------------------------------------------------------------------------ bandyopadhyay_r_aph.tex AIP Conf. Proc., 2008, in press Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Virus-Scanned: by UF Astronomy Mail Virus Scanner ks/14/4/2005 X-MailScanner-Information: Please contact the postmaster@aoc.nrao.edu for more information X-MailScanner: Found to be clean X-MailScanner-SpamCheck: not spam, SpamAssassin (not cached, score=0, required 5, autolearn=disabled) X-MailScanner-From: reba@astro.ufl.edu X-Spam-Status: No % astro-ph/0803.2721 \documentclass[12pt,twoside]{article} \usepackage{xrb2007} \pagestyle{myheadings} \usepackage{natbib,graphics,graphicx,subfigure} \newcommand{\eg}{{\sl e.g.~}} \newcommand{\ie}{{\sl i.e.~}} \newcommand{\etal}{{\sl et al.}} \newcommand{\Msun}{\mbox{$M_{\odot}$}} \newcommand{\Rsun}{\mbox{R$_{\odot}$}} \newcommand{\Lsun}{\mbox{L$_{\odot}$}} \newcommand{\JHK}{$J\!H\!K~$} \newcommand{\degs}{$^{\circ}~$} \newcommand{\pms}{$\pm$} \newcommand{\ergs}{$\,$erg$\,$s$^{-1}$} \newcommand{\Chandra}{{\it Chandra~}} \begin{document} % select your session by uncommenting the appropriate line %\session{Jets} %\session{Jet and Black Hole Binaries} \session{Faint Galactic XRB Populations} %\session{Faint XRBs and Galactic LMXBs} %\session{Obscured XRBs and INTEGRAL Sources} %\session{ULXs} %\session{Extragalactic Populations} %\session{Future Missions and Surveys} %\session{Population Synthesis} \shortauthor{Bandyopadhyay \etal} \shorttitle{The Path to Buried Treasure} \title{The Path to Buried Treasure: Paving the Way to the FLAMINGOS-2 Galactic Center Survey with IR and X-ray Observations} \vspace{-2mm} \author{Reba M. Bandyopadhyay, Stephen S. Eikenberry, Curtis Dewitt} \affil{Dept. of Astronomy, University of Florida, Gainesville, FL 32611} \author{Andrew J. Gosling} \affil{Dept. of Astrophysics, University of Oxford, Oxford OX1 3RH, UK} \author{Michael P. Muno} \affil{California Institute of Technology, Pasadena, CA 91125} \author{and the F2GCS Team} \begin{abstract} I describe the IR and X-ray campaign we have undertaken to determine the nature of the faint discrete X-ray source population discovered by \Chandra in the Galactic Center. These results will provide the input to the FLAMINGOS-2 Galactic Center Survey (F2GCS). With FLAMINGOS-2's multi-object IR spectrograph we will obtain 1000s of IR spectra of candidate X-ray source counterparts, allowing us to efficiently identify the nature of these sources, and thus dramatically increase the number of known X-ray binaries and CVs in the Milky Way. \end{abstract} \vspace{-8mm} \section{Introduction} \vspace{-1mm} The unprecedented sensitivity and angular resolution of \Chandra has been utilized by Wang \etal~ (2002; hereafter W02) and Muno \etal~ (2003; hereafter M03) to investigate the X-ray source population of the Galactic Center (GC). The W02 ACIS-I survey of the central 0.8\degs$\times$2\degs of the GC revealed a large population of previously undiscovered discrete weak sources with X-ray luminosities of $10^{32}-10^{35}$\ergs. M03 imaged the central 40 pc$^{2}$ (at 8 kpc) around Sgr A* (Fig. 1a), finding an additional $\sim$2300 discrete point sources down to a limiting flux of $10^{31}$ erg/s. Some individual sources have been identified as X-ray transients, high-mass stars, LMXBs, and CVs. However, the nature of the majority of these newly detected sources is as yet unknown. We have undertaken an IR and X-ray observational campaign to determine the nature of the faint discrete X-ray source population discovered by \Chandra in the GC. Data obtained to date includes a deep \Chandra survey of the Nuclear Bulge; deep, high resolution IR imaging from VLT/ISAAC, CTIO/ISPI, and the UKIDSS Galactic Plane Survey; and IR spectroscopy from VLT/ISAAC and IRTF/SpeX. By cross-correlating the X-ray imaging with our IR surveys, we initially identify candidate counterparts to the X-ray sources via astrometry. Using a detailed IR extinction map, we are deriving magnitudes and colours for all the candidates. Having thus established a target list, we will use the multi-object IR spectrograph FLAMINGOS-2 on Gemini South to carry out a spectroscopic survey of the candidates, to search for the emission lines which are a hallmark of accreting binaries. By determining the nature of these sources, this FLAMINGOS-2 Galactic Center Survey will have a dramatic impact on our knowledge of the Galactic accreting binary population. \vspace{-2mm} \section{X-ray/IR Cross-Correlation} \vspace{-1mm} We have cross-correlated the source catalog derived from the W02 \Chandra survey with \JHK images of 26 selected 2.5\arcsec$^{2}$ regions obtained with ISAAC on the VLT to identify candidate IR counterparts to the X-ray sources \citep{reba}. IR spectroscopy to search for accretion signatures will be required for definitive identifications. Using this technique, we were able to conclusively identify the IR counterpart to ``Edd-1'', one of these newly discovered low-luminosity X-ray sources \citep{edd1}. Cross-correlation of our CTIO ISPI \JHK image (Fig. 1b) of the central 10 pc$^{2}$ of the GC with the deep Sgr A* image of M03 is underway; this will produce a large number of IR candidate counterparts to the X-ray sources. Due to the extremely high stellar density in the Nuclear Bulge, many of these astrometric ``matches'' are likely to be chance superpositions. With thousands of candidate counterparts, traditional long-slit single-target spectroscopy would be a prohibitively inefficient method by which to identify true counterparts. Thus we will need to follow-up with multi-object IR spectroscopy to find the true matches: this is the work which will be performed with the FLAMINGOS-2 Galactic Center Survey (F2GCS). \begin{figure} \centering \subfigure[\Chandra/ACIS-I] % caption for subfigure a { \label{fig:sub:a} \scalebox{0.5}{\includegraphics{bandyopadhyay_r_fig1.jpg}} } \hspace{1cm} \subfigure[CTIO/ISPI] % caption for subfigure b { \label{fig:sub:b} \scalebox{0.4}{\includegraphics{bandyopadhyay_r_fig2.jpg}} } \caption{(a) \Chandra image of the central 8.5\arcmin$^{2}$ of the GC \citep{muno03}. (b) Our CTIO/ISPI \JHK image of the same region.} \label{fig:sub} \vspace{-3mm} \end{figure} \vspace{-2mm} \section{Extinction} \vspace{-1mm} We find that the IR extinction in the GC can vary on scales as small as 5\arcsec~ (0.2-0.6 pc at 8 kpc; \cite{ajg06}). Some areas show little evidence of this ``granularity'', while others are highly structured. To obtain reddening-corrected stellar photometry, a local value for the \JHK extinction (on scales $< 20\arcsec$) must be measured and applied. The relationship of extinction to wavelength in the IR is a power law with slope $\alpha$ \citep{mw90}. The ``canonical'' value for $\alpha$ is $\sim$2 (\eg \cite{rl85,nish}). In contrast, for the GC we find a mean value of $\alpha = 2.64\pm0.52$; and furthermore, along any given line of sight to the GC $\alpha$ varies substantially, ranging from $\sim$1.8--3.6 \citep{ajg08}. Thus we find that the ``universal'' IR extinction law is {\it not} universal in the GC! \vspace{-2mm} \section{Combining UKIDSS and \Chandra} \vspace{-1mm} \begin{figure} \centering \subfigure[UKIDSS GPS mosaic of the GC] % caption for subfigure a { \label{fig:sub:a} \scalebox{0.2}{\includegraphics{bandyopadhyay_r_fig3.jpg}} } \hspace{1cm} \subfigure[UKIDSS extinction map of the GC] % caption for subfigure b { \label{fig:sub:b} \scalebox{0.27}{\includegraphics{bandyopadhyay_r_fig4.jpg}} } \caption{(a) UKIDSS GPS \JHK mosaic of the GC which covers the entire W02 survey area \citep{gps}. (b) Extinction map of the same area of the GC, derived from the GPS data; black areas indicate regions which have not yet been fully included in the UKIDSS second data release (Gosling \etal, {\it these proceedings}).} \label{fig:sub} \vspace{-3mm} \end{figure} The United Kingdom Infrared Deep Sky Survey (UKIDSS) is an imaging survey covering 7500\degs$^{2}$ of the northern sky using a wide-field IR camera (WFCAM) on UKIRT. The Galactic Plane Survey (GPS) component of UKIDSS covers the Nuclear Bulge (-2\degs$