Optical studies of ultraluminous X-ray sources (ULX) in nearby galaxies have turned out to be instrumental in discriminating between various models including the much advertised intermediate mass black hole hypothesis and various beaming scenarios. Here we report on ESO VLT and SUBARU observations of ULX that have revealed the parent stellar clusters with ages of some 60 million years in two cases. Thus we are able to derive upper limits of about 8 M$_{\odot}$ for the mass donors in these systems. The optical counterparts are dominated by X-ray heated accretion disks, and the discovery of the He II $\gamma$4686 emission line now allows to derive dynamical masses in these systems. Apparent radial velocity variations of 300 km/s have been detected in NGC 1313 X-2 which, if confirmed by further observations, would exclude the presence of IMBH in these systems.

We present the results of ROSAT and XMM-Newton observations of the recurrent ultraluminous X-ray source (ULX) NGC 253 ULX1. This transient is one of the few ULXs that was detected during several outbursts. The luminosity reached 1.4$\,{\times}\,10^{39}$ \hbox{erg s$^{-1}$ and $0.5\,{\times}\,10^{39}$ erg s$^{-1}$ in the detections by ROSAT and XMM-Newton, respectively, indicating a black hole X-ray binary (BHXRB) with a mass of the compact object of $>$11 M$_{\odot}$. In the ROSAT detection NGC 253 ULX1 showed significant variability, whereas the luminosity was constant in the detection from XMM-Newton. The XMM-Newton EPIC spectra are well-fit by a bremsstrahlung model (kT$=2.24\,$keV, $N_H=1.74\times10^{20} \mbox{cm}^{-2}$), which can be used to describe a comptonized plasma. No counterpart was detected in the optical I, R, B, NUV and FUV bands to limits of 22.9, 24.2, 24.3, 22 and 23 mag, respectively, pointing at a XRB with a low mass companion.

We detail a model we have created to describe the optical emission from a ULX in terms of an irradiated companion star and disk. We apply this model to optical observations of ULX X-7 in NGC 4559. We revise the parameters of the companion star in this system to be older, less massive and of a later spectral type than previously reported. We find the black hole to be of a few hundred solar masses at most.

We present optical observations of an ultraluminous X-ray source (ULX) in Holmberg IX, a dwarf galaxy near M81. The ULX has an average X-ray luminosity of some $10^{40}$ erg/s. It is located in a huge (400pc$\,{\times}\,$300pc) ionized nebula being much larger than normal supernova remnants. From the observed emission lines (widths and ratios) we find that the structure is due to collisional excitation by shocks, rather than by photoionization.

We identify the optical counterpart to be a 22.8 mag blue star ($M_V=-5.0$) belonging to a small stellar cluster. From isochrone fitting of our multi-colour photometry we determine a cluster age of some 60 Myr. We also discovered strong stellar HeII$\lambda$4686 emission (equivalent width of $\mbox{10 \AA}$) which proves the identification with the X-ray source, and which suggests the presence of an X-ray heated accretion disc around the putative black hole.

Accumulating evidence indicates that some of ultra-luminous X-ray sources (ULXs) are intermediate mass black holes (IMBHs), but the formation process of IMBHs is unknown. One possibility is that they were formed as remnants of population III (Pop III) stars, but it has been thought that the probability of being an ULX is too low for IMBHs distributed in galactic haloes to account for the observed number of ULXs. Here we show that the number of ULXs can be explained by such halo IMBHs passing through a dense molecular cloud, if Pop III star formation is very efficient as recently suggested by the excess of the cosmic near-infrared background radiation that cannot be accounted for by normal galaxy populations. We calculate the luminosity function of X-ray sources in our scenario and find that it is consistent with observed data. Our scenario can explain that ULXs are preferentially found at outskirts of large gas concentrations in star forming regions. A few important physical effects are pointed out and discussed, including gas dynamical friction, radiative efficiency of accretion flow, and radiative feedback to ambient medium. ULXs could last for ${\sim}10^{5-6}$ yr to emit a total energy of ${\sim}10^{53}$ erg, which is sufficient to power the ionized expanding nebulae found by optical observations.

We present results of optical follow-up observations of candidate ultra-luminous X-ray sources (ULXs). Using Keck optical spectroscopy, 17 of the candidates from the Colbert & Ptak (2002) catalog have been identified; this is one of the largest sets of optical identifications of such objects thus far. 15 are background active galactic nuclei (AGN); 2 are foreground stars in our Galaxy. These findings are compared with background and foreground object expectations, as derived from log $N$-log $S$ relations. Also, the results are briefly discussed in terms of the spiral-galaxy/ULX connection.

We report the serendipitous discovery of several ULX candidates in XMM-Newton observations. Such discoveries suggest that ULXs are not a negligible component of the extragalactic X-ray source population.

We present the results of an on-going program for the identification and characterization of optical counterparts of Ultra-Luminous X-ray (ULX) sources. The targets have been selected from the catalogues by Colbert & Ptak (2002) and Swartz et al. (2004). A clear identification based on unambiguous optical spectral features was possible for 26 objects. A large number of objects result to be QSOs at higher redshift than the putative parent galaxy, and other ULXs seem to be associated to HII regions. In a few cases the optical counterpart results a foreground star in our galaxy. The observational program will continue to obtain a representative sample for statistical studies.

This paper will review the main astrophysical results obtained in the field of high energy Galactic sources with the INTEGRAL/IBIS Gamma-ray Imager (Ubertini et al. 2003) on-board INTEGRAL (Winkler et al. 2003), the ESA space Observatory successfully launched the 17th October 2002 from Baikonur with a Proton vehicle. In view of the high sensitivity of the two gamma ray instruments IBIS and SPI and their capability to provide at the same time image, spectra and time profiles of all the sources in their wide field of view, a key project was approved as “Core Programme” to deeply observe the Galactic Centre (GCDE) and to exploit regular scans of the whole Galaxy Plane. The major results obtained in terms of classes of high energy emitters are shortly outlined.

INTEGRAL is the first gamma-ray astronomy mission with a sufficient sensitivity and angular resolution combination appropriate to the detection and identification of considerable numbers of gamma-ray emitting sources. The large field of view enables INTEGRAL to survey the Galactic Plane on a regular ($\sim$weekly) basis as part of the core programme. The first source catalogue, based on the 1st year of core programme data ($\sim$5 Msec) has been completed and published (Bird et al. 2004). It contained 123 $\gamma$-ray sources (24 HMXB, 54 LMXB, 28 “unknown”, plus 17 others) - sufficient numbers for a reasonable statistical analysis of their global properties. The detection of previously unknown $\gamma$-ray emitting sources generally exhibiting high intrinsic absorption, which do not have readily identifiable counterparts at other wavelengths, is intriguing. The substantial fraction of unclassified $\gamma$-ray sources suggests they must constitute a significant family of objects. In this paper we review the global characteristics of the known galactic sources as well as the unclassified objects.

We present the first results of our X-ray monitoring campaign using the X-ray satellites XMM-Newton and Chandra on the 1.7 square degree region centered on Sgr A*. The purpose of this campaign is to monitor the X-ray behavior of X-ray sources (both persistent and transient ones) which are too faint to be detected by monitoring instruments aboard other satellites currently in orbit (e.g., RXTE, Integral, Swift). Our first observations were obtained on June 5, 2005, using the HRC-I detector aboard Chandra. Here we briefly discuss our results on the very faint X-ray transients we detected during these observations. We detected new outbursts (with peak luminosities close to $10^{36}$ erg s$^{-1}$) of the two very faint transients GRS 1741.9–2853 and XMM J174457-2850.3, as well as a potential new very faint X-ray transient with an outburst luminosity of a few times $10^{34}$ erg s$^{-1}$.

Shell-like supernova remnants (SNRs) are primary candidates for the origin of Galactic cosmic rays. However, among the known SNRs (about 220), only a small fraction has been known to exhibit the synchrotron X-ray spectrum, that is considered to be a piece of evidence for high energy particle acceleration. Synchrotron X-ray emitting SNRs are known to be systematically radio-quiet compared to the SNRs that do not emit synchrotron X-rays. Therefore, most synchrotron X-ray emitting SNR candidates may have escaped detection in the previous systematic radio surveys. On the other hand, hard X-ray surveys are effective to search for synchrotron X-ray emitting SNRs, because of its penetration power. Thus we have searched for SNRs in the ASCA Galactic Plane Survey data, the first Galactic imaging survey in $>$2 keV, and discovered 14 candidates. Deep follow-up observations with ASCA, XMM, or Chandra on 5 of them revealed 2 sources to be synchrotron X-ray emitting SNRs. Furthermore we confirmed non-thermal X-ray spectra from the other 3 sources, though the origin is yet unknown. We report the observational results and discuss the X-ray origin.

Recent studies suggested that there might be a correlation between unidentified $\gamma$-ray sources from the third EGRET catalogue and OB associations. Moreover, when extrapolating the fluxes measured by EGRET at energies above 100 MeV with a power-law down to the energy range of ISGRI, the expected count rates should be large enough to be detectable with INTEGRAL. Most of those OB associations being located in the Galactic plane, they are monitored by INTEGRAL as part of the Core Program during both the Galactic Plane Scans and the Galactic Center Deep Exposure. Combining public and CP data, we have performed a search for gamma-ray emission from OB associations and the first results are presented.

We study high mass X-ray binaries (HMXBs) in the Galaxy using data of the INTEGRAL observatory. High sensitivity survey of the whole Galaxy with a possibility to detect absorbed sources significantly enlarged our sample of HMXBs in a comparison with the previous studies. Large fraction of detected high mass X-ray binaries is highly photoabsorbed. We investigated the HMXBs distribution along the Galactic plane and found their strong concentrations toward Galactic spiral arms, confirming previous results of Grimm et al. (2002) obtained using smaller sample of sources. We conclude that the mapping of Galactic HMXBs should be important tool to trace the star formation regions at the opposite side of the Galaxy.

An analysis of the high-energy emission from IGR J16393–4643 is presented using data from INTEGRAL and XMM-Newton. The source is persistent in the 20–40 keV band at an average flux of $5.1\times10^{-11}$ergs cm$^{-2}$ s$^{-1}$, with variations in intensity by at least an order of magnitude. A pulse period of 912$\,{\pm}\,$3 s was discovered in the ISGRI and EPIC light curves. The source spectrum is a strongly-absorbed ($N_{\mathrm{H}}=(2.5\,{\pm}\,0.2)\times10^{23}$ cm$^{-2}$) power law that features a high-energy cutoff above 15 keV. Two iron emission lines at 6.4 and 7.1 keV, an iron absorption edge $\gtrsim$7.1 keV, and a soft excess emission of $7\times10^{-15}$ergs cm$^{-2}$ s$^{-1}$ between 0.5–2 keV, are detected in the EPIC spectrum. The shape of the spectrum does not change with the pulse. Its persistence, pulsation, and spectrum place IGR J16393–4643 among the class of heavily-absorbed HMXBs. The improved position from EPIC is R.A. (J2000)$=16^{\mathrm{h}}39^{\mathrm{m}}05.4^{\mathrm{s}}$ and Dec.$=-46^{\circ}42'12''$ ($4''$ uncertainty) which is compatible with that of 2MASS J16390535–4642137.

IGR J17252–3616 is the hard X-ray counterpart of EXO 1722–363. The regular monitoring by INTEGRAL shows that IGR J17252–3616 is a persistent source with an average count rate of $\sim$6.4 mCrab in the 20–60 keV energy band. A follow-up observation with XMM-Newton showed that the source is located at R.A. (2000.0) $=17^{h}25^{m}11.4^{s}$ and Dec. $=-36{\hbox{$^\circ$}} 16 {\hbox{$^\prime$}} 58.6 \hbox{$^{\prime\prime}$}$ with an uncertainty of $4 \hbox{$^{\prime\prime}$}.

The source is a binary X-ray pulsar with a spin period of 413.7 s. The spectral shape is typical for an accreting pulsar except that a huge intrinsic absorption and a cold iron fluorescence line are detected. The absorbing column density and cold iron line do not vary with the pulse period. The observations suggest that the source is a wind-fed accreting pulsar accompanied by a supergiant star.

We present the preliminary results of simultaneous XMM-Newton and INTEGRAL observations of the highly absorbed INTEGRAL source IGR J16320-4751. We refine the X-ray position with XMM-Newton, and then examine the spectral properties of the source using both satellites, separating two periods visible in the lightcurves, an initial flare and a more steady period. We show that the source spectrum and its behaviour are compatible with IGR J16320-4751 being a pulsar accreting from a high mass companion.

During the first year in operation, INTEGRAL has detected more than 28 new bright sources which emit the bulk of their emission above 10 keV. Follow-up observations of a subset of these sources in the X-ray band with XMM-Newton indicate that 80% of them are very strongly absorbed. More than half of these absorbed sources show strong pulsations with long periods ranging from 139 to 1300s, i.e., they are slow X-ray pulsars. Many of these new sources are super-giant high-mass X-ray binaries (HMXB) in which the stellar wind of the companion star is accreted onto the compact object. The large local absorption in these new sources can be understood if the compact objects are buried deep in their stellar winds. These new objects represent half of the population of active super-giant HMXB.

I will discuss the connection between the luminosity function of a population of high-mass X-ray binaries in a galaxy and the star formation rate in the this galaxy. The understanding of this connection provides on the one hand an independent measure of an important galaxy property, and on the other hand new insights into populations of high-mass X-ray binaries. In particular, observations with the Chandra X-ray telescope are uniquely suited to investigate the X-ray part of this connection and I will present examples of this.

NGC 5102 has an unusually low number of XRBs. The deficit of LMXBs is even more striking because some of these sources may in fact be HMXBs, produced in one of the two recent bursts of star formation $\sim$15 and $\sim$300 Myr ago. Our UV-optical spectral synthesis analysis demonstrates that a significant fraction ($>$50%) of the stars in this galaxy are comparatively young ($<$3 Gyr). NGC 5102 also has an usually low number of globular clusters for its mass, luminosity and environment.

We discuss the relationship between the XRB population, the globular cluster population and the relative youth of the majority of the stars in this galaxy. We intend to extend our investigation of the relationship between XRB populations, star-formation history and globular clusters to a sample of ten early-type galaxies with a range of star-formation histories and investigate the implications for models of LMXB formation and evolution.