IGR J17511-3057 is a low-mass X-ray binary hosting a neutron star and is one of the few accreting millisecond X-ray pulsars with X-ray bursts. We report on a 20 ks Chandra grating observation of IGR ...J17511-3057, performed on 2009 September 22. We determine the most accurate X-ray position of IGR J17511-3057, alpha sub(J2000) = 17 super(h)51 super(m)08. super(s)66, delta sub(J2000) = -30degrees57'41."0 (90% uncertainty of 0."6). During the observation, a ~54 s long type-I X-ray burst is detected. The persistent (non-burst) emission has an absorbed 0.5-8 keV luminosity of 1.7 x 10 super(36) erg s super(-1) (at 6.9 kpc) and can be well described by a thermal Comptonization model of soft, ~0.6 keV, seed photons upscattered by a hot corona. The type-I X-ray burst spectrum, with average luminosity over the 54 s duration L sub(0.5-8 keV) = 1.6 x 10 super(37) erg s super(-1), can be well described by a blackbody with kT sub(bb) ~ 1.6 keV and R sub(bb) ~ 5 km. While an evolution in temperature of the blackbody can be appreciated throughout the burst (average peak kT sub(bb) = 2.5 super(+0.8) sub(- 0.4) keV to tail kT sub(bb) = 1.3 super(+0.2) sub(- 0.1) keV), the relative emitting surface shows no evolution. The overall persistent and type-I burst properties observed during the Chandra observation are consistent with what was previously reported during the 2009 outburst of IGR J17511-3057.
IGR J18179-1621 is an obscured accreting X-ray pulsar discovered by INTEGRAL on 2012 February 29. We report on our 20 ks Chandra-High Energy Transmission Gratings Spectrometer observation of the ...source performed on 2012 March 17, on two short contemporaneous Swift observations, and on our two near-infrared (K sub(s), H sub(h), and J sub(n)) observations performed on 2012 March 13 and 26. We determine the most accurate X-ray position of IGR J18179-1621, alpha sub(J2000) = 18 super(h) 17 super(m) 52 super(s).18, delta sub(J2000) = -16degrees21'31"./68 (90% uncertainty of 0".6). A strong periodic variability at 11.82 s is clearly detected in the Chandra data, confirming the pulsating nature of the source, with the light-curve softening at the pulse peak. In many ways IGR J18179-1621 is unusual: its combination of a several week long outburst (without evidence of repeated outbursts in the historical record), high absorption column (a large fraction of which is likely local to the system), and 11.82 s period does not fit neatly into existing X-ray binary categories.
The X-ray transient source XTE J1818−245 went through an outburst in 2005 that was observed during a multiwavelength campaign from radio to soft γ-rays. Observations in V and R optical bands with the ...1-m Swope telescope allowed the discovery of a new bright source. As we aimed to reveal the nature of the companion star, we performed new optical observations with the ESO/NTT telescope at La Silla, both in photometry and spectroscopy. We confirm the optical counterpart found by the Swope telescope, but the spectral type of the secondary star could not be identified. The spectrum showed a blue-dominated shape and an Hα emission line was detected, indicating that the optical emission was dominated by the presence of an accretion disc. The broad-band spectral energy distribution revealed that the outer parts of the accretion disc had to be irradiated by its inner parts to explain the optical emission. New observations of XTE J1818−245 in quiescence are needed to find the nature of the companion star. Moreover, radio-to-X-ray strictly simultaneous observations of transient black holes are needed to disentangle the importance of jets and irradiated accretion discs in the infrared-optical-ultraviolet domain.
We present an analysis of a 37 ks observation of the supergiant fast X-ray transient IGR J17391--3021 (= XTE J1739--302) gathered with Suzaku. The source evolved from quiescence to a low-activity ...level culminating in three weak flares lasting ~3 ks each in which the peak luminosity is only a factor of five times that of the pre-flare luminosity. The minimum observed luminosity was 1.3 X 1033 erg s--1(d/2.7 kpc)2 in the 0.5-10 keV range. The weak flares are accompanied by significant changes in the spectral parameters including a column density (N H =(4.1+0.4 --0.5) X 1022 cm--2) that is ~2-9 times the absorption measured during quiescence. Accretion of obscuring clumps of stellar wind material can explain both the small flares and the increase in N H. Placing this observation in the context of the recent Swift monitoring campaign, we find that weak-flaring episodes, or at least epochs of enhanced activity just above the quiescent level but well below the moderately bright or high-luminosity outbursts, represent more than 60% ? 5% of all observations in the 0.5-10 keV energy range making this the most common state in the emission behavior of IGR J17391--3021.
Context. AX J1749.1-2733 is an unclassified transient X-ray source discovered during surveys by ASCA in 1993–1999. The transient behaviour and the short and bright flares of the source have led to ...the idea that it is part of the recently revealed subclass of supergiant fast X-ray transients. Aims. A multi-wavelength study in NIR, optical, X-rays, and hard X-rays of AX J1749.1-2733 is undertaken in order to determine its nature. Methods. Public INTEGRAL data and our target of opportunity observation with XMM-Newton were used to study the high-energy source through timing and spectral analysis. Multi-wavelength observations in optical and NIR with the ESO/NTT telescope were also performed to search for the counterpart. Results. AX J1749.1-2733 is a new high-mass X-ray binary pulsar with an orbital period of $185.5\pm1.1$ days (or $185.5/f$ with $f=2,3\ \mathrm{or}\ 4$) and a spin period of ~66 s, parameters typical of a Be/X-ray binary. The outbursts last ~12 d. A spin-down of $\dot{P}=0.08~\pm\,\,0.02$ s yr -1 is also observed, very likely due to the propeller effect. The most accurate X-ray position is RA (2000) $=17^{\mathrm{h}}49^{\mathrm{m}}06.8^{\mathrm{s}}$ and Dec $=-27\degr32\arcmin32.5\arcsec\ (\mathrm{uncertainty}\ 2\arcsec)$. The high-energy broad-band spectrum is well-fitted with an absorbed powerlaw and a high-energy cutoff with values $N_{\mathrm{H}}=20.1_{-1.3}^{+1.5}\times10^{22}$ cm-2, $\Gamma=1.0_{-0.3}^{+0.1}$, and $E_{\mathrm{cut}}=21_{-3}^{+5}$ keV. The only optical/NIR candidate counterpart within the X-ray error circle has magnitudes of $R=21.9\pm0.1$, $I=20.92\pm0.09$, $J=17.42\pm0.03$, $H=16.71\pm0.02$, and $K_{\mathrm{s}}=15.75\pm0.07$, which points towards a Be star located far away (>8.5 kpc) and highly absorbed ($N_{\mathrm{H}}\sim 1.7\times10^{22}$ cm-2). The average 22–50 keV luminosity is 0.4–$0.9\times10^{36}\ \mathrm{erg}\,{\rm s}^{-1}$ during the long outbursts and $3\times10^{36}\ \mathrm{erg}\,{\rm s}^{-1}$ during the bright flare that occurred on MJD 52891 for an assumed distance of 8.5 kpc.
We report multiwavelength observations of the soft X-ray transient XTE J1118+480, which we observed with UKIRT, Hubble Space Telescope (HST), RXTE, Extreme Ultraviolet Explorer (EUVE) and many other ...instruments and facilities. Adding radio (Ryle Telescope, VLA), submillimetre (JCMT) and X-ray (Chandra and SAX) data from the literature, we assembled the most complete spectral energy distribution (SED) of this source yet published. We followed the evolution of this source for 1 yr, including six observations performed during the outburst, and one observation at the end of the outburst. Because of the unusually high galactic latitude of XTE J1118+480, it suffers from very low extinction, and its SED is nearly complete, including extreme ultraviolet observations. XTE J1118+480 exhibits an unusually low low/hard state (estimated inner radius of 350Rs) and a strong non-thermal contribution in the radio to optical domain, which is likely to be due to synchrotron emission. We discuss the interstellar column density and show that it is low, between 0.80 and 1.30 × 1020 cm−2. We analyse the evolution of the SED during the outburst, including the contributions from the companion star, the accretion disc, the outflow, and relating irradiation and variability of the source in different bands to the SED. We find no significant spectral variability during the outburst evolution, consistent with the presence of a steady outflow. An analysis of its outflow to accretion energy ratio suggests that the microquasar XTE J1118+480 is analogous to radio-quiet quasars. This, combined with the inverted spectrum from radio to optical, makes XTE J1118+480 very similar to other microquasar sources, e.g. GRS 1915+105 and GX 339−4 in their low/hard state. We model the high-energy emission with a hot disc model, and discuss different accretion models for the broad-band spectrum of XTE J1118+480.
We present HST, RXTE, and UKIRT observations of the broad-band spectra of the black hole X-ray transient XTE J1859+226 during the decline from its 1999–2000 outburst. Our UV spectra define the 2175-Å ...interstellar absorption feature very well and based on its strength we estimate E(B-V)=0.58±0.12. Hence we deredden our spectra and follow the evolution of the spectral energy distribution on the decline from outburst. We find that the UV and optical data, and the X-ray thermal component when detectable, can be fitted with a simple blackbody model of an accretion disc heated by internal viscosity and X-ray irradiation, and extending to close to the last stable orbit around the black hole, although the actual inner radius cannot be well constrained. During the decline we see the disc apparently evolving from a model with the edge dominated by irradiative heating towards one where viscous heating is dominant everywhere. The outer disc radius also appears to decrease during the decline; we interpret this as evidence of a cooling wave moving inwards and discuss its implications for the disc instability model. Based on the normalization of our spectral fits we estimate a likely distance range of 4.6–8.0 kpc, although a value outside this range cannot securely be ruled out.
Context. While pulsar wind nebulae (PWNe) and their associated isolated pulsars are commonly detected at X-ray energies, they are much rarer at near infrared (nIR) and optical wavelengths. Aims. Here ...we examine three PWN systems in the Galactic plane – IGR J14003−6326, HESS J1632 − 478 and IGR J18490 − 0000 – in a bid to identify optical/nIR emission associated with either the extended PWNe or their previously detected X-ray point sources. Methods. We obtain optical/nIR images of the three fields with the ESO – New Technology Telescope and apply standard photometric and astrometric calibrations. Results. We find no evidence of any extended emission associated with the PWNe in any of the fields; neither do we find any new counterparts to the X-ray point sources, except to confirm the magnitude of the previously identified counterpart candidate of IGR J18490 − 0000. Conclusions. Further observations are required to confirm the association of the nIR source to IGR J18490 − 0000 and to detect counterparts to IGR J14003−6326 and HESS J1632 − 478, while a more accurate X-ray position is required to reduce the probability of a chance superposition in the field of the latter.
An analysis of IGR J16207-5129 is presented based on observations taken with Suzaku. The data set represents {approx}80 ks of effective exposure time in a broad energy range between 0.5 and 60 keV, ...including unprecedented spectral sensitivity above 15 keV. The average source spectrum is well described by an absorbed power law in which we measured a large intrinsic absorption of N {sub H} =(16.2{sup +0.9} {sub -1.1}) x 10{sup 22} cm{sup -2}. This confirms that IGR J16207-5129 belongs to the class of absorbed high-mass X-ray binaries. We were able to constrain the cutoff energy at 19{sup +8} {sub -4} keV, which argues in favor of a neutron star as the primary. Our observation includes an epoch in which the source count rate is compatible with no flux suggesting a possible eclipse. We discuss the nature of this source in light of these and of other recent results.
PDF
