The X-ray emissivity (i.e., luminosity per unit stellar mass) of globular clusters are an important indicator of their dynamical evolution history. Based on deep archival \textit{Chandra} observations, we report a stacking analysis of 44 globular clusters (GCs) with 0.5-8 keV luminosities $L_{\rm X} \lesssim 10^{35} {\rm~erg~s^{-1}}$ in the M\,31 bulge, which are supposed to be dominated by cataclysmic variables (CVs) and coronally active binaries (ABs). We obtain a significant detection at $5σ$ level in 0.5-8 keV band. The average X-ray luminosity per GC and the average X-ray emissivity are determined to be $5.3 \pm 1.6\times10^{33}\rm~erg~s^{-1}$ and $13.2\pm4.3\times10^{27}\rm~erg~{\rm s^{-1}~M^{-1}_{\odot}}$, respectively. Both of these values are consistent with those of MW GCs. What's more, the measured emissivity of M31 GCs is also consistent with that of the MW field stars. Massive GCs have X-ray luminosities which are marginally higher with less massive ones. Massive GCs also show a lower emissivity ($4.5\pm 2.4\times10^{27}{\rm~erg~s^{-1}~M^{-1}_{\odot}}$) than less massive ones ($15.0\pm 7.8\times10^{27}{\rm~erg~s^{-1}~M^{-1}_{\odot}}$), which is consistent with the scenario that the (progenitors of) CVs and ABs were more efficiently destructed via stellar encounters in the more massive GCs. No dependence of the X-ray emissivity on GC color or on the projected galactocentric distance of GCs were found.
We study the statistical properties of faint X-ray sources detected in the Chandra Bulge Field. The unprecedented sensitivity of the Chandra observations allows us to probe the population of faint Galactic X-ray sources down to luminosities L(2-10 keV)~1e30 erg/sec at the Galactic Center distance. We show that the luminosity function of these CBF sources agrees well with the luminosity function of sources in the Solar vicinity (Sazonov et al. 2006). The cumulative luminosity density of sources detected in the CBF in the luminosity range 1e30-1e32 erg/sec per unit stellar mass is L(2-10 keV)/M*…
As part of the Chandra Galactic Bulge Survey (GBS), we present a catalogue of optical sources in the GBS footprint. This consists of two regions centered at Galactic latitude b = 1.5 degrees above and below the Galactic Centre, spanning (l x b) = (6x1) degrees. The catalogue consists of 2 or more epochs of observations for each line of sight in r', i' and Hα filters. It is complete down to r' = 20.2 and i' = 19.2 mag; the mean 5σ depth is r' = 22.5 and i' = 21.1 mag. The mean root-mean-square residuals of the astrometric solutions is 0.04 arcsec. We cross-correlate this optical catalogue with …
Based on archival {\it Chandra} data, we have carried out an X-ray survey of 69, or nearly half the known population of, Milky Way globular clusters (GCs), focusing on weak X-ray sources, mainly cataclysmic variables (CVs) and coronally active binaries (ABs). Using the cumulative X-ray luminosity per unit stellar mass (i.e., X-ray emissivity) as a proxy of the source abundance, we demonstrate a paucity (lower by $41\%\pm27\%$ on average) of weak X-ray sources in most GCs relative to the field, which is represented by the Solar neighborhood and Local Group dwarf elliptical galaxies. We also rev…
We discuss radio sources in the Chandra Galactic Bulge survey region. By cross-matching the X-ray sources in this field with the NVSS archival data, we find 12 candidate matches. We present a classification scheme for radio/X-ray matches in surveys taken in or near the Galactic Plane, taking into account other multi-wavelength data. We show that none of the matches found here is likely to be due to coronal activity from normal stars because the radio to X-ray flux ratios are systematically too high. We show that one of the sources could be a radio pulsar, and that one could be a planetary nebu…
We studied the stellar population in the central 6.6x6.6arcmin,region of the ultra-deep (1Msec) Chandra Galactic field - the "Chandra bulge field" (CBF) approximately 1.5 degrees away from the Galactic Center - using the Hubble Space Telescope ACS/WFC blue (F435W) and red (F625W) images. We mainly focus on the behavior of red clump giants - a distinct stellar population, which is known to have an essentially constant intrinsic luminosity and color. By studying the variation in the position of the red clump giants on a spatially resolved color-magnitude diagram, we confirm the anomalous total-t…
We present optical lightcurves of variable stars consistent with the positions of X-ray sources identified with the Chandra X-ray Observatory for the Chandra Galactic Bulge Survey. Using data from the Mosaic-II instrument on the Blanco 4m Telescope at CTIO, we gathered time-resolved photometric data on timescales from $\sim2$ hr to 8 days over the $\frac{3}{4}$ of the X-ray survey containing sources from the initial GBS catalog. Among the lightcurve morphologies we identify are flickering in interacting binaries, eclipsing sources, dwarf nova outbursts, ellipsoidal variations, long period vari…
We report the detection of a weak X-ray point source coincident with the nucleus of the bulgeless disk galaxy NGC 3621, recently discovered by Spitzer to display high ionization mid-infrared lines typically associated with AGN. These Chandra observations provide confirmation for the presence of an AGN in this galaxy, adding to the growing evidence that black holes do form and grow in isolated bulgeless disk galaxies. Although the low signal-to-noise ratio of the X-ray spectrum prevents us from carrying out a detailed spectral analysis of the nuclear source, the X-ray results, combined with the…
If the mysterious Fermi-LAT GeV gamma-ray excess is due to an unresolved population of millisecond pulsars (MSP) in the Galactic bulge, one expects this very same population to shine in X rays. For the first time, we address the question of what is the sensitivity of current X-ray telescopes to an MSP population in the Galactic bulge. To this end, we create a synthetic population of Galactic MSPs, building on an empirical connection between gamma- and X-ray MSP emission based on observed source properties. We compare our model with compact sources in the latest Chandra source catalog, applying…
We provide the Chandra source list for the last ~quarter of the area covered by the Galactic Bulge Survey (GBS). The GBS targets two strips of 6\degr x 1\degr (12 square degrees in total), one above (1\degr<b<2\degr) and one below (-2\degr<b<-1\degr) the Galactic plane in the direction of the Galactic Center at X-ray, optical and near-infrared wavelengths. For the X-ray part of the survey we use 2 ks per Chandra pointing. We find 424 X-ray sources in the 63 Chandra observations we report on here. These sources are in addition to the 1216 X-ray sources discovered in the first part of the GBS su…
We present the first X-ray study of Collinder 261 (Cr261), which at an age of 7 Gyr is one of the oldest open clusters known in the Galaxy. Our observation with the Chandra X-ray Observatory is aimed at uncovering the close interacting binaries in Cr261, and reaches a limiting X-ray luminosity of Lx ~ 4e29 erg/s (0.3-7 keV) for stars in the cluster. We detect 107 sources within the cluster half-mass radius r_h, and we estimate that among the sources with Lx >~ 1e30 erg/s, ~26 are associated with the cluster. We identify a mix of active binaries and candidate active binaries, candidate cataclys…
Recently an unidentified emission line at 3.55 keV has been detected in X-ray spectra of clusters of galaxies. The line has been discussed as a possible decay signature of 7.1 keV sterile neutrinos, which have been proposed as a dark matter (DM) candidate. We aim to further constrain the line strength and its implied mixing angle under the assumption that all DM is made of sterile neutrinos. The X-ray observations of the Limiting Window (LW) towards the Galactic bulge (GB) offer a unique dataset for exploring DM lines. We characterize the systematic uncertainties of the observation and the fit…
We present a detailed spectroscopic study of the hot gas toward the Galactic bulge along the 4U 1820-303 sight line by a combination analysis of emission and absorption spectra. In addition to the absorption lines of OVII Kalpha, OVII Kbeta, OVIII Kalpha and NeIX Kalpha by Chandra LTGS as shown by previous works, Suzaku detected clearly the emission lines of OVII, OVIII, NeIX and NeX from the vicinity. We used simplified plasma models with constant temperature and density. Evaluation of the background and foreground emission was performed carefully, including stellar X-ray contribution based o…
The diffuse hard X-ray emission that fills the Galactic center, bulge, and ridge is believed to arise from unresolved populations of X-ray binary systems. However, the identity of the dominant class of accreting objects in each region remains unclear. Recent studies of Fe line properties and the low-energy (<10 keV) X-ray continuum of the bulge indicate a major population fraction of non-magnetic cataclysmic variables (CVs), in particular quiescent dwarf novae. This is in contrast to previous high-energy (>10 keV) X-ray measurements of the bulge and ridge, which indicate a dominant population …
Based on the Chandra data archive as of March 2016, we have identified 314 candidate active galactic nuclei in 719 galaxies located closer than 50 Mpc, among them late-type (Hubble types Sc and later) galaxies that previously had been classified from optical observations as containing star-forming (H II) nuclei. These late-type galaxies comprise a valuable subsample to search for low-mass (<~ 10^6 solar masses) central black holes. For the sample as a whole, the overall dependence of the fraction of active nuclei on galaxy type and nuclear spectral classification is consistent with previous re…
IGR J17591-2342 is an accreting millisecond X-ray pulsar discovered in 2018 August in scans of the Galactic bulge and center by the INTEGRAL X-ray and gamma-ray observatory. It exhibited an unusual outburst profile with multiple peaks in the X-ray, as observed by several X-ray satellites over three months. Here we present observations of this source performed in the X-ray/gamma-ray and near infrared domains, and focus on a simultaneous observation performed with the Chandra-High Energy Transmission Gratings Spectrometer (HETGS) and the Neutron Star Interior Composition Explorer (NICER). HETGS …
(Abridged) We have discovered an intermediate polar (IP) in the 100 ks Chandra observation of Baade's Window (BW), a low extinction region at ~ 4 deg south of the Galactic Center. The source exhibits large X-ray modulations at a period of 1028.4 s in the 0.3 - 8 keV band. The X-ray spectral fit with a power law model shows the integrated spectrum is intrinsically hard (photon index ~ 0.44) and moderately absorbed (NH22 ~ 0.15). Quantile analysis reveals that the modulations in the X-ray flux strongly correlate with spectral changes that are dominated by varying internal absorption. The X-ray s…
We present Chandra studies of the X-ray binary (XRB) populations in the bulge and ring regions of the ring galaxy NGC 1291. We detect 169 X-ray point sources in the galaxy, 75 in the bulge and 71 in the ring, utilizing the four available Chandra observations totaling an effective exposure of 179 ks. We report photometric properties of these sources in a point-source catalog. There are ~40% of the bulge sources and ~25% of the ring sources showing >3σlong-term variability in their X-ray count rate. The X-ray colors suggest that a significant fraction of the bulge (~75%) and ring (~65%) sources …
Presence of the apparently extended hard (2-10 keV) X-ray emission along the Galactic plane has been known since the early 1980s. With a deep X-ray exposure using the Chandra X-ray Observatory of a slightly off-plane region in the Galactic bulge, most of the extended emission was resolved into faint discrete X-ray sources in the Fe K band (Revnivtsev et al.,2009). The major constituents of these sources have long been considered to be X-ray active stars and magnetic cataclysmic variables (CVs). However, recent works including our NIR imaging and spectroscopic studies (Morihana et al.,2013, 201…
We present a systematic study of periodic X-ray sources in the Limiting Window (LW), a $\sim$70 arcmin$^2$ field representative of the inner Galactic bulge and the target of $\sim$1 Ms Chandra observations. Using the Gregory-Loredo algorithm, which applies Bayes's theorem to the phase-folded light curve and is well-suited for irregularly sampled X-ray data, we detect 25 periodic signals in 23 discrete sources, among which 15 signals are new discoveries and two sources show dual periods. The vast majority of the 23 periodic sources are classified as magnetic cataclysmic variables (CVs), based o…
We introduce the Galactic Bulge Survey (GBS) and we provide the Chandra source list for the region that has been observed to date. Among the goals of the GBS are constraining the neutron star equation of state and the black hole mass distribution via the identification of eclipsing neutron star and black hole low-mass X-ray binaries. The latter goal will, in addition, be obtained by significantly enlarging the number of black hole systems for which a black hole mass can be derived. Further goals include constraining X-ray binary formation scenarios, in particular the common envelope phase and …
We present optical lightcurves, spectroscopy, and classification of five X-ray sources in the Chandra Galactic Bulge Survey [CXOGBS J174009.1-284725 (CX5), CXOGBS J173935.7-272935 (CX18), CXOGBS J173946.9-271809 (CX28), CXOGBS J173729.1-292804 (CX37), CXOGBS J174607.6-261547 (CX561)]. These objects were selected based on bright optical counterparts which were quickly found to have emission lines in their optical spectra. This paper presents an illustration of GBS optical follow-up, targeting emission line objects. Of these five objects, four exhibit photometric variability in the Sloan r' band…
The Galactic Bulge Survey is a wide but shallow X-ray survey of regions above and below the Plane in the Galactic Bulge. It was performed using the Chandra X-ray Observatory's ACIS camera. The survey is primarily designed to find and classify low luminosity X-ray binaries. The combination of the X-ray depth of the survey and the accessibility of optical and infrared counterparts makes this survey ideally suited to identification of new symbiotic X-ray binaries in the Bulge. We consider the specific case of the X-ray source CXOGBS J173620.2-293338. It is coincident to within 1 arcsec with a ver…
We present a new X-ray study of NGC188, one of the oldest open clusters known in our Galaxy (7 Gyr). Our observation with the Chandra X-ray Observatory is aimed at uncovering the population of close interacting binaries in NGC188. We detect 84 sources down to a luminosity of Lx ~ 4e29 erg/s (0.3-7 keV), of which 73 are within the half-mass radius rh. Of the 60 sources inside rh with more than 5 counts, we estimate that ~38 are background sources. We detected 55 new sources, and confirmed 29 sources previously detected by ROSAT and/or XMM-Newton. A total of 13 sources detected are cluster membe…
Relativistic jets propagating through an ambient medium must produce some observational effects along their side boundaries because of interactions across the large velocity gradient. One possible effect of such an interaction would be a sheared magnetic field structure at the jet boundaries, leading to a characteristic radio polarization pattern. As proposed by Ostrowski, another effect can come from the generation of a high energy cosmic ray component at the boundary, producing dynamic effects on the medium surrounding the jet and forming a cocoon dominated by cosmic rays with a decreased th…
[Abridged] We have carried out a deep X-ray and optical survey with Chandra and HST of low-extinction regions in the Galactic bulge. Here we present the results of a search for low-luminosity (L_X <~ 1e34 ergs/s) accreting binaries in the region closest to the Galactic Center, at an angular offset of 1.4deg, that we have named the Limiting Window. Based on their blue optical colors, excess Halpha fluxes, and high X-ray--to--optical flux ratios, we identify three likely cataclysmic variables (CVs). Distance estimates put these systems farther than >~2 kpc. Based on their Halpha-excess fluxes an…
We observed an extended X-ray source designated as Suzaku J1759-3450 with the Suzaku and Chandra observations towards 1RXS J175911.0-344921, which is an unidentified X-ray source listed in the ROSAT Bright Source Catalogue. A conspicuous emission line at 6 keV was also found in the Suzaku J1759-3450 spectrum. Assuming the emission line to be K emission from He-like Fe ions, we inferred Suzaku J1759-3450 to be an extragalactic object located at z = 0.13. The radial profile of the surface brightness in the 0.5-10 keV band was explained well with an isothermal beta-model of r_c = 1.61' and beta =…
Binary active galactic nuclei (AGNs) provide clues to how gas-rich mergers trigger and fuel AGNs and how supermassive black hole (SMBH) pairs evolve in a gas-rich environment. While significant effort has been invested in their identification, the detailed properties of binary AGNs and their host galaxies are still poorly constrained. In a companion paper, we examined the nature of ionizing sources in the double nuclei of four kpc-scale binary AGNs with redshifts between 0.1~0.2. Here, we present their host galaxy morphology based on F336W (U-band) and F105W (Y-band) images taken by the Wide F…
The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an accreting neutron star. It shows highly ionized absorption features, with a blueshift of $\sim$ 400 km s$^{-1}$ and an outflow-mass rate similar to the accretion rate. Many other X-ray sources exhibit warm absorption features, and they all show periodic dipping behavior at the same time. Recently, a dipping periodicity has also been determined for GX 13+1 using long-term X-ray folded light-curves, leading to a clear identification of one of such periodic dips in an archival Chandra observation. We give the first spectr…
WR~102-1 was detected by Suzaku as a conspicuous point source in the 6.7 keV intensity map of the central region of the Milky Way. The source was suggested as a possible Wolf-Rayet binary based on its X-ray and infrared spectral characteristics. The iron line emission is expected to originate in the Wolf-Rayet star's dynamic stellar-wind when colliding the companion's mild stellar wind. Here, we report the result of a long-term X-ray monitoring of WR~102-1 since 1998 using archival data of ASCA, XMM-Newton, Chandra, Suzaku, and Swift to reveal variations of the iron K-emission line and the cir…
Previous X-ray observations toward the Nuclear Star Cluster (NSC) at the Galactic center have discovered thousands of point sources, most of which were believed to be cataclysmic variables (CVs), i.e., a white dwarf (WD) accreting from a low-mass companion. However, the population properties of these CVs remain unclear, which otherwise contain important information about the evolutionary history of the NSC. In this work we utilize ultradeep archival \textit{Chandra} observations to study the spectral properties of the NSC CVs, in close comparison with those in the Solar vicinity. We find that …
We present the Chandra and Suzaku observations of 1RXS J170047.8$-$314442, located towards the Galactic bulge, to reveal a wide-band ($0.3$-$10$ keV) X-ray morphology and spectrum of this source. With the Chandra observation, no point source was found at the position of 1RXS J170047.8$-$314442. Alternatively, we revealed the presence of diffuse X-ray emission by the wide-band X-ray image obtained from the Suzaku XIS. Although the X-ray emission had a nearly circular shape with a spatial extent of $\sim 3.5'$, the surface brightness profile was not axisymmetric; a bright spot-like emission was …
We present an analysis of the new, deep (94\,ksec) {\it Chandra} ACIS-S observation of radio-loud active galaxy CGCG\,292$-$057, characterized by a LINER-type nucleus and a complex radio structure that indicates intermittent jet activity. On the scale of the host galaxy bulge, we detected excess X-ray emission with a spectrum best fit by a thermal plasma model with a temperature of $\sim 0.8$\,keV. We argue that this excess emission results from compression and heating of the hot diffuse fraction of the interstellar medium displaced by the expanding inner, $\sim 20$\, kpc-scale lobes observed …
The eclipsing low-mass X-ray binary X1822-371 is the prototype of the accretion disc corona (ADC) sources. We analyse two Chandra observations and one XMM-Newton observation to study the discrete features and their variation as a function of the orbital phase, deriving constraints on the temperature, density, and location of the plasma responsible for emission lines. The HETGS and XMM/Epic-pn observed X1822-371 for 140 and 50 ks, respectively. We extracted an averaged spectrum and five spectra from five selected orbital-phase intervals that are 0.04-0.25, 0.25-0.50, 0.50-0.75, 0.75-0.95, and, …
We have identified a new transient luminous low-mass X-ray binary, NGC 6440 X-2, with Chandra/ACIS, RXTE/PCA, and Swift/XRT observations of the globular cluster NGC 6440. The discovery outburst (July 28-31, 2009) peaked at L_X~1.5*10^36 ergs/s, and lasted for <4 days above L_X=10^35 ergs/s. Four other outbursts (May 29-June 4, Aug. 29-Sept. 1, Oct. 1-3, and Oct. 28-31 2009) have been observed with RXTE/PCA (identifying millisecond pulsations, Altamirano et al. 2009a) and Swift/XRT (confirming a positional association with NGC 6440 X-2), with similar peak luminosities and decay times. Optical a…
This paper is devoted to determining the nature of the hard X-ray source IGR J17463-2854 located toward the Galactic bulge. Using data from the INTEGRAL and Chandra X-ray observatories, we show that five point X-ray sources with approximately identical fluxes in the 2-10 keV energy band are detected in the error circle of the object under study. In addition, significant absorption at low energies has been detected in the spectra of all these sources. Based on data from the VVV (VISTA/ESO) infrared Galactic Bulge Survey, we have unambiguously identified three of the five sources, determined the…
We identified a new X-ray transient CXOM31 004252.457+411631.17 (T13) in M31 during a 2013 June Chandra observation. This system is particularly exciting because it is located within 100" of M31*; it is thought that this region of the bulge is sufficiently dense to form X-ray binaries dynamicaly, but only systems with black hole accretors and/or short periods are expected to survive. A follow-up XMM-Newton observation yielded a soft spectrum, well described by a 0.39+/-0.02 keV disk blackbody; applying this model to the Chandra observation yielded an observed 0.3--10 keV luminosity peak of 6.2…
Galaxies can grow through their mutual gravitational attraction and subsequent union. While orbiting a regular high-surface-brightness galaxy, the body of a low-mass galaxy can be stripped away. However, the stellar heart of the infalling galaxy, if represented by a tightly-bound nuclear star cluster, is more resilient. From archival Hubble Space Telescope images, we have discovered a red, tidally-stretched star cluster positioned ~5 arcseconds (~400 pc in projection) from, and pointing toward the center of, the post-merger spiral galaxy NGC 4424. The star cluster, which we refer to as `Nikhul…
Detailed spectral analysis of the Galactic X-ray background emission, or the Galactic Ridge X-ray Emission (GRXE), is presented. To study the origin of the emission, broad-band and high-quality GRXE spectra were produced from 18 pointing observations with Suzaku in the Galactic bulge region, with the total exposure of 1 Ms. The spectra were successfully fitted by a sum of two major spectral components; a spectral model of magnetic accreting white dwarfs with a mass of 0.66 (0.59-0.75) solar, and a softer optically-thin thermal emission with a plasma temperature of 1.2-1.5 keV which is attribut…
(Abridged) We present the LogN-LogS and spatial distributions of X-ray point sources in seven Galactic Bulge (GB) fields within 4 deg from the Galactic Center (GC). We compare the properties of 1159 X-ray point sources discovered in our deep (100 ks) Chandra observations of three low extinction Window fields near the GC with the X-ray sources in the other GB fields centered around Sgr B2, Sgr C, the Arches Cluster and Sgr A* using Chandra archival data. To reduce the systematic errors induced by the uncertain X-ray spectra of the sources coupled with field-and-distance dependent extinction, we…
We present the first large sample investigation of the properties of jets in Fanaroff and Riley type I radio galaxies (FR-I) based on data from the Chandra archive. We explore relations between the properties of the jets and the properties of host galaxies in which they reside. We find previously unknown correlations to exist, relating photon index, volume emissivity, jet volume and luminosity, and find that the previously long held assumption of a relationship between luminosities at radio and X-ray wavelengths is linear in nature when bona fide FR-I radio galaxies are considered. In addition…
Using a spectral analysis of bright Chandra X-ray sources located in 27 nearby galaxies and maps of star-formation rate (SFR) and ISM surface densities for these galaxies, we constructed the intrinsic X-ray luminosity function (XLF) of luminous high-mass X-ray binaries (HMXBs), taking into account absorption effects and the diversity of HMXB spectra. The XLF per unit SFR can be described by a power law dN/d log L=2.0(L/10^39 erg/s)^(-0.6) (M_Sun/yr)^(-1) from L=10^38 to 10^40.5 erg/s, where L is the unabsorbed luminosity at 0.25-8 keV. The intrinsic number of luminous HMXBs per unit SFR is a f…
