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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 1308.8 ± 0.4 s giving a period derivative $\dot{P}\sim 2\times {10}^{-8}$ 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 yr. 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 α line, as well as negative residuals around 14 keV from a candidate cyclotron line detected at a significance of 5 σ . 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 days thanks to Swift/BAT monitoring data from 2005–2022.

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 1308.8 ± 0.4 s giving a period derivative P ̇ ∼ 2 × 10 − 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 yr. 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 α line, as well as negative residuals around 14 keV from a candidate cyclotron line detected at a significance of 5 σ. 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 days thanks to Swift/BAT monitoring data from 2005–2022. [ABSTRACT FROM AUTHOR]

We present the two-point cross-correlation function between high-mass X-ray binaries (HMXBs) in the Small Magellanic Cloud (SMC) and their likely birthplaces (OB associations: OBAs). This function compares the spatial correlation between the observed HMXB and OBA populations against mock catalogs in which the members are distributed randomly across the sky. A significant correlation (∼15 σ) is found for the HMXB and OBA populations when compared with a randomized catalog in which the OBAs are distributed uniformly over the SMC. A less significant correlation (4 σ) is found for a randomized catalog of OBAs built with a bootstrap method. However, no significant correlation is detected when the randomized catalogs assume the form of a Gaussian ellipsoid or a distribution that reflects the star formation history from 40 Myr ago. Based on their observed distributions and assuming a range of migration timescales, we infer that the average value of the kick velocity inherited by an HMXB during the formation of its compact object is 2â€"34 km sâˆ'1. This is considerably less than the value obtained for their counterparts in the Milky Way hinting that the galactic environment affecting stellar evolution plays a role in setting the average kick velocity of HMXBs. [ABSTRACT FROM AUTHOR]

We report on Chandra, NuSTAR, and MDM observations of two International Gamma-ray Astrophysics Laboratory (INTEGRAL) sources, namely IGR J17528−2022 and IGR J20063+3641. IGR J17528−2022 is an unidentified INTEGRAL source, while IGR J20063+3641 was recently identified as a magnetic cataclysmic variable (mCV) by Halpern et al. The Chandra observation of IGR J17528−2022 has allowed us to locate the optical counterpart to the source and to obtain its optical spectrum, which shows a strong Hα emission line. The optical spectrum and flickering observed in the optical time-series photometry in combination with the X-ray spectrum, which is well fit by an absorbed partially covered thermal bremsstrahlung model, suggest that this source is a strong mCV candidate. The X-ray observations of IGR J20063+3641 reveal a clear modulation with a period of 172.46 ± 0.01 s, which we attribute to the white dwarf spin period. Additional MDM spectroscopy of the source has also allowed for a clear determination of the orbital period at 0.731 ± 0.015 days. The X-ray spectrum of this source is also well fit by an absorbed partially covered thermal bremsstrahlung model. The X-ray spectrum, spin periodicity, and orbital periodicity allow this source to be further classified as an intermediate polar. [ABSTRACT FROM AUTHOR]

Here, we report on X-ray observations of ten 17–60 keV sources discovered by the International Gamma-Ray Astrophysics Laboratory satellite. The primary new information is sub-arcsecond positions obtained by the Chandra X-ray Observatory. In six cases (IGR J17040-4305, IGR J18017-3542, IGR J18112-2641, IGR J18434-0508, IGR J19504+3318, and IGR J20084+3221), a unique Chandra counterpart is identified with a high degree of certainty, and for five of these sources (all but J19504), Gaia distances or proper motions indicate that they are Galactic sources. For four of these, the most likely classifications are that the sources are magnetic cataclysmic variables (CVs). J20084 could be either a magnetic CV or a high-mass X-ray binary. We classify the sixth source (J19504) as a likely active galactic nucleus (AGN). In addition, we find likely Chandra counterparts to IGR J18010-3045 and IGR J19577+3339, and the latter is a bright radio source and probable AGN. The other two sources, IGR J12529-6351 and IGR J18013-3222, do not have likely Chandra counterparts, indicating that they are transient, highly variable, or highly absorbed. [ABSTRACT FROM AUTHOR]

The Arches cluster is a young, densely packed massive star cluster in our Galaxy that shows a high level of star formation activity. The nature of the extended non-thermal X-ray emission around the cluster remains unclear. The observed bright Fe Kα line emission at 6.4 keV from material that is neutral or in a low ionization state can be produced either by X-ray photoionization or by cosmic-ray particle bombardment or both. In this paper, we report on the first detection of the extended emission around the Arches cluster above 10 keV with the NuSTAR mission, and present results on its morphology and spectrum. The spatial distribution of the hard X-ray emission is found to be consistent with the broad region around the cluster where the 6.4 keV line is observed. The interpretation of the hard X-ray emission within the context of the X-ray reflection model puts a strong constraint on the luminosity of the possible illuminating hard X-ray source. The properties of the observed emission are also in broad agreement with the low-energy cosmic-ray proton excitation scenario. [ABSTRACT FROM AUTHOR]

Detecting gamma-rays from microquasars is a challenging but worthwhile endeavor for understanding particle acceleration and the jet mechanism and for constraining leptonic/hadronic emission models. We present results from a likelihood analysis on timescales of 1 day and 10 days of ∼4 yr worth of gamma-ray observations (0.1-10 GeV) by Fermi-LAT of Cyg X-1, Cyg X-3, GRS 1915+105, and GX 339–4. Our analysis reproduced all but one of the previous gamma-ray outbursts of Cyg X-3 as reported with Fermi or AGILE, plus five new days on which Cyg X-3 is detected at a significance of ∼5σ that are not reported in the literature. In addition, Cyg X-3 is significantly detected on 10 day timescales outside of known gamma-ray flaring epochs, which suggests that persistent gamma-ray emission from Cyg X-3 has been detected for the first time. For Cyg X-1 we find three low-significance excesses (∼3-4σ) on daily timescales that are contemporaneous with gamma-ray flares reported (also at low significance) by AGILE. Two other microquasars, GRS 1915+105 and GX 339–4, are not detected, and we derive 3σ upper limits of 2.3 × 10–8 photons cm–2 s–1 and 1.6 × 10–8 photons cm–2 s–1, respectively, on the persistent flux in the 0.1-10 GeV range. These results enable us to define a list of the general conditions that are necessary for the detection of gamma-rays from microquasars. [ABSTRACT FROM AUTHOR]

We have performed the first sensitive X-ray observation of the low-mass X-ray binary (LMXB) SAX J1750.8 -2900 in quiescence with XMM-Newton. The spectrum was fit to both a classical blackbody model, and a non-magnetized, pure hydrogen neutron star (NS) atmosphere model. A power-law component was added to these models, but we found that it was not required by the fits. The distance to SAX J1750.8-2900 is known to be D = 6.79 kpc from a previous analysis of photospheric radius expansion bursts. This distance implies a bolometric luminosity (as given by the NS atmosphere model) of (1.05 ±0.12) x 1034 (D/6.79 kpc)² erg s-1, which is the highest known luminosity for a NS LMXB in quiescence. One simple explanation for this surprising result could be that the crust and core of the NS were not in thermal equilibrium during the observation. We argue that this was likely not the case, and that the core temperature of the NS in SAX J1750.8-2900 is unusually high. [ABSTRACT FROM AUTHOR]

We present the first direct measurement of the spatial cross-correlation function of high-mass X-ray binaries (HMXBs) and active OB star-forming complexes in the Milky Way. This result relied on a sample containing 79 hard X-ray-selected HMXBs and 458 OB associations. Clustering between the two populations is detected with significance above 7σ for distances < 1 kpc. Thus, HMXBs closely trace the underlying distribution of the massive star-forming regions that are expected to produce the progenitor stars of HMXBs. The average offset of 0.4 ± 0.2 kpc between HMXBs and OB associations is consistent with being due to natal kicks at velocities of the order of 100 ± 50 km s-1. The characteristic scale of the correlation function suggests an average kinematical age (since the supernova phase) of ~4 Myr for the HMXB population. Despite being derived from a global view of our Galaxy, these signatures of HMXB evolution are consistent with theoretical expectations as well as observations of individual objects. [ABSTRACT FROM AUTHOR]

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 8σ 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 between ~0.5 and 1.5 days. Near-infrared 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-χ2 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 $N_{\rm H} = (2.8^{+2.3}_{-1.4})\times 10^{23}$ cm–2, $(9^{+15}_{-7})\times 10^{22}$ cm–2, and $(1.7^{+1.7}_{-0.9})\times 10^{23}$ 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 >1034 erg s–1. 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., Astronomy

We present a source catalog from the first deep hard X-ray (E > 10 keV) survey of the Small Magellanic Cloud (SMC), the Nuclear Spectroscopic Telescope Array (NuSTAR) Legacy Survey of the SMC. We observed three fields, for a total exposure time of 1 Ms, along the bar of this nearby star-forming galaxy. Fields were chosen for their young stellar and accreting binary populations. We detected 10 sources above a 3σ significance level (4–25 keV) and obtained upper limits on an additional 40 sources. We reached a 3σ limiting luminosity in the 4–25 keV band of ∼1035 erg s−1, allowing us to probe fainter X-ray binary (XRB) populations than has been possible with other extragalactic NuSTAR surveys. We used hard X-ray colors and luminosities to constrain the compact-object type, exploiting the spectral differences between accreting black holes and neutron stars at E > 10 keV. Several of our sources demonstrate variability consistent with previously observed behavior. We confirmed pulsations for seven pulsars in our 3σ sample. We present the first detection of pulsations from a Be-XRB, SXP 305 (CXO J005215.4–73191), with an X-ray pulse period of 305.69 ± 0.16 s and a likely orbital period of ∼1160–1180 days. Bright sources (≳5 × 1036 erg s−1) in our sample have compact-object classifications consistent with their previously reported types in the literature. Lower-luminosity sources (≲5 × 1036 erg s−1) have X-ray colors and luminosities consistent with multiple classifications. We raise questions about possible spectral differences at low luminosity between SMC pulsars and the Galactic pulsars used to create the diagnostic diagrams. [ABSTRACT FROM AUTHOR]

We present 15 high-mass X-ray binary (HMXB) candidates in the disk of M31 for which we are able to infer compact object type, spectral type of the donor star, and age using multiwavelength observations from NuSTAR, Chandra, and the Hubble Space Telescope. The hard X-ray colors and luminosities from NuSTAR permit the tentative classification of accreting X-ray binary systems by compact object type, distinguishing black hole from neutron star systems. We find hard-state black holes, pulsars, and non-magnetized neutron stars associated with optical point-source counterparts with similar frequency. We also find nine non-magnetized neutron stars coincident with globular clusters and an equal number of pulsars with and without point-source optical counterparts. We perform spectral energy distribution (SED) fitting for the most likely optical counterparts to the HMXB candidates, finding seven likely high-mass stars and one possible red helium-burning star. The remaining seven HMXB optical counterparts have poor SED fits, so their companion stars remain unclassified. Using published star formation histories, we find that the majority of HMXB candidates—X-ray sources with UV-bright point-source optical counterpart candidates—are found in regions with star formation bursts less than 50 Myr ago, and three are associated with young stellar ages (<10 Myr). This is consistent with similar studies of HMXB populations in the Magellanic Clouds, M33, NGC 300, and NGC 2403. [ABSTRACT FROM AUTHOR]

We present the first survey results of hard X-ray point sources in the Galactic Center (GC) region by NuSTAR. We have discovered 70 hard (3–79 keV) X-ray point sources in a 0.6 deg region around Sgr A* with a total exposure of 1.7 Ms, and 7 sources in the Sgr B2 field with 300 ks. We identify clear Chandra counterparts for 58 NuSTAR sources and assign candidate counterparts for the remaining 19. The NuSTAR survey reaches X-ray luminosities of ∼4× and ∼8 × 10 erg s at the GC (8 kpc) in the 3–10 and 10–40 keV bands, respectively. The source list includes three persistent luminous X-ray binaries (XBs) and the likely run-away pulsar called the Cannonball. New source-detection significance maps reveal a cluster of hard (>10 keV) X-ray sources near the Sgr A diffuse complex with no clear soft X-ray counterparts. The severe extinction observed in the Chandra spectra indicates that all the NuSTAR sources are in the central bulge or are of extragalactic origin. Spectral analysis of relatively bright NuSTAR sources suggests that magnetic cataclysmic variables constitute a large fraction (>40%–60%). Both spectral analysis and logN–logS distributions of the NuSTAR sources indicate that the X-ray spectra of the NuSTAR sources should have kT > 20 keV on average for a single temperature thermal plasma model or an average photon index of Γ = 1.5–2 for a power-law model. These findings suggest that the GC X-ray source population may contain a larger fraction of XBs with high plasma temperatures than the field population. [ABSTRACT FROM AUTHOR]

The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was observed by the Nuclear Spectroscope Telescope Array 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 keV. This allowed us to measure the magnetic field strength of the neutron star for the first time: B = (2.5 ± 0.1) × 1012 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 s s−1 (−0.6 s per year, or a frequency derivative of Hz s−1). 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 2 × 108 cm. [ABSTRACT FROM AUTHOR]

We report on 0.3–10 keV observations with the Chandra X-ray Observatory of eight hard X-ray sources discovered within 8° of the Galactic plane by the International Gamma-ray Astrophysics Laboratory 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 Γ = 0.6 ± 0.4 and –, 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 , and it is moderately absorbed, NH = (4 ± 1) × 1022 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. 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. [ABSTRACT FROM AUTHOR]

Polarization of the keV hard tail of the microquasar Cygnus X-1 has been independently reported by INTEGRAL/Imager on Board the INTEGRAL Satellite (IBIS), and INTEGRAL/SPectrometer on INTEGRAL and interpreted as emission from a compact jet. These conclusions were, however, based on the accumulation of all INTEGRAL data regardless of the spectral state. We utilize additional INTEGRAL exposure accumulated until 2012 December, and include the AMI/Ryle (15 GHz) radio data in our study. We separate the observations into hard, soft, and intermediate/transitional states and detect radio emission from a compact jet in hard and intermediate states (IS), but not in the soft. The 10–400 keV INTEGRAL (JEM-X and IBIS) state resolved spectra are well modeled with thermal Comptonization and reflection components. We detect a hard tail in the 0.4–2 MeV range for the hard state only. We extract the state dependent polarigrams of Cyg X-1, which are all compatible with no or an undetectable level of polarization except in the 400–2000 keV range in the hard state where the polarization fraction is 75% ± 32% and the polarization angle 40.°0 ± 14.°3. An upper limit on the 0.4–2 MeV soft state polarization fraction is 70%. Due to the short exposure, we obtain no meaningful constraint for the IS. The likely detection of a keV polarized tail in the hard state, together with the simultaneous presence of a radio jet, reinforce the notion of a compact jet origin of the keV emission. [ABSTRACT FROM AUTHOR]

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) × 10–14 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 = cm–2 and Γ = 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) × 103M☼ pc–3. [ABSTRACT FROM AUTHOR]

We present a catalog of 1415 X-ray sources identified in the Norma Arm Region Chandra Survey (NARCS), which covers a 2° × 0.°8 region in the direction of the Norma spiral arm to a depth of ≈20 ks. Of these sources, 1130 are point-like sources detected with ⩾3σ 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 ∼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 cataclysmic variables dominates the Scutum-Crux and near Norma arms, while intermediate polars 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 fX ≈ 10–14 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. [ABSTRACT FROM AUTHOR]

Results are presented for an initial survey of the Norma Arm gathered with the focusing hard X-Ray Telescope NuSTAR. The survey covers 0.2 deg2 of sky area in the 3-79 keV range with a minimum and maximum raw depth of 15 ks and 135 ks, respectively. Besides a bright black-hole X-ray binary in outburst (4U 1630–47) and a new X-ray transient (NuSTAR J163433–473841), NuSTAR locates three sources from the Chandra survey of this region whose spectra are extended above 10 keV for the first time: CXOU J163329.5–473332, CXOU J163350.9–474638, and CXOU J163355.1–473804. Imaging, timing, and spectral data from a broad X-ray range (0.3-79 keV) are analyzed and interpreted with the aim of classifying these objects. CXOU J163329.5–473332 is either a cataclysmic variable or a faint low-mass X-ray binary. CXOU J163350.9–474638 varies in intensity on year-long timescales, and with no multi-wavelength counterpart, it could be a distant X-ray binary or possibly a magnetar. CXOU J163355.1–473804 features a helium-like iron line at 6.7 keV and is classified as a nearby cataclysmic variable. Additional surveys are planned for the Norma Arm and Galactic Center, and those NuSTAR observations will benefit from the lessons learned during this pilot study. [ABSTRACT FROM AUTHOR]

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 (TÜBİTAK 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 Hα emission lines, one is a late-type star, and the other is an 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. [ABSTRACT FROM AUTHOR]

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 × 10–13 erg cm–2 s–1 with a photon index of 2.9 ± 0.8 (NH = 1.0 × 1023 cm–2). This flux is approximately consistent with that measured at the same time after the source's outburst in 1998. With measurements spanning three 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 ± 0.1 and 59 ± 6 days, and a thermal cooling model fit suggests that SGR 1627–41 may have a hot core (Tc ∼ 2 × 108 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 ≳ 1013 G) in relation to their spin-inferred surface magnetic field strength and find a possible trend between the two quantities. [ABSTRACT FROM AUTHOR]

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 subarcsecond 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° of the plane, four of the IGR sources are active galactic nuclei (AGNs; IGR J01545+6437, IGR J15391-5307, IGR J15415-5029, and IGR J21565+5948) and four others are likely AGNs (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 AGNs selected by their 2-10 keV emission in previous studies and find that these IGR AGNs 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 (1.4+1.0– 0.4) × 1032 erg s–1, 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 R☼. 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. [ABSTRACT FROM AUTHOR]

Results are presented for XMM-Newton observations of five hard X-ray sources discovered by INTEGRAL in the direction of the Scutum Arm. Each source received ≳20 ks of effective exposure time. We provide refined X-ray positions for all five targets enabling us to pinpoint the most likely counterpart in optical/infrared archives. Spectral and timing information (much of which is provided for the first time) allow us to give a firm classification for IGR J18462–0223 and to offer tentative classifications for the others. For IGR J18462–0223, we discovered a coherent pulsation period of 997 ± 1 s, which we attribute to the spin of a neutron star in a highly obscured (NH =2 × 1023 cm–2) high-mass X-ray binary (HMXB). This makes IGR J18462–0223 the seventh supergiant fast X-ray transient candidate with a confirmed pulsation period. IGR J18457+0244 is a highly absorbed (NH =8 × 1023 cm–2) source in which the possible detection of an iron line suggests an active galactic nucleus (AGN) of type Sey-2 situated at z = 0.07(1). A periodic signal at 4.4 ks could be a quasi-periodic oscillation which would make IGR J18457+0244 one of a handful of AGNs in which such features have been claimed, but a slowly rotating neutron star in an HMXB cannot be ruled out. IGR J18482+0049 represents a new obscured HMXB candidate with NH =4 × 1023 cm–2. We tentatively propose that IGR J18532+0416 is either an AGN or a pulsar in an HMXB system. The X-ray spectral properties of IGR J18538–0102 are consistent with the AGN classification that has been proposed for this source. [ABSTRACT FROM AUTHOR]

The Chandra High Resolution Camera observed the fields of five hard X-ray sources in order to help us obtain X-ray coordinates with subarcsecond precision. These observations provide the most accurate X-ray positions known for IGR J16393–4643 and IGR J17091–3624. The obscured X-ray pulsar IGR J16393–4643 lies at R.A. (J2000) = 16h39m05.ˢ47, and decl. = –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 Two Micron All Sky Survey star. The black hole candidate IGR J17091–3624 was observed during its 2011 outburst providing coordinates of R.A. = 17h09m07.ˢ59, and decl. = –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 3σ upper limits of, respectively, 1.7, 1.8, and 1.5 × 10–12 erg cm–2 s–1 on their 2-10 keV fluxes. [ABSTRACT FROM AUTHOR]