From October 1999 to the present Chandra has taken nearly monthly snap-shot observations of M31. The first 3 years of this dataset in combination with deeper but less frequent XMM observations has allowed the detection of 45 X-ray transients within M31. By analogy to our Galaxy, many of these transients are likely black hole candidates. We have indentified a few optical counterparts of these possible black holes via simultaneous HST imaging. The census allows a study of the endpoints of stellar evolution in our nearest neighbor galaxy. When stacked, the observations also allow a deep study of the M31 X-ray source population, which turns up a few surprises. The supermassive black hole in the center of M31 is the next nearest one after Sgr A*. Chandra and HST observations allow the detection of a weak X-ray source at the position of this SMBH. These observations provide some of the most secure and severe limitations on accretion in SMBH.

We have conducted a survey of X-ray sources in XMM-Newton observations of M31, examining their power density spectra (PDS) and spectral energy distributions (SEDs). Our automated source detection yielded 535 good X-ray sources; to date, we have studied 225 of them. In particular, we examined the PDS because low mass X-ray binaries (LMXBs) exhibit two distinctive types of PDS. At low accretion rates, the PDS is characterised by a broken power law, with the spectral index changing from $\sim$0 to $\sim$1 at some frequency in the range $\sim$0.01–1 Hz; we refer to such PDS as Type A. At higher accretion rates, the PDS is described by a simple power law; we call these PDS Type B. Of the 225 sources studied to date, 75 exhibit Type A variability, and are almost certainly LMXBs, while 6 show Type B but not Type A, and are likely LMXBs. Of these 81 candidate LMXBs, 71 are newly identified in this survey; furthermore, they are mostly found near the centre of M31. Furthermore, most of the X-ray population in the disc are associated with the spiral arms, making them likely high mass X-ray binaries (HMXBs). In general these HMXBs do not exhibit Type A variability, while many central X-ray sources (LMXBs) in the same luminosity range do. Hence the PDS may distinguish between LMXBs and HMXBs in this luminosity range.

Using 24 overlapping XMM-Newton observations of the Local Group spiral galaxy M 33, we have detected 447 sources in each individual pointing and in deep combined images. A total of 61 sources exhibit significant flux variations by a factor of up to 144, on time scales of hours to months or years. The detected variability, together with the hardness ratio (HR) method and optical identification (when available), is used to classify the sources as X-ray binaries (XRBs), supernova remnants (SNRs) and super-soft sources (SSS) in M 33, as well as background AGN and foreground stars in the field of view. The majority of sources can only be classified as ‘hard’, according to their HRs. We find that the luminosity distribution of the detected SNRs and SNR candidates in M 33 is similar to M 31, and slightly steeper than that of the LMC.

As shown in our first poster, in a recent survey of M33 with XMM-Newton we detected the X-ray source population of this nearby spiral galaxy down to the (0.2-4.5) keV luminosity of $10^{35}$ erg s$^{-1}$, a factor of 10 deeper than in previous observations. The majority of the detected sources was classified using, in many cases, only their X-ray properties. In particular, 8 new X-ray binary (XRB) candidates were selected, based on their long-term X-ray light curves. We also classified supernova remnants (SNRs), super-soft sources (SSS), AGN, foreground stars and a population of ‘hard’ sources using the hardness ratio (HR) method. A detailed spectral and timing analysis of the brightest sources is in progress. We present a few examples of spectra for particular source classes. We find that bright ‘hard’ sources can be divided into two broad families: one best modelled by a powerlaw with photon index in the range of 1.0–2.0, and the other displaying disk blackbody spectra with kT of 0.8 to 1.5 keV.

XMM-Newton and Chandra have ushered in a new era for the study of dwarf galaxies in the Local Group. We provide an overview of the opportunities, challenges, and some early results. The large number of background sources relative to galaxy sources is a major theme. Despite this challenge, the identification of counterparts has been possible, providing hints that the same mechanisms producing X-ray sources in larger galaxies are active in dwarf galaxies. A supersoft X-ray source within 2″ of the supermassive black hole in M32 may be a remnant of the tidal disruption of a giant, although other explanations cannot be ruled out.

The SMC is now known to contain many more transient X-ray pulsars than would be expected based on a simple scaling of the number of such sources in the Galaxy by the relative mass of the SMC. We have been conducting regular monitoring observations of the SMC with the Proportional Counter Array on the Rossi X-ray Timing Explorer since 1997. This has resulted in the discovery of many of these X-ray pulsars and also provided orbital period measurements from detections of regular outbursts. We can now investigate the differences and similarities of the Galactic and SMC X-ray pulsar populations and consider the origin of the huge SMC X-ray pulsar over-abundance.

We report the results of a deep Chandra survey of the Sculptor dwarf spheroidal galaxy. We find five X-ray sources with $L_X$ of at least $6\times10^{33}$ ergs/sec with optical counterparts establishing them as members of Sculptor. These X-ray luminosities indicate that these sources are X-ray binaries, as no other known class of Galactic point sources can reach 0.5-8 keV luminosities this high. Finding these systems proves definitively that such objects can exist in an old stellar population without stellar collisions. Three of these objects have highly evolved optical counterparts (giants or horizontal branch stars), as do three other sources whose X-ray luminosities are in the range which includes both quiescent low mass X-ray binaries and the brightest magnetic cataclysmic variables. We predict that large area surveys of the Milky Way should also turn up large numbers of quiescent X-ray binaries.

Today's sensitive, high-resolution Chandra X-ray observations allow the study of many populations of X-ray sources. The traditional astronomical tools of photometric diagrams and luminosity functions are now applied to these populations, and provide the means for classifying the X-ray sources and probing their evolution. While overall stellar mass drives the amount of X-ray binaries in old stellar populations, the amount of sources in star forming galaxies is related to the star formation rate. Short-lived, luminous, high mass binaries (HMXBs) dominate these young X-ray populations.

We present X-ray properties of NGC 300 point sources, extracted from 66 ksec of XMM-Newton data taken in 2000 December and 2001 January. A total of 163 sources was detected in the energy range of 0.3–6 keV. We report on the global properties of the sources detected inside the $D_{25}$ optical disk, such as the hardness ratio and X-ray fluxes, and spectral fitting of the brightest sources. We also present some properties of their optical counterparts found in B, V, and R images from the 2.2 m MPG/ESO telescope. Furthermore, we cross-correlate the X-ray sources with SIMBAD, the USNO-A2.0 catalog, and radio catalogues.

The X-ray populations of spiral galaxies consist almost entirely of accreting X-ray binaries and supernova remnants. For the most luminous sources, it is possible to use X-ray spectroscopy and variability studies to gain insights into the nature of the sources. However, without unambiguously identified optical counterparts, it is impossible to definitively classify sources as, e.g. high-mass or low-mass X-ray binaries. The nearby interacting galaxy M51 is one of the best-studied galaxies across all wavelengths. At a distance of around 8 Mpc, it is possible to resolve features on scales of a few parsecs both in the X-ray and the optical. Recently, M51 was observed with the Advanced Camera for Surveys on the Hubble Space Telescope as part of a Hubble Legacy program. M51 has also been observed 3 times with the Chandra X-ray observatory. Combining these two datasets, we present initial results on the optical environments of M51 X-ray sources as the first part of a truly multi-wavelength study of X-ray sources in nearby galaxies.

In May/June 2005, Chandra observed M81 fifteen times, roughly once every three days, as part of our proposal to explore the days to weeks timescale of variability for extragalactic point sources. Each observation reached a sensitivity of $5\,{\times}\,10^{36}$ erg/s. Because these observations probe the timescale on which X-ray binaries typically evolve, we can now compare extragalactic sources to Galactic X-ray binaries on a more equal level than has been possible in the past. In addition, we can measure and quantify any possible time variability of M81's X-ray luminosity function and investigate alternative methods to characterize a galaxy's dynamic X-ray source population. We present here preliminary results of the observations.

We present the results of a Chandra observation made of the nearby spiral galaxy NGC 7793: the effective exposure time of this observation was 49094 seconds. Twenty-two discrete sources were identified at a minimum of a 3$\sigma$ level to an estimated limiting luminosity of ∼$2\times10^{36}$ ergs sec$^{-1}$. We have performed a spectral analysis of the known ultraluminous X-ray source (ULX) in this galaxy: statistically-acceptable fits to the spectrum can be obtained with either a power law model, a bremsstrahlung model or a DISKBB model. We have also searched for counterparts at multiple wavelengths to these sources: based on this search, we have classified two supernova remnants, one HII region and two foreground stars.

The massive star clusters identified by S. Larsen are compared to the available Chandra observations of face-on spiral galaxies. In each galaxy, a few percent of the Larsen-identified clusters match X-ray-emitting point sources. An additional few match knots of emission in the diffuse emission. The cluster properties are examined to ascertain whether massive star clusters are X-ray sources.

Chandra observations have allowed the detection of a large number of low mass X-ray binaries (LMXBs) in early-type galaxies. Comparison to catalogs of globular clusters (GCs) from Hubble Space Telescope observations have shown that a high fraction of the LMXBs in early-type galaxies are associated with GCs. The fraction of LMXBs associated with globular clusters increases along the Hubble sequence from spiral bulges to S0s to Es to cDs. On the other hand, the fraction of globular clusters which contain X-ray sources appears to be roughly constant ($\sim$4% for $L_X \gtrsim 10^{38}$ ergs/s, $\sim$10% for $L_X \gtrsim 10^{37}$ ergs/s). There is a strong tendency for the X-ray sources to be associated with the optically more luminous GCs. There is a trend for the X-ray sources to be found preferentially in redder, more metal-rich GCs, which is independent of optical luminosity correlation.

The relative role of formation of LMXBs in GCs and in situ formation in the field is uncertain. One of the best ways to study this is to compare the spatial distribution of GC-LMXBs, field LMXBs, GCs, and optical light in the galaxies. Theoretical models and results of fits to the observed distributions are presented.

The recent evidence of a peculiar distribution of X-ray sources in the elliptical galaxy NGC 4261 reported by Zezas et al. has prompted us to study this galaxy combining archive X-ray and optical observations, from Chandra, INT, and HST. We find that a sizable fraction of the X-ray sources has a globular cluster as optical counterpart. This together with the shape of the luminosity function of the X-ray sources suggest that they are accreting low-mass binaries. We further show a remarkable similarity in the anisotropy of the projected spatial distributions of the optical and X-ray sources, which leads us to conclude that the spatial anisotropy of the X-ray sources in NGC 4261 is due to the anisotropy of the globular cluster population.

Chandra observations of early-type galaxies have resolved large populations of low-mass X-ray binaries (LMXBs) in early-type galaxies. The majority of these observations have been snapshots on the order of a day or less. In our own Galaxy, LMXBs are known to exhibit a range of luminosity and spectral variability. Multi-epoch observations of early-type galaxies are just beginning to explore the regime of variability on timescales of days to years. We present results for NGC 4365 and NGC 4697, and compare them to the Milky Way.

In the local Universe most massive black holes at the centers of galaxies are not luminous quasars. Is this because (1) they are starved of gas, (2) they accrete without emitting radiation, (3) they refuse to eat, ejecting the incoming material, or (4) they are storing up matter in an accretion disk to feast later?

With Chandra ACIS we have imaged a pilot sample of 6 nearby (D $<$ 30 Mpc) elliptical galaxies chosen to be especially quiescent based on the careful optical spectroscopy of Ho, measured black hole masses (Mbh$\,{>}\,$10(7)Msol), and with existing X-ray upper limits (Lx$\,{<}\,$10(40)erg/s) implying far sub-Eddington accretion. In these galaxies we can measure, or limit, the diffuse hot interstellar medium, and so constrain the Bondi accretion rate.

Faint X-ray emission is detected at or around the nucleus in each galaxy. The morphology of these weak X-ray sources is complex. The X-ray colors of the sources can be determined, and a moderate quality spectrum for one was obtained. We discuss these results against the possible explanations of black hole quiescence.

On the other hand, a few percent of all galaxies shows evidence for nuclear activity and a brief review of the high energy emission from Active Galactic Nuclei is given.

As the nearest galaxies around us, the Local Group systems offer especially good opportunities for observations of their nuclear X-ray radiation. Certain or possible nuclear X-ray sources in the Local Group suggest a minimum luminosity for activity to become manifest.

We report the results from an homogeneous analysis of the X-ray (ACIS-S/Chandra) data available for a sample of 52 LINER galaxies. The X-ray morphology has been classified attending to their nuclear compactness in the hard band (4.5–8.0 keV), into 2 categories: AGN-like nuclei (with a clearly identified unresolved nuclear source) and Starburst-like nuclei (without a clear nuclear source). 60% of the total sample are classified as AGNs, with a median luminosity of $\rm{L_{X}(2.0\hbox{-}10.0 keV)=2.5\times 10^{40} erg s^{-1}}$, which is an order of magnitude higher than that for SB-like nuclei. The spectral fitting allows to conclude that most of the objects need a non-negligible thermal contribution. When no spectral fitting can be performed (low signal-to-noise ratio), the Color-Color diagrams allow us to compute physical parameters such as density column, temperature of the thermal model or spectral index and therefore to analyze the origin of the X-ray emission. All X-ray morphology, spectral fitting and Color-Color diagrams allow conclude that a high percentage of LINER galaxies host AGN nuclei.

The status of Very High Energy (VHE, E$\,{>}\,$50 GeV) gamma-ray observations of BL Lacertae objects is presented. The catalogue of well-established BL Lacertae objects detected at VHE energies contains seven members, and there have been recent reports of the detection of another four. All are nearby, X-ray bright sources. The temporal, spectral and broadband multi-wavelength properties of the sources are reviewed and possible implications for the gamma-ray production mechanism discussed. The most recent detections provide more stringent constraints on the cosmic extragalactic background light level and imply that the Universe is more transparent to VHE gamma radiation than previously thought.