead94} Readhead, A.~C. 1994, \apj, 426, 51 \bibitem[\protect\citeauthoryear{Reich \& F\"urst}{1984}]{rf84} Reich, W.\ \& F\"urst, E. 1984, \aaps, 57, 165 %G357.7+0.3 and G359.1-0.5 - Two Shell-type Supernova Remnants in the Galactic Centre Region \bibitem[\protect\citeauthoryear{Turolla, Possenti, \& Treves}{Turolla et al.}{2005}]{tpt05} Turolla, R., Possenti, A., \& Treves, A. 2005, \apj, in press; astro-ph/0506199 % Is the Bursting Radio-source GCRT J1745-3009 a Double % Neutron Star Binary? \bibitem[\protect\citeauthoryear{Yusef-Zadeh, Hewitt, \& Cotton}{Yusef-Zadeh et al.}{2004}]{y-zhc04} Yusef-Zadeh, F., Hewitt, J.~W., \& Cotton, W. 2004, \apjs, 155, 421 \end{thebibliography} \clearpage \begin{figure} \epsscale{0.45} \rotatebox{-90}{\plotone{f1.eps}} %\includegraphics[angle=270,width=6.0in]{Fig1.eps} \caption[]{Image of the Galactic center field at~330~MHz from the discovery observations on 2002 September~30 \citep{hlkrmy-z05}. The transient source \src\ is indicated by the small box below the approximately 20\arcmin\ diameter shell of \protect\objectname[]{SNR~359.1$-$00.5}. The resolution and sensitivity of the image are $48\arcsec \times 39\arcsec$ and 15~\mjybm, respectively. \src\ appears as a 100~mJy source here since it is averaged over five, short ($\sim 10$~min.), roughly 1~Jy bursts out of a total of a 6-hr observation. Other sources in the field of view include the sources to the west which are part of \protect\objectname[]{Sgr~\hbox{E}}, the \protect\objectname[]{Snake} is the linear feature to the north, and the \protect\objectname[]{Mouse} is northeast of \src.} \label{fig:chart} \end{figure} \begin{figure} \begin{center} \epsscale{0.45} \rotatebox{-90}{\plotone{f2a.ps}}\, \rotatebox{-90}{\plotone{f2b.ps}}\\ \rotatebox{-90}{\plotone{f2c.ps}} \end{center} \caption[]{A 330~MHz VLA image of the field surrounding \src\ and the source Sgr~E~46 (at upper right in each image) made in 5-min.\ intervals just before (top left), during (top right), and just after (bottom) the fourth burst detected in~2002 September~30. The fourth burst is shown because sampling of it is complete. Most of the bursts detected in this epoch were sampled only partially due to (unfortunately-timed) interruptions for phase calibration. The transient is located at the bottom left in the center image. The contour levels are $-0.4$, 0.4, 0.7, 1.0, and~1.3~Jy~beam${}^{-1}$.} \label{fig:contour2002} \end{figure} \begin{figure} \epsscale{1.0} \plotone{f3.eps} \caption{The light curves of \src. The top panel shows the single detected burst from~2003 September~28 with 17-s sampling; the remaining panels show the bursts from~2002 September~30 with 30-s sampling \citep{hlkrmy-z05}, arranged with the fifth burst shown in the second panel to the first burst in the bottom panel. For the 2002 September~30 bursts, the light curve has been folded at the apparent 77.1~min.\ periodicity. For the 2003 September~28 burst, the light curve has been aligned in time to be consistent with the decay portions of the 2002 bursts. In many cases, because the existence of \src\ was not known at the time of the observation, the full burst is not captured because the observations were interrupted for calibration observations. The 2003 September~28 observations consisted of one 7~min.\ scan per hour for several hours. The arrows represent 3$\sigma$ upper limits for nondetections.} \label{fig:lightcurve} \end{figure} %\begin{figure} %\plotone{Fig4.ps} %\caption[]{330 MHz GMRT image of the single burst detected during its $\sim$2 min decay %phase on 2003 September 28. The contour levels are 1~Jy beam$^{-1} \times (-0.15, 0.15, %0.2, 0.25, 0.3$, and 0.35).} %\label{fig:contour2003} %\end{figure} %\begin{figure} %\plotone{Fig4.eps} %\caption[]{The 2003 burst light curve (circles) normalized by a factor of 3.2 %to the average light curve %of the five 2002 bursts (squares) reported in \cite{hlkrmy-z05}. %We have aligned the 2003 light curve so that the decay is %consistent with that of the 2002 average light curve.} %\label{fig:lc2003norm} %\end{figure} \begin{figure} \plotone{f4.eps} \caption[]{The fourth burst detected in the 2002 September~30 observations shown with the full 5-s sampling. The arrows represent 3$\sigma$ upper limits for nondetections.} \label{fig:5seclightcurve} \end{figure} %\begin{figure} %\plotone{Fig7.eps} %\caption[]{1.4 GHz image (reference) showing emission from the shell of SNR~G359.1$-$0.5 %near the location of \src.} %\label{fig:20cmchart} %\end{figure} \clearpage \begin{deluxetable}{lcccc} \tablecaption{330 MHz Observational Log\label{tab:log}} %Message-ID: <1112467536.424ee850ddc5d@webmail.sbc.edu> %From: shyman@sbc.edu %Subject: GCRT epochs %Date: Sat, 2 Apr 2005 13:45:36 -0500 \tablewidth{0pc} \tabletypesize{\small} \tablehead{ % & % & % & % & % & \\ % & \colhead{Image} \\ \multicolumn{2}{c}{Epoch\tablenotemark{a}} & \colhead{Telescope\tablenotemark{b}} % & \colhead{Frequency} & \colhead{Bandwidth} & \colhead{Duration} \\ % & \colhead{Noise Level} \\ & & % & \colhead{(MHz)} & \colhead{(MHz)} & \colhead{(min.)}} % & \colhead{(\mjybm)}} \startdata 1989 March~18 & 09:49:40 & VLA:B & 12.5 & 331.8 \\ \\ 1995 October~14 & 22:49:40 & VLA:B & 1.6 & 30.7 \\ \\ 1996 October~19 & 23:03:30 & VLA:A & 6.2 & 173.5 \\ 1996 October~19 & 19:48:50 & VLA:A & 6.2 & 176.0 \\ \\ 1997 February~06 & 13:57:10 & VLA:BnA & 6.2 & 76.7 \\ \\ 1998 November~29 & 16:52:20 & VLA:C & 3.1 & 380.7 \\ 1998 September~26 & 02:24:30 & VLA:B & 3.1 & 124.0 \\ 1998 September~25 & 21:08:00 & VLA:B & 3.1 & 277.5 \\ 1998 March~14 & 14:40:50 & VLA:A & 3.1 & 98.8 \\ 1998 March~14 & 10:06:10 & VLA:A & 3.1 & 231.8 \\ \\ 1999 May~31 & 04:53:00 & VLA:D & 3.1 & 376.7 \\ \\ 2001 September~05 & 00:40:60 & VLA:C & 3.1 & 181.3 \\ \\ 2002 March~26 & 10:46:30 & VLA:A & 6.2 & 29.7 \\ 2002 March~26 & 11:18:50 & VLA:A & 6.2 & 34.7 \\ 2002 April~27 & 09:10:40 & VLA:A & 6.2 & 83.8 \\ 2002 May~17 & 08:51:50 & VLA:AB & 6.2 & 83.8 \\ 2002 June~24 & 09:22:00 & VLA:B & 6.2 & 34.5 \\ 2002 July~21 & 06:05:60 & VLA:B & 6.2 & 59.2 \\ %detected 2002 September~30\tablenotemark{c} & 20:48:30 & VLA:BC & 6.2 & 289.7 \\ % detected 2002 October~01\tablenotemark{c} & 02:34:45 & VLA:BC & 6.2 & 53.8 \\ \\ 2003 January~20 & 15:27:40 & VLA:CD & 6.2 & 187.8 \\ %2003 June~17\tablenotemark{d} % & 04:58:20 & VLA:A & 6.2 & 30.0 \\ 2003 July~05 & 08:09:50 & VLA:A & 6.2 & 34.5 \\ 2003 July~08 & 06:28:10 & VLA:A & 6.2 & 59.2 \\ 2003 July~12 & 04:12:50 & VLA:A & 6.2 & 34.7 \\ 2003 July~14 & 04:04:60 & VLA:A & 6.2 & 34.5 \\ 2003 July~28 & 07:09:20 & VLA:A & 6.2 & 34.5 \\ 2003 August~09 & 01:52:50 & VLA:A & 6.2 & 59.2 \\ 2003 August~18 & 23:43:40 & VLA:A & 6.2 & 59.2 \\ % (detected 1 burst) 2003 September~28\tablenotemark{c,d} & 10:06:26 & GMRT & 15 & 63 \\ 2003 September~29\tablenotemark{d} & 11:51:10 & GMRT & 15 & 45 \\ 2003 October~14 & 00:32:50 & VLA:AB & 6.2 & 59.2 \\ 2003 October~21\tablenotemark{d} & 07:27:09 & GMRT & 15 & 45 \\ 2003 October~24 & 00:23:30 & VLA:B & 6.2 & 59.0 \\ 2003 November~23 & 21:16:40 & VLA:B & 6.2 & 39.7 \\ %2003 December~20 & 19:30:30 & VLA:B & 6.2 & 64.1 \\ 2003 December~29 & 18:55:10 & VLA:B & 6.2 & 64.0 \\ \\ 2005 March~25 & 13:13:30 & VLA:B & 5.0 & 72.0 \\ \enddata \tablenotetext{a}{We provide the IAT start time of the observation for use in a later analysis. However, depending upon the observing program, the duration of the observation may not have been obtained in a single observation, but in multiple shorter ones at the epoch.} \tablenotetext{b}{The notation ``VLA:A'' refers to the A configuration of the \hbox{VLA}.} \tablenotetext{c}{Source detected at this epoch.} \tablenotetext{d}{This epoch consists of multiple, short scans taken over several hours.} \end{deluxetable} \end{document}