Search

We report on a timing and spectral analysis of a 50-ks NuSTAR observation of IGR J16320-4751 (= AX J1631.9-4752); a high-mass X-ray binary hosting a slowly-rotating neutron star. In this observation from 2015, the spin period was 1,308.8+/-0.4 s giving a period derivative dP/dt ~ 2E-8 s s-1 when compared with the period measured in 2004. In addition, the pulsed fraction decreased as a function of energy, as opposed to the constant trend that was seen previously. This suggests a change in the accretion geometry of the system during the intervening 11 years. The phase-averaged spectra were fit with the typical model for accreting pulsars: a power law with an exponential cutoff. This left positive residuals at 6.4 keV attributable to the known iron K-alpha line, as well as negative residuals around 14 keV from a candidate cyclotron line detected at a significance of 5-sigma. We found no significant differences in the spectral parameters across the spin period, other than the expected changes in flux and component normalizations. A flare lasting around 5 ks was captured during the first half of the observation where the X-ray emission hardened and the local column density decreased. Finally, the binary orbital period was refined to 8.9912+/-0.0078 d thanks to Swift/BAT monitoring data from 2005-2022., Comment: 17 pages, 11 figures, Referee-revised version accepted for publication in the Astrophysical Journal

The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite has detected in excess of 1000 sources in the ~20-100 keV band during its surveys of the sky over the past 17 years. We obtained 5 ks observations of 15 unclassified IGR sources with the Chandra X-ray Observatory in order to localize them, to identify optical/IR counterparts, to measure their soft X-ray spectra, and to classify them. For 10 of the IGR sources, we detect Chandra sources that are likely (or in some cases certain) to be the counterparts. IGR J18007-4146 and IGR J15038-6021 both have Gaia parallax distances, placing them at 2.5+0.5-0.4 and 1.1+1.5-0.4 kpc, respectively. We tentatively classify both of them as intermediate polar-type Cataclysmic Variables. Also, IGR J17508-3219 is likely to be a Galactic source, but it is unclear if it is a Dwarf Nova or another type of transient. For IGR J17118-3155, we provide a Chandra localization, but it is unclear if the source is Galactic or extragalactic. Based on either near-IR/IR colors or the presence of extended near-IR emission, we classify four sources as Active Galactic Nuclei (IGR J16181-5407, IGR J16246-4556, IGR J17096-2527, and IGR J19294+1327), and IGR J20310+3835 and IGR J15541-5613 are AGN candidates. In addition, we identified an AGN in the INTEGRAL error circle of IGR J16120-3543 that is a possible counterpart., Comment: Accepted by ApJ

AX J1949.8+2534 is a candidate supergiant fast X-ray transient (SFXT) observed in outburst by INTEGRAL (IGR J19498+2534). We report on the results of six Neil Gehrels Swift-XRT, one Chandra, and one NuSTAR observation of the source. We find evidence of rapid X-ray variability on a few ks timescales. Fortunately, Chandra observed the source in a relatively bright state, allowing us to confidently identify the optical/NIR counterpart of the source. We also obtained an optical spectrum of this counterpart, which shows an H$\alpha$ emission line and He I absorption features. The photometry and spectrum of the source allow us to constrain its distance, $\sim7-8$ kpc, and reddening, $A_V=8.5-9.5$. We find that the star is likely an early B-type Ia supergiant, confirming that AX J1949.8+2534 is indeed an SFXT., Comment: 9 pages, 5 Figures, Accepted for publication in the Astrophysical Journal

The NuSTAR serendipitous survey has already uncovered a large number of Active Galactic Nuclei (AGN), providing new information about the composition of the Cosmic X-ray Background. For the AGN off the Galactic plane, it has been possible to use the existing X-ray archival data to improve source localizations, identify optical counterparts, and classify the AGN with optical spectroscopy. However, near the Galactic Plane, better X-ray positions are necessary to achieve optical or near-IR identifications due to the higher levels of source crowding. Thus, we have used observations with the Chandra X-ray Observatory to obtain the best possible X-ray positions. With eight observations, we have obtained coverage for 19 NuSTAR serendips within 12 deg of the plane. One or two Chandra sources are detected within the error circle of 15 of the serendips, and we report on these sources and search for optical counterparts. For one source (NuSTAR J202421+3350.9), we obtained a new optical spectrum and detected the presence of hydrogen emission lines. The source is Galactic, and we argue that it is likely a Cataclysmic Variable. For the other sources, the Chandra positions will enable future classifications in order to place limits on faint Galactic populations, including high-mass X-ray binaries and magnetars., Comment: 16 pages, accepted by ApJ

We present a detailed X-ray spectral study of the quasar PG 1211+143 based on Chandra High Energy Transmission Grating Spectrometer (HETGS) observations collected in a multi-wavelength campaign with UV data using the Hubble Space Telescope Cosmic Origins Spectrograph (HST-COS) and radio bands using the Jansky Very Large Array (VLA). We constructed a multi-wavelength ionizing spectral energy distribution using these observations and archival infrared data to create XSTAR photoionization models specific to the PG 1211+143 flux behavior during the epoch of our observations. Our analysis of the Chandra-HETGS spectra yields complex absorption lines from H-like and He-like ions of Ne, Mg and Si which confirm the presence of an ultra-fast outflow (UFO) with a velocity ~ $-$17,300 km s$^{-1}$ (outflow redshift $z_{\rm out}$ ~ $-$0.0561) in the rest frame of PG 1211+143. This absorber is well described by an ionization parameter $\log \xi$ ~ 2.9 erg s$^{-1}$ cm and column density $\log N_{\rm H}$ ~ 21.5 cm$^{-2}$. This corresponds to a stable region of the absorber's thermal stability curve, and furthermore its implied neutral hydrogen column is broadly consistent with a broad Ly$\alpha$ absorption line at a mean outflow velocity of ~ $-$16,980 km s$^{-1}$ detected by our HST-COS observations. Our findings represent the first simultaneous detection of a UFO in both X-ray and UV observations. Our VLA observations provide evidence for an active jet in PG 1211+143, which may be connected to the X-ray and UV outflows; this possibility can be evaluated using very-long-baseline interferometric (VLBI) observations., Comment: 17 pages, 10 figures, published in The Astrophysical Journal, proof corrections applied

Relativistic plasma jets are observed in many accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched. Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot be easily disentangled from other accreting components. Here, we show that rapid optical flux variations from a Galactic black-hole binary are delayed with respect to X-rays radiated from close to the black hole by ~0.1 seconds, and that this delayed signal appears together with a brightening radio jet. The origin of these sub-second optical variations has hitherto been controversial. Not only does our work strongly support a jet origin for the optical variations, it also sets a characteristic elevation of <~10$^3$ Schwarzschild radii for the main inner optical emission zone above the black hole, constraining both internal shock and magnetohydrodynamic models. Similarities with blazars suggest that jet structure and launching physics could potentially be unified under mass-invariant models. Two of the best-studied jetted black hole binaries show very similar optical lags, so this size scale may be a defining feature of such systems., Comment: Authors' version of Letter published in Nature Astronomy on Oct 30, including supplementary information. Definitive version to be available at the journal website (nature.com/natastron)

We present a hard X-ray observation of the TeV gamma-ray binary candidate HESS J1832-093 coincident with supernova remnant (SNR) G22.7-0.2 using the Nuclear Spectroscopic Telescope Array (NuSTAR). Non-thermal X-ray emission from XMMU J183245-0921539, the X-ray source associated with HESS J1832-093, is detected up to ~30 keV and is well-described by an absorbed power-law model with the best-fit photon index $\Gamma = 1.5\pm0.1$. A re-analysis of archival Chandra and XMM-Newton data finds that the long-term X-ray flux increase of XMMU J183245-0921539 is $50^{+40}_{-20}$% (90% C.L.), much less than previously reported. A search for a pulsar spin period or binary orbit modulation yields no significant signal to a pulse fraction limit of fp < 19% in the range 4 ms < P < 40 ks. No red noise is detected in the FFT power spectrum to suggest active accretion from a binary system. While further evidence is required, we argue that the X-ray and gamma-ray properties of XMMU J183245-0921539 are most consistent with a non-accreting binary generating synchrotron X- rays from particle acceleration in the shock formed as a result of the pulsar and stellar wind collision. We also report on three nearby hard X-ray sources, one of which may be associated with diffuse emission from a fast-moving supernova fragment interacting with a dense molecular cloud., Comment: ApJ in press. 9 pages

The Nuclear Spectroscopic Telescope Array (NuSTAR) provides an improvement in sensitivity at energies above 10 keV by two orders of magnitude over non-focusing satellites, making it possible to probe deeper into the Galaxy and Universe. Lansbury and collaborators recently completed a catalog of 497 sources serendipitously detected in the 3-24 keV band using 13 deg2 of NuSTAR coverage. Here, we report on an optical and X-ray study of 16 Galactic sources in the catalog. We identify eight of them as stars (but some or all could have binary companions), and use information from Gaia to report distances and X-ray luminosities for three of them. There are four CVs or CV candidates, and we argue that NuSTAR J233426-2343.9 is a relatively strong CV candidate based partly on an X-ray spectrum from XMM-Newton. NuSTAR J092418-3142.2, which is the brightest serendipitous source in the Lansbury catalog, and NuSTAR J073959-3147.8 are LMXB candidates, but it is also possible that these two sources are CVs. One of the sources is a known HMXB, and NuSTAR J105008-5958.8 is a new HMXB candidate, which has strong Balmer emission lines in its optical spectrum and a hard X-ray spectrum. We discuss the implications of finding these HMXBs for the surface density (logN-logS) and luminosity function of Galactic HMXBs. We conclude that, with the large fraction of unclassified sources in the Galactic plane detected by NuSTAR in the 8-24 keV band, there could be a significant population of low luminosity HMXBs., Comment: 16 pages, accepted by ApJ

We present a catalog of hard X-ray sources in a square-degree region surveyed by NuSTAR in the direction of the Norma spiral arm. This survey has a total exposure time of 1.7 Ms, and typical and maximum exposure depths of 50 ks and 1 Ms, respectively. In the area of deepest coverage, sensitivity limits of $5\times10^{-14}$ and $4\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$ in the 3-10 and 10-20 keV bands, respectively, are reached. Twenty-eight sources are firmly detected and ten are detected with low significance; eight of the 38 sources are expected to be active galactic nuclei. The three brightest sources were previously identified as a low-mass X-ray binary, high-mass X-ray binary, and pulsar wind nebula. Based on their X-ray properties and multi-wavelength counterparts, we identify the likely nature of the other sources as two colliding wind binaries, three pulsar wind nebulae, a black hole binary, and a plurality of cataclysmic variables (CVs). The CV candidates in the Norma region have plasma temperatures of $\approx$10-20 keV, consistent with the Galactic Ridge X-ray emission spectrum but lower than temperatures of CVs near the Galactic Center. This temperature difference may indicate that the Norma region has a lower fraction of intermediate polars relative to other types of CVs compared to the Galactic Center. The NuSTAR log$N$-log$S$ distribution in the 10-20 keV band is consistent with the distribution measured by Chandra at 2-10 keV if the average source spectrum is assumed to be a thermal model with $kT\approx15$~keV, as observed for the CV candidates., Comment: 42 pages, 12 figures, 11 tables

We present results of long NuSTAR (200 ks) and XMM-Newton (100 ks) observations of the Arches stellar cluster, a source of bright thermal (kT~2 keV) X-rays with prominent Fe XXV K_alpha 6.7 keV line emission and a nearby molecular cloud, characterized by an extended non-thermal hard X-ray continuum and fluorescent Fe K_alpha 6.4 keV line of a neutral or low ionization state material around the cluster. Our analysis demonstrates that the non-thermal emission of the Arches cloud underwent a dramatic change, with its homogeneous morphology, traced by fluorescent Fe K_alpha line emission, vanishing after 2012, revealing three bright clumps. The declining trend of the cloud emission, if linearly fitted, is consistent with half-life decay time of ~8 years. Such strong variations have been observed in several other molecular clouds in the Galactic Centre, including the giant molecular cloud Sgr B2, and point toward a similar propagation of illuminating fronts, presumably induced by the past flaring activity of Sgr A*., Comment: 15 pages, 14 figures, 7 tables, submitted to MNRAS; comments welcome

We report on FORS2 spectroscopy aiming at the identification of four Galactic Plane sources discovered by $INTEGRAL$, IGR J18088-2741, IGR J18381-0924, IGR J17164-3803, and IGR J19173+0747, complemented by $XMM$-$Newton$ spectroscopy for IGR J18381-0924. The presence of broad emission lines of neutral H and He and a flat Balmer decrement H{\alpha}/H{\beta} show that IGR J18088-2741 is a cataclysmic variable located beyond 8 kpc. For IGR J18381-0924, the detection of red-shifted H{\alpha} and neutral O emission signatures and the absence of narrow forbidden emission lines point towards a low-luminosity Seyfert 1.9 nature at $z=0.031\pm0.002$. Its $XMM$-$Newton$ spectrum, best-fit by an absorbed ${\Gamma}=1.19\pm0.07$ power law combined with a $z=0.026_{-0.016}^{+0.008}$ red-shifted iron emission feature, is in agreement with this classification. The likely IGR J17164-3803 optical counterpart is an M2 III star at 3 to 4 kpc which, based on the X-ray spectrum of the source, is the companion of a white dwarf in an X-ray faint symbiotic system. Finally, we challenge the accepted identification of IGR J19173+0747 as an high mass X-ray binary. Indeed, the USNO optical counterpart is actually a blend of two objects located at the most likely 3 kpc distance, both lying within the error circle of the Swift position. The first is a cataclysmic variable, which we argue is the real nature of IGR J19173+0747. However, we cannot rule out the second one that we identify as an F3 V star that, if associated to IGR J19173+0747, likely belongs to a quiescent X-ray binary., Comment: 10 pages, 8 figures, 4 tables. Accepted for publication in MNRAS

Recently, unresolved hard (20-40 keV) X-ray emission has been discovered within the central 10 pc of the Galaxy, possibly indicating a large population of intermediate polars (IPs). Chandra and XMM-Newton measurements in the surrounding ~50 pc imply a much lighter population of IPs with $\langle M_{\rm WD} \rangle \approx 0.5 M_\odot$. Here we use broad-band NuSTAR observations of two IPs: TV Columbae, which has a fairly typical but widely varying reported mass of $M_{\rm WD} \approx 0.5-1.0 M_\odot$, and IGR J17303-0601, with a heavy reported mass of $M_{\rm WD} \approx 1.0-1.2 M_\odot$. We investigate how varying spectral models and observed energy ranges influence estimated white dwarf mass. Observations of the inner 10 pc can be accounted for by IPs with $\langle M_{\rm WD} \rangle \approx 0.9 M_\odot$, consistent with that of the CV population in general, and the X-ray observed field IPs in particular. The lower mass derived by Chandra and XMM-Newton appears to be an artifact of narrow energy band fitting. To explain the (unresolved) CHXE by IPs requires an X-ray (2-8 keV) luminosity function (XLF) extending down to at least $5\times10^{31}$ erg/s. The CHXE XLF, if extended to the surrounding ~50 pc observed by Chandra and XMM-Newton, requires at least ~20-40% of the $\sim$9000 point sources are IPs. If the XLF extends just a factor of a few lower in luminosity, then the vast majority of these sources are IPs. This is in contrast to recent observations of the Galactic ridge, where the bulk of the 2-8 keV emission is ascribed to dwarf novae., Comment: 17 pages, accepted for publication in Astrophysical Journal

We report on a simultaneous NuSTAR and Swift observation of the neutron star low-mass X-ray binary 4U 1728-34. We identified and removed four Type I X-ray bursts during the observation in order to study the persistent emission. The continuum spectrum is hard and well described by a black body with $kT=$ 1.5 keV and a cutoff power law with $\Gamma=$ 1.5 and a cutoff temperature of 25 keV. Residuals between 6 and 8 keV provide strong evidence of a broad Fe K$\alpha$ line. By modeling the spectrum with a relativistically blurred reflection model, we find an upper limit for the inner disk radius of $R_{\rm in}\leq2 R_{\rm ISCO}$. Consequently we find that $R_{\rm NS}\leq23$ km, assuming $M=1.4{\mbox{$\rm\,M_{\mathord\odot}$}}$ and $a=0.15$. We also find an upper limit on the magnetic field of $B\leq2\times10^8$ G., Comment: 9 pages, 8 figures

IGR J14091-6108 is a Galactic X-ray source known to have an iron emission line, a hard X-ray spectrum, and an optical counterpart. Here, we report on X-ray observations of the source with XMM-Newton and NuSTAR as well as optical spectroscopy with ESO/VLT and NOAO/SOAR. In the X-rays, this provides data with much better statistical quality than the previous observations, and this is the first report of the optical spectrum. Timing analysis of the XMM data shows a very significant detection of 576.3+/-0.6 s period. The signal has a pulsed fraction of 30%+/-3% in the 0.3-12 keV range and shows a strong drop with energy. The optical spectra show strong emission lines with significant variability in the lines and continuum, indicating that they come from an irradiated accretion disk. Based on these measurements, we identify the source as a magnetic Cataclysmic Variable of Intermediate Polar (IP) type where the white dwarf spin period is 576.3 s. The X-ray spectrum is consistent with the continuum emission mechanism being due to thermal Bremsstrahlung, but partial covering absorption and reflection are also required. In addition, we use the IP mass (IPM) model, which suggests that the white dwarf in this system has a high mass, possibly approaching the Chandrasekhar limit., Comment: 14 pages, 7 figures, 4 tables, accepted by MNRAS

The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was observed by NuSTAR in the 3-79 keV energy band for a net exposure time of 50 ks. We present the results of this observation which enabled the discovery of a cyclotron resonant scattering feature with a centroid energy of 29.3(+1.1/-1.3) keV. This allowed us to measure the magnetic field strength of the neutron star for the first time: B = (2.5+/-0.1)e12 G. The known pulsation period is now observed at 904.0+/-0.1 s. Since 2006, the neutron star has undergone a long-term spin-up trend at a rate of P' = -2e-8 s/s (-0.6 s per year, or a frequency derivative of nu' = 3e-14 Hz/s ). In the power density spectrum, a break appears at the pulse frequency which separates the zero slope at low frequency from the steeper slope at high frequency. This addition of angular momentum to the neutron star could be due to the accretion of a quasi-spherical wind, or it could be caused by the transient appearance of a prograde accretion disk that is nearly in corotation with the neutron star whose magnetospheric radius is around 2e8 cm., Comment: Accepted for publication in the Astrophysical Journal, 7 pages, 8 figures, 2 tables

We report on the results of NuSTAR and XMM-Newton observations of the persistent X-ray source 1E1743.1-2843, located in the Galactic Center region. The source was observed between September and October 2012 by NuSTAR and XMM-Newton, providing almost simultaneous observations in the hard and soft X-ray bands. The high X-ray luminosity points to the presence of an accreting compact object. We analyze the possibilities of this accreting compact object being either a neutron star (NS) or a black hole, and conclude that the joint XMM-Newton and NuSTAR spectrum from 0.3 to 40 $\mathrm{keV}$ fits to a black body spectrum with $kT\sim1.8~\mathrm{keV}$ emitted from a hot spot or an equatorial strip on a neutron star surface. This spectrum is thermally Comptonized by electrons with $kT_{e}\sim4.6~\mathrm{keV}$. Accepting this neutron star hypothesis, we probe the Low Mass (LMXB) or High Mass (HMXB) X-ray Binary nature of the source. While the lack of Type-I bursts can be explained in the LMXB scenario, the absence of pulsations in the 2 mHz - 49 Hz frequency range, the lack of eclipses and of an IR companion, and the lack of a $K_{\alpha}$ line from neutral or moderately ionized iron strongly disfavor interpreting this source as a HMXB. We therefore conclude that 1E1743.1-2843 is most likely a NS-LMXB located beyond the Galactic Center. There is weak statistical evidence for a soft X-ray excess possibly indicating thermal emission from an accretion disk. However, the disk normalization remains unconstrained due to the high hydrogen column density ($N_{H}\sim1.6\times10^{23}~\mathrm{cm^{-2}}$)., Comment: Accepted for publication on ApJ on March 8th 2016

We report on 0.3-10 keV observations with the Chandra X-ray Observatory of eight hard X-ray sources discovered within 8 degrees of the Galactic plane by the INTEGRAL satellite. The short (5 ks) Chandra observations of the IGR source fields have yielded very likely identifications of X-ray counterparts for three of the IGR sources: IGR J14091-6108, IGR J18088-2741, and IGR J18381-0924. The first two have very hard spectra in the Chandra band that can be described by a power-law with photon indices of Gamma = 0.6+/-0.4 and -0.7(+0.4)(-0.3), respectively (90% confidence errors are given), and both have a unique near-IR counterpart consistent with the Chandra position. IGR J14091-6108 also displays a strong iron line and a relatively low X-ray luminosity, and we argue that the most likely source type is a Cataclysmic Variable (CV), although we do not completely rule out the possibility of a High Mass X-ray Binary. IGR J18088-2741 has an optical counterpart with a previously measured 6.84 hr periodicity, which may be the binary orbital period. We also detect five cycles of a possible 800-950 s period in the Chandra light curve, which may be the compact object spin period. We suggest that IGR J18088-2741 is also most likely a CV. For IGR J18381-0924, the spectrum is intrinsically softer with Gamma = 1.5(+0.5)(-0.4), and it is moderately absorbed, nH = (4+/-1)e22 cm-2. There are two near-IR sources consistent with the Chandra position, and they are both classified as galaxies, making it likely that IGR J18381-0924 is an Active Galactic Nucleus (AGN). For the other five IGR sources, we provide lists of nearby Chandra sources, which may be used along with further observations to identify the correct counterparts, and we discuss the implications of the low inferred Chandra count rates for these five sources., Comment: Accepted by ApJ, 14 pages

We report on a multiwavelength observational campaign of the black hole X-ray binary Swift J1753.5-0127 that consists of an ESO/X-shooter spectrum supported by contemporaneous Swift/XRT+UVOT and ATCA data. ISM absorption lines in the X-shooter spectrum allows us to determine E(B-V)=0.45+/-0.02 along the line-of-sight to the source. We also report detection of emission signatures of He II at 4686 angstrom, H alpha, and, for the first time, H I at 10906 angstrom and Paschen Beta. The double-peaked morphology of these four lines is typical of the chromosphere of a rotating accretion disk. Nonetheless, the paucity of disk features points towards a low level of irradiation in the system. This is confirmed through spectral energy distribution modeling and we find that the UVOT+X-shooter continuum mostly stems from the thermal emission of a viscous disk. We speculate that the absence of reprocessing is due to the compactness of an illumination-induced envelope that fails to reflect enough incoming hard X-ray photons back to the outer regions. The disk also marginally contributes to the Compton-dominated X-ray emission and is strongly truncated, with an inner radius about a thousand times larger than the black hole's gravitational radius. A near-infrared excess is present, and we associate it with synchrotron radiation from a compact jet. However, the measured X-ray flux is significantly higher than what can be explained by the optically thin synchrotron jet component. We discuss these findings in the framework of the radio quiet versus X-ray bright hypothesis, favoring the presence of a residual disk, predicted by evaporation models, that contributes to the X-ray emission without enhancing the radio flux., Comment: 12 pages, 11 figures, 8 tables. Accepted for publication in the Astrophysical Journal

We present spectral and timing analysis of NuSTAR observations of the accreting X-ray pulsar 2RXP J130159.6-635806. The source was serendipitously observed during a campaign focused on the gamma-ray binary PSR B1259-63 and was later targeted for a dedicated observation. The spectrum has a typical shape for accreting X-ray pulsars, consisting of a simple power law with an exponential cutoff starting at ~7 keV with a folding energy of E_fold=~18 keV. There is also an indication of the presence of a 6.4 keV iron line in the spectrum at the ~3 sigma significance level. NuSTAR measurements of the pulsation period reveal that the pulsar has undergone a strong and steady spin-up for the last 20 years. The pulsed fraction is estimated to be ~80%, and is constant with energy up to 40 keV. The power density spectrum shows a break towards higher frequencies relative to the current spin period. This, together with steady persistent luminosity, points to a long-term mass accretion rate high enough to bring the pulsar out of spin equilibrium., Comment: 13 pages, 7 figures, accepted for publication in The Astrophysical Journal

We present results from multi-wavelength simultaneous X-ray and radio observations of the black hole X-ray binary V404 Cyg in quiescence. Our coverage with NuSTAR provides the very first opportunity to study the X-ray spectrum of V404 Cyg at energies above 10 keV. The unabsorbed broad-band (0.3--30 keV) quiescent luminosity of the source is 8.9$\times$10$^{32}$ erg s$^{-1}$ for a distance of 2.4 kpc. The source shows clear variability on short time scales (an hour to a couple of hours) in radio, soft X-ray and hard X-ray bands in the form of multiple flares. The broad-band X-ray spectra obtained from XMM-Newton and NuSTAR can be characterized with a power-law model having photon index $\Gamma$=2.12$\pm$0.07 (90\% confidence errors); however, residuals at high energies indicate spectral curvature significant at a 3$\sigma$ confidence level with e-folding energy of the cutoff to be 20$^{+20}_{-7}$ keV. Such curvature can be explained using synchrotron emission from the base of a jet outflow. Radio observations using the VLA reveal that the spectral index evolves on very fast time-scales (as short as 10 min.), switching between optically thick and thin synchrotron emission, possibly due to instabilities in the compact jet or stochastic instabilities in accretion rate. We explore different scenarios to explain this very fast variability., Comment: 16 pages, 11 figures and 3 tables, Accepted for publication in The Astrophysical Journal

(abridged) We report on multi-wavelength measurements of Swift J1753.5-0127 in the hard state at L=2.7e36 erg/s (assuming d=3 kpc) in 2014. The radio emission is optically thick synchrotron, presumably from a compact jet. We take advantage of the low extinction and model the near-IR to UV emission with a multi-temperature disk model. Assuming a BH mass of M_BH=5 Msun and a system inclination of 40 deg, the fits imply an inner radius for the disk of Rin/Rg>212 d_3 (5Msun/M_BH). The outer radius is R_out/R_g=90,000 d_3 (5Msun/M_BH), which corresponds to 6.6e10 d_3 cm, consistent with the expected size of the disk. The 0.5-240 keV spectrum measured by Swift/XRT, Suzaku, and NuSTAR is relatively well characterized by a power-law with a photon index of Gamma=1.722+/-0.003, but a significant improvement is seen when a second continuum component is added. Reflection is a possibility, but no iron line is detected, implying a low iron abundance. We are able to fit the entire SED with a multi-temperature disk component, a Comptonization component, and a broken power-law, representing the emission from the compact jet. The broken power-law cannot significantly contribute to the soft X-ray emission, and this may be related to why Swift J1753.5-0127 is an outlier in the radio/X-ray correlation. The broken power-law might dominate above 20 keV, which would constrain the break frequency to be between 2.4e10 Hz and 3.6e12 Hz. Although the fits to the full SED do not include significant thermal emission in the X-ray band, previous observations have consistently seen such a component, and we find that there is evidence at the 3.1-sigma level for a disk-blackbody component with a temperature of 150(+30)(-20) eV and an inner radius of 5-14 R_g. If this component is real, it might imply the presence of an inner accretion disk in addition to the strongly truncated (R_in>212 R_g) disk., Comment: 18 pages, 9 figures, accepted by ApJ

Here, we report on observations of two hard X-ray sources that were originally discovered with the INTEGRAL satellite: IGR J04059+5416 and IGR J08297-4250. We use the Chandra X-ray Observatory to localize the sources and then archival near-IR images to identify the counterparts. Both sources have counterparts in the catalog of extended 2 Micron All-Sky Survey sources, and the counterpart to IGR J04059+5416 has been previously identified as a galaxy. Thus, we place IGR J04059+5416 in the class of Active Galactic Nuclei (AGN), and we suggest that IGR J08297-4250 is also an AGN. If this identification is correct, the near-IR images suggest that the host galaxy of IGR J08297-4250 may be merging with a smaller nearby galaxy. For IGR J04059+5416, the 0.3-86 keV spectrum from Chandra and INTEGRAL is consistent with an absorbed power-law with a column density of N_H = 3.1(+2.0)(-1.5)e22 cm-2 and a photon index of Gamma = 1.4+/-0.7, and we suggest that it is a Seyfert galaxy. For IGR J08297-4250, the photon index is similar, Gamma = 1.5+/-0.8, but the source is highly absorbed (N_H = 6.1(+10.1)(-4.3)e23 cm-2)., Comment: 10 pages, 5 figures, 5 tables

Multi-wavelength analysis of the young massive cluster VVV CL077 is presented for the first time. Our Chandra survey of this region enabled the detection of three X-ray emitting stellar members of the cluster, as well as a possible diffuse X-ray component that extends a few arcseconds from the cluster core with an intrinsic flux of (9+/-3)x10^-14 erg cm^-2 s^-1 in the 0.5-10 keV band. Infrared spectra we obtained for two of these X-ray point sources show absorption lines typical of the atmospheres of massive O stars. The X-ray spectrum from the visible extent of VVV CL077 i.e., a 15"-radius around the cluster, can be modeled with an absorbed power law with nH = (6+/-4)x10^22 cm^-2 and gamma = 2+/-1. In addition, the X-ray core of VVV CL077 coincides with diffuse emission seen in the infrared band and with a local maximum in the radio continuum map. A possible association with a neighboring H II region would place VVV CL077 at a distance of around 11 kpc; on the far side of the Norma Arm. At this distance, the cluster is 0.8 pc wide with a mass density of (1-4)x10^3 Msol pc^-3., Comment: accepted for publication in The Astrophysical Journal: 9 pages, 5 figures

We aim at characterising the structure of the gas and dust around the high mass X-ray binary GX 301-2, a highly obscured X-ray binary hosting a hypergiant star and a neutron star, in order to better constrain its evolution. We used Herschel PACS to observe GX 301-2 in the far infrared and completed the spectral energy distribution of the source using published data or catalogs, from the optical to the radio range (0.4 to 4x10^4 micrometer). GX 301-2 is detected for the first time at 70 and 100 micrometer. We fitted different models of circumstellar environments to the data. All tested models are statistically acceptable, and consistent with a hypergiant star at ~3 kpc. We found that the addition of a free-free emission component from the strong stellar wind is required and could dominate the far infrared flux. Through comparisons with similar systems and discussion on the estimated model parameters, we favour a disk-like circumstellar environment of ~8 AU that would enshroud the binary system. The temperature goes down to ~200 K at the edge of the disk, allowing for dust formation. This disk is probably a rimmed viscous disk with an inner rim at the temperature of the dust sublimation temperature (~1500 K). The similarities between the hypergiant GX 301-2, B[e] supergiants and the highly obscured X-ray binaries (in particular IGR J16318-4848) are strengthened. GX 301-2 might represent a transition stage in the evolution of massive stars in binary systems, connecting supergiant B[e] systems to luminous blue variables., Comment: accepted for publication in ApJ (tentatively scheduled for the December 1, 2014, V796 - 2 issue), 15 pages with emulateapj style

We present a catalog of 1415 X-ray sources identified in the Norma arm region Chandra survey (NARCS), which covers a 2 deg x 0.8 deg region in the direction of the Norma spiral arm to a depth of $\approx$20 ks. Of these sources, 1130 are point-like sources detected with $\geq3\sigma$ confidence in at least one of three energy bands (0.5-10, 0.5-2, and 2-10 keV), five have extended emission, and the remainder are detected at low significance. Since most sources have too few counts to permit individual classification, they are divided into five spectral groups defined by their quantile properties. We analyze stacked spectra of X-ray sources within each group, in conjunction with their fluxes, variability, and infrared counterparts, to identify the dominant populations in our survey. We find that $\sim$50% of our sources are foreground sources located within 1-2 kpc, which is consistent with expectations from previous surveys. Approximately 20% of sources are likely located in the proximity of the Scutum-Crux and near Norma arm, while 30% are more distant, in the proximity of the far Norma arm or beyond. We argue that a mixture of magnetic and nonmagnetic CVs dominates the Scutum-Crux and near Norma arms, while intermediate polars (IPs) and high-mass stars (isolated or in binaries) dominate the far Norma arm. We also present the cumulative number count distribution for sources in our survey that are detected in the hard energy band. A population of very hard sources in the vicinity of the far Norma arm and active galactic nuclei dominate the hard X-ray emission down to $f_X\approx10^{-14}$ erg cm$^{-2}$ s$^{-1}$, but the distribution curve flattens at fainter fluxes. We find good agreement between the observed distribution and predictions based on other surveys., Comment: 27 pages, 19 figures, 12 tables, catalog will be made available in published ApJ version and by personal request

We report on CTIO/NEWFIRM and CTIO/OSIRIS photometric and spectroscopic observations of 20 new X-ray (0.5-10 keV) emitters discovered in the Norma Arm Region Chandra Survey (NARCS). NEWFIRM photometry was obtained to pinpoint the near-infrared counterparts of NARCS sources, while OSIRIS spectroscopy was used to help identify 20 sources with possible high mass X-ray binary properties. We find that (1) two sources are WN8 Wolf-Rayet stars, maybe in colliding wind binaries, part of the massive star cluster Mercer 81; (2) two are emission-line stars, possibly in X-ray binaries, that exhibit near- and mid-infrared excesses either due to free-free emission from the decretion discs of Be stars or warm dust in the stellar winds of peculiar massive stars such as B[e] supergiants or luminous blue variables; (3) one is a B8-A3 IV-V star that could be in a quiescent high mass X-ray binary system; (4) two are cataclysmic variables including one intermediate polar; (5) three may be neutron star symbiotic binaries; (6) five are most likely white dwarf symbiotic binaries; and (7) five exhibit properties more consistent with isolated giant/dwarf stars. The possible detection of one to three high mass X-ray binaries is in good agreement with our predictions. However, our study illustrates the difficulty of clearly differentiating quiescent or intermediate X-ray luminosity systems from isolated massive stars, which may lead to an underestimation of the number of known high mass X-ray binaries., Comment: 17 pages, 14 figures, 7 tables. Accepted for publication in A&A

As a complement to our optical and near-infrared study of the continuum properties of GX 339-4 in the two hard and one soft state observations made by the ESO/VLT FORS2 and ISAAC in early 2010, we report here on the results of our spectral line analysis for the same observations. In the soft state, the presence of strong Balmer, Paschen and Brackett emission lines points to the optical and near-infrared spectra stemming from the irradiated chromosphere of the optically thick and geometrically thin accretion disc. Most of these HI features are still detected in emission in both hard states but are veiled by the compact jets continuum. We also confirm the presence of a broad Hbeta absorption feature, prominent in the soft state and shallower in the first hard state, which we argue forms in the deep layers of the optically thick accretion disc. However, this trough is absent in the second hard state, a likely consequence of the formation of a geometrically thick extended envelope that arises above the disc plane and eventually enshrouds the region where the Hbeta absorption feature forms. We detect this envelope through the presence of a broad Pabeta emission line, which is constant during the first hard state but correlates with the underlying continuum during the second hard state, pointing to changing physical properties. We consider that this behaviour may be consistent with the launch of a thermally-driven accretion disc wind during the second hard state., Comment: 9 pages, 6 figures, 4 tables. Accepted for publication in MNRAS

During hard X-ray observations of the Norma spiral arm region by the Nuclear Spectroscopic Telescope Array (NuSTAR) in 2013 February, a new transient source, NuSTAR J163433-4738.7, was detected at a significance level of 8-sigma in the 3-10 keV bandpass. The source is consistent with having a constant NuSTAR count rate over a period of 40 ks and is also detected simultaneously by Swift at lower significance. The source is not significantly detected by NuSTAR, Swift, or Chandra in the days before or weeks after the discovery of the transient, indicating that the strong X-ray activity lasted for between ~0.5 and 1.5 days. Near-IR imaging observations were carried out before and after the X-ray activity, but we are not able to identify the counterpart. The combined NuSTAR and Swift energy spectrum is consistent with a power-law with a photon index of Gamma = 4.1(+1.5)(-1.0) (90% confidence errors), a blackbody with kT = 1.2+/-0.3 keV, or a bremsstrahlung model with kT = 3.0(+2.1)(-1.2) keV. The reduced-chi2 values for the three models are not significantly different, ranging from 1.23 to 1.44 for 8 degrees of freedom. The spectrum is strongly absorbed with NH = 2.8(+2.3)(-1.4)e23 cm-2, 9(+15)(-7)e22 cm-2, and 1.7(+1.7)(-0.9)e23 cm-2, for the power-law, blackbody, and bremsstrahlung models, respectively. Although the high column density could be due to material local to the source, it is consistent with absorption from interstellar material along the line of sight at a distance of 11 kpc, which would indicate an X-ray luminosity >1e34 erg/s. Although we do not reach a definitive determination of the nature of NuSTAR J163433-4738.7, we suggest that it may be an unusually bright active binary or a magnetar., Comment: 6 pages, 4 figures, published as ApJ, 785, 4. The name of the source has been revised to NuSTAR J163433-4738.7 in order to comply with the IAU format for NuSTAR sources

Since it started observing the sky, the INTEGRAL satellite has discovered new categories of high mass X-ray binaries (HMXB) in our Galaxy. These observations raise important questions on the formation and evolution of these rare and short-lived objects. We present here new infrared observations from which to reveal or constrain the nature of 15 INTEGRAL sources, which allow us to update and discuss the Galactic HMXB population statistics. After previous photometric and spectroscopic observing campaigns in the optical and near-infrared, new photometry and spectroscopy was performed in the near-infrared with the SofI instrument on the ESO/NTT telescope in 2008 and 2010 on a sample of INTEGRAL sources. These observations, and specifically the detection of certain features in the spectra, allow the identification of these high-energy objects by comparison with published nIR spectral atlases of O and B stars. We present photometric data of nine sources (IGR J10101-5654, IGR J11187-5438, IGR J11435-6109, IGR J14331-6112, IGR J16328-4726, IGR J17200-3116, IGR J17354-3255, IGR J17404-3655, and IGR J17586-2129) and spectroscopic observations of 13 sources (IGR J10101-5654, IGR J11435-6109, IGR J13020-6359, IGR J14331-6112, IGR J14488-5942, IGR J16195-4945, IGR J16318-4848, IGR J16320-4751, IGR J16328-4726, IGR J16418-4532, IGR J17354-3255, IGR J17404-3655, and IGR J17586-2129). Our spectroscopic measurements indicate that: five of these objects are Oe/Be high-mass X-ray binaries (BeHMXB), six are supergiant high-mass X-ray binaries (sgHMXB), and two are sgB[e]. From a statistical point of view, we estimate the proportion of confirmed sgHMXB to be 42% and that of the confirmed BeHMXB to be 49%. The remaining 9% are peculiar HMXB., Comment: Accepted for publication in A&A (in press)

We present preliminary results on Herschel/PACS mid/far-infrared photometric observations of INTEGRAL supergiant High Mass X-ray Binaries (HMXBs), with the aim of detecting the presence and characterizing the nature of absorbing material (dust and/or cold gas), either enshrouding the whole binary systems, or surrounding the sources within their close environment. These unique observations allow us to better characterize the nature of these HMXBs, to constrain the link with their environment (impact and feedback), and finally to get a better understanding of the formation and evolution of such rare and short-living supergiant HMXBs in our Galaxy., Comment: Proceedings of the 9th INTEGRAL Workshop and celebration of the 10th anniversary of the launch "An INTEGRAL view of the high-energy sky (the first 10 years)", accepted for publication in Proceedings of Science (editors: F. Lebrun, A. Goldwurm and C. Winkler), 4 pages, 2 figures

We present results from a coordinated IR-to-X-ray spectral campaign of the QSO IRAS 13349+2438. Optical spectra reveal extreme Eigenvector-1 characteristics, but the H-beta line width argues against a NLS1 classification; we refine z=0.10853 based on [O III]. We estimate a BH mass=10^9 Msun using 2 independent methods (H-beta line width & SED fits). Blue-shifted absorption (-950km/s & -75km/s) is seen for the 1st time in STIS UV spectra from Ly-alpha, NV, & CIV. The higher velocity UV lines are coincident with the lower-ionisation (xi~1.6) X-ray warm absorber lines. A dusty multiple ionization absorber blueshifted by 700-900km/s is required to fit the X-ray data. Theoretical models comparing different ionising SEDs reveal that a UV-inclusive (i.e., the accretion disc) ionising continuum strongly impacts conclusions for the thermodynamic stability of the warm absorber. Specific to IRAS13349, an Xray-UV ionising SED favors a continuous distribution of ionisation states in a smooth flow (this paper), versus discrete clouds in pressure equilibrium (work by others where UV is omitted). Direct dust detections are seen in both the IR: PAH emission at (7.7 & 11.3)micron which may also be blended with forsterite, and (10 & 18)micron silicate emission, and X-rays: iron dust with a dust-to-gas ratio > 90%. We develop a geometrical model whereby the QSO nuclear region is viewed through the upper atmosphere of an obscuring torus. This sight line is obscured by dust that blocks a direct view of the UV/optical emission region but is largely transparent in X-rays since the gas is ionised. In our model, 20% of the intrinsic UV/optical continuum is scattered into our sight line by the far wall of an obscuring torus. An additional 2.4% of the direct light, which likely dominates the UV emission, is Thomson-scattered into our line-of-sight by another off-plane component of highly ionized gas., Comment: Accepted for publication to MNRAS 2013 January 8. Received 2013 January 7; in original form 2012 August 20. 31 MNRAS pages

We report on the optical spectroscopic follow up observations of the candidate counterparts to four INTEGRAL sources: IGR J04069+5042, IGR J06552-1146, IGR J21188+4901 and IGR J22014+6034. The candidate counterparts were determined with Chandra, and the optical observations were performed with 1.5-m RTT-150 telescope (T\"{U}B\.{I}TAK National Observatory, Antalya, Turkey) and 2.4-m Hiltner Telescope (MDM Observatory, Kitt Peak, Arizona). Our spectroscopic results show that one of the two candidates of IGR J04069+5042 and the one observed for IGR J06552-1146 could be active late-type stars in RS CVn systems. However, according to the likelihood analysis based on Chandra and INTEGRAL, two optically weaker sources in the INTEGRAL error circle of IGR J06552-1146 have higher probabilities to be the actual counterpart. The candidate counterparts of IGR J21188+4901 are classified as an active M-type star and a late-type star. Among the optical spectra of four candidates of IGR J22014+6034, two show H\alpha emission lines, one is a late-type star and the other is a M type. The likelihood analysis favors a candidate with no distinguishing features in the optical spectrum. Two of the candidates classified as M type dwarfs are similar to some IGR candidates claimed to be symbiotic stars. However, some of the prominent features of symbiotic systems are missing in our spectra, and their NIR colors are not consistent with those expected for giants. We consider the IR colors of all IGR candidates claimed to be symbiotic systems and find that low resolution optical spectrum may not be enough for conclusive identification., Comment: 24 pages, 12 figures; accepted for publication in ApJ

We report on an observation of SGR 1627-41 made with the Chandra X-ray Observatory on 2011 June 16. Approximately three years after its outburst activity in 2008, the source's flux has been declining, as it approaches its quiescent state. For an assumed power-law spectrum, we find that the absorbed 2--10 keV flux for the source is $1.0^{+0.3}_{-0.2} \times 10^{-13} erg cm^{-2} s^{-1}$ with a photon index of $2.9 \pm 0.8$ ($N_H=1.0\times10^{23}$ cm^{-2}). This flux is approximately consistent with that measured at the same time after the source's outburst in 1998. With measurements spanning 3 years after the 2008 outburst, we analyze the long-term flux and spectral evolution of the source. The flux evolution is well described by a double exponential with decay times of 0.5 $\pm$ 0.1 and 59 $\pm$ 6 days, and a thermal cooling model fit suggests that SGR 1627-41 may have a hot core ($T_c ~ 2\times 10^8$ K). We find no clear correlation between flux and spectral hardness as found in other magnetars. We consider the quiescent X-ray luminosities of magnetars and the subset of rotation-powered pulsars with high magnetic fields ($B >~ 10^{13}$ G) in relation to their spin-inferred surface magnetic-field strength, and find a possible trend between the two quantities., Comment: 25 pages, 5 figures, 3 tables. Accepted for publication in ApJ

We report on Chandra observations of 18 hard X-ray (>20 keV) sources discovered with the INTEGRAL satellite near the Galactic plane. For 14 of the INTEGRAL sources, we have uncovered one or two potential Chandra counterparts per source. These provide soft X-ray (0.3-10 keV) spectra and sub-arcsecond localizations, which we use to identify counterparts at other wavelengths, providing information about the nature of each source. Despite the fact that all of the sources are within 5 degrees of the plane, four of the IGR sources are AGN (IGR J01545+6437, IGR J15391-5307, IGR J15415-5029, and IGR J21565+5948) and four others are likely AGN (IGR J03103+5706, IGR J09189-4418, IGR J16413-4046, and IGR J16560-4958) based on each of them having a strong IR excess and/or extended optical or near-IR emission. We compare the X-ray and near-IR fluxes of this group of sources to those of AGN selected by their 2-10 keV emission in previous studies and find that these IGR AGN are in the range of typical values. There is evidence in favor of four of the sources being Galactic (IGR J12489-6243, IGR J15293-5609, IGR J16173-5023, and IGR J16206-5253), but only IGR J15293-5609 is confirmed as a Galactic source as it has a unique Chandra counterpart and a parallax measurement from previous optical observations that puts its distance at 1.56+/-0.12 kpc. The 0.3-10 keV luminosity for this source is 1.4e32 erg/s, and its optical/IR spectral energy distribution is well described by a blackbody with a temperature of 4200-7000 K and a radius of 12.0-16.4 Rsun. These values suggest that IGR J15293-5609 is a symbiotic binary with an early K-type giant and a white dwarf accretor. We also obtained likely Chandra identifications for IGR J13402-6428 and IGR J15368-5102, but follow-up observations are required to constrain their source types., Comment: 17 pages, accepted by ApJ

A new class of X-ray binaries has been recently discovered by the high energy observatory, INTEGRAL. It is composed of intrinsically obscured supergiant high mass X-ray binaries, unveiled by means of multi-wavelength X-ray, optical, near- and mid-infrared observations, in particular photometric and spectroscopic observations using ESO facilities. However the fundamental questions about these intriguing sources, namely their formation, evolution, and the nature of their environment, are still unsolved. Among them, IGR J16318-4848 - a compact object orbiting around a supergiant B[e] star - seems to be one of the most extraordinary celestial sources of our Galaxy. We present here new ESO/VLT VISIR mid-infrared (MIR) spectroscopic observations of this source. First, line diagnostics allow us to confirm the presence of absorbing material (dust and cold gas) enshrouding the whole binary system, and to characterise the nature of this material. Second, by fitting broadband near to mid-infrared Spectral Energy Distribution - including ESO NTT/SofI, VLT/VISIR and Spitzer data - with a phenomenological model for sgB[e] stars, we show that the star is surrounded by an irradiated rim heated to a temperature of 3800-5500 K, along with a viscous disk component at an inner temperature of 750 K. VISIR data allow us to exclude the spherical geometry for the dust component. This detailed study will allow us in the future to get better constraints on the formation and evolution of such rare and short-living high mass X-ray binary systems in our Galaxy., Comment: To appear in ApJ, 2012 June 1, vol. 751-2; 19 pages, 8 figures, 4 tables

We report on Chandra X-ray and Parkes radio observations of IGR J11014-6103, which is a possible pulsar wind nebula with a complex X-ray morphology and a likely radio counterpart. With the superb angular resolution of Chandra, we find evidence that a portion of the extended emission may be related to a bow shock due to the putative pulsar moving through the interstellar medium. The inferred direction of motion is consistent with IGR J11014-6103 having been born in the event that produced the supernova remnant (SNR) MSH 11-61A. If this association is correct, then previous constraints on the expansion of MSH 11-61A imply a transverse velocity for IGR J11014-6103 of 2,400-2,900 km/s, depending on the SNR model used. This would surpass the kick velocities of any known pulsars and rival or surpass the velocities of any compact objects that are associated with SNRs. While it is important to confirm the nature of the source, our radio pulsation search did not yield a detection., Comment: Accepted for ApJ Letters, 5 pages, 5 figures

The Chandra High Resolution Camera observed the fields of five hard X-ray sources in order to help us obtain X-ray coordinates with sub-arcsecond precision. These observations provide the most accurate X-ray positions known for IGR J16393-4643 and for IGR J17091-3624. The obscured X-ray pulsar IGR J16393-4643 lies at R.A. (J2000) = 16:39:05.47, and Dec. = -46:42:13.0 (error radius of 0.6" at 90% confidence). This position is incompatible with the previously-proposed counterpart 2MASS J16390535-4642137, and it points instead to a new counterpart candidate that is possibly blended with the 2MASS star. The black hole candidate IGR J17091-3624 was observed during its 2011 outburst providing coordinates of R.A. = 17:09:07.59, and Dec. = -36:24:25.4. This position is compatible with those of the proposed optical/IR and radio counterparts, solidifying the source's status as a microquasar. Three targets, IGR J14043-6148, IGR J16358-4726, and IGR J17597-2201, were not detected. We obtained 3{\sigma} upper limits of, respectively, 1.7, 1.8, and 1.5 \times 10^-12 erg cm^-2 s^-1 on their 2-10 keV fluxes., Comment: 10 pages, 3 figures, accepted for publication in ApJ

We report on a Spitzer/IRS (mid-infrared), RXTE/PCA+HEXTE (X-ray), and Ryle (radio) simultaneous multiwavelength study of the microquasar Cygnus X-1, which aimed at an investigation of the origin of its mid-infrared emission. Compact jets were present in two out of three observations, and we show that they strongly contribute to the mid-infrared continuum. During the first observation, we detect the spectral break - where the transition from the optically thick to the optically thin regime takes place - at about 2.9e13 Hz. We then show that the jet's optically thin synchrotron emission accounts for the Cygnus X-1's emission beyond 400 keV, although it cannot alone explain its 3-200 keV continuum. A compact jet was also present during the second observation, but we do not detect the break, since it has likely shifted to higher frequencies. In contrast, the compact jet was absent during the last observation, and we show that the 5-30 micron mid-infrared continuum of Cygnus X-1 stems from the blue supergiant companion star HD 226868. Indeed, the emission can then be understood as the combination of the photospheric Raleigh-Jeans tail and the bremsstrahlung from the expanding stellar wind. Moreover, the stellar wind is found to be clumpy, with a filling factor f {\infty}~0.09-0.10. Its bremsstrahlung emission is likely anti-correlated to the soft X-ray emission, suggesting an anti-correlation between the mass-loss and mass-accretion rates. Nevertheless, we do not detect any mid-infared spectroscopic evidence of interaction between the jets and the Cygnus X-1's environment and/or companion star's stellar wind., Comment: 46 pages, 10 figures, 8 tables. Accepted for publication in ApJ

GRS1915+105 is a very peculiar black hole binary that exhibits accretion-related states that are not observed in any other stellar-mass black hole system. One of these states, however -- referred to as the plateau state -- may be related to the canonical hard state of black hole X-ray binaries. Both the plateau and hard state are associated with steady, relatively lower X-ray emission and flat/inverted radio emission, that is sometimes resolved into compact, self-absorbed jets. However, while generally black hole binaries quench their jets when the luminosity becomes too high, GRS1915+105 seems to sustain them despite the fact that it accretes at near- or super-Eddington rates. In order to investigate the relationship between the plateau and the hard state, we fit two multi-wavelength observations using a steady-state outflow-dominated model, developed for hard state black hole binaries. The data sets consist of quasi-simultaneous observations in radio, near-infrared and X-ray bands. Interestingly, we find both significant differences between the two plateau states, as well as between the best-fit model parameters and those representative of the hard state. We discuss our interpretation of these results, and the possible implications for GRS 1915+105's relationship to canonical black hole candidates., Comment: accepted for publication in MNRAS

Context: The INTEGRAL hard X-ray observatory has revealed an emerging population of highly obscured X-ray binary systems through multi-wavelength observations. Previous studies have shown that many of these sources are high-mass X-ray binaries hosting neutron stars orbiting around luminous and evolved companion stars. Aims: To better understand this newly-discovered population, we have selected a sample of sources for which an accurate localisation is available to identify the stellar counterpart and reveal the nature of the companion star and of the binary system. Methods: We performed an intensive study of a sample of thirteen INTEGRAL sources, through multi-wavelength optical to NIR photometric and spectroscopic observations, using EMMI and SofI instruments at the ESO NTT telescope. We performed accurate astrometry and identified candidate counterparts for which we give the optical and NIR magnitudes. We detected many spectral lines allowing us to determine the spectral type of the companion star. We fitted with stellar black bodies the mid-infrared to optical spectral energy distributions of these sources. From the spectral analysis and SED fitting we identified the nature of the companion stars and of the binary systems. (abridged)., Comment: A&A in press; The official date of acceptance is 15/12/2007; 25 pages, 6 figures, 8 tables. New version with language editing required by editor

Context: The INTEGRAL mission has led to the discovery of a new type of supergiant X-ray binaries (SGXBs), whose physical properties differ from those of previously known SGXBs. Those sources are in the course of being unveiled by means of multi-wavelength X-rays, optical, near- and mid-infrared observations, and two classes are appearing. The first class consists of obscured persistent SGXBs and the second is populated by the so-called supergiant fast X-ray transients (SFXTs). Aims: We report here mid-infrared (MIR) observations of the companion stars of twelve SGXBs from these two classes in order to assess the contribution of the star and the material enshrouding the system to the total emission.} Methods: We used data from observations we carried out at ESO/VLT with VISIR, as well as archival and published data, to perform broad-band spectral energy distributions of the companion stars and fitted them with a combination of two black bodies representing the star and a MIR excess due to the absorbing material enshrouding the star, if there was any. Results: We detect a MIR excess in the emission of IGR J16318-4848, IGR J16358-4726, and perhaps IGR J16195-4945. The other sources do not exhibit any MIR excess even when the intrinsic absorption is very high. (abridged), Comment: A&A in press, The official date of acceptance is 25/01/2008, 17 pages, 4 figures, 9 tables. New version with english language editing required by editor, note added in proof

A new type of sources has been discovered by INTEGRAL. These sources are in the course of being unveiled by means of multi-wavelength optical, near- and mid-infrared observations. Among the high-energy binary sources, two distinct classes are appearing. The first class is constituted of intrinsically obscured high-energy sources, of which IGR J16318-4848 seems to be the archetype. The second class is populated by the so-called supergiant fast X-ray transients, with IGR J17544-2619 being the archetype. We report here on multi-wavelength observations of sources from these two classes, focusing on optical to mid-infrared observations. We show that in the case of the obscured sources IGR J16318-4848 and IGR J16195-4945, our observations suggest the presence of absorbing material (dust and/or cold gas) enshrouding the whole binary system. We then discuss the nature of these two different types of sources., Comment: Proceedings of The 6th INTEGRAL Workshop, The Obscured Universe, Space Research Institute, Moscow, Russia, July 2-8, 2006, to be published by ESA's Publications Division in December 2006 as Special Publication SP-622, 8 pages, 7 figures

We present a catalog of hard X-ray sources in a square-degree region surveyed by the Nuclear Spectroscopic Telescope Array (NuSTAR) in the direction of the Norma spiral arm. This survey has a total exposure time of 1.7 Ms, and the typical and maximum exposure depths are 50 ks and 1 Ms, respectively. In the area of deepest coverage, sensitivity limits of 5 x 10(exp -14) and 4 x 10(exp -14) ergs/s/sq cm in the 3-10 and 10-20 keV bands, respectively, are reached. Twenty-eight sources are firmly detected, and 10 are detected with low significance; 8 of the 38 sources are expected to be active galactic nuclei. The three brightest sources were previously identified as a low-mass X-ray binary, high-mass X-ray binary, and pulsar wind nebula. Based on their X-ray properties and multiwavelength counterparts, we identify the likely nature of the other sources as two colliding wind binaries, three pulsar wind nebulae, a black hole binary, and a plurality of cataclysmic variables (CVs). The CV candidates in the Norma region have plasma temperatures of approx. 10-20 keV, consistent with the Galactic ridge X-ray emission spectrum but lower than the temperatures of CVs near the Galactic center. This temperature difference may indicate that the Norma region has a lower fraction of intermediate polars relative to other types of CVs compared to the Galactic center. The NuSTAR logN-logS distribution in the 10-20keV band is consistent with the distribution measured by Chandra at 2-10 keV if the average source spectrum is assumed to be a thermal model with kT approx. =15 keV, as observed for the CV candidates.