ABSTRACT We report on a 40 ks long, uninterrupted X-ray observation of the candidate supergiant fast X-ray transient (SFXT) IGRJ16418-4532 performed with XMM-Newton on 2011 February 23. This ...high-mass X-ray binary lies in the direction of the Norma arm, at an estimated distance of 13kpc. During the observation, the source showed strong variability exceeding two orders of magnitudes, never observed before from this source. Its X-ray flux varied in the range from 0.1 to 15 counts s-1, with several bright flares of different durations (from a few hundred to a few thousand seconds) and sometimes with a quasi-periodic behaviour. This finding supports the previous suggestion that IGRJ16418-4532 is a member of the SFXT class. In our new observation we measured a pulse period of 1212 ± 6s, thus confirming that this binary contains a slowly rotating neutron star. During the periods of low luminosity the source spectrum is softer and more absorbed than during the flares. A soft excess is present below 2keV in the cumulative flare spectrum, possibly due to ionized wind material at a distance similar to the neutron star accretion radius. The kind of X-ray variability displayed by IGRJ16418-4532, its dynamic range and time-scale, together with the sporadic presence of quasi-periodic flaring, are all suggestive of a transitional accretion regime between pure wind accretion and full Roche lobe overflow. We discuss here for the first time this hypothesis to explain the behaviour of IGRJ16418-4532 and, possibly, of other SFXTs with short orbital periods.
Fast X-ray Transients (FXTs) are believed to be non-recurrent bright X-ray sources lasting less than a day and occuring at serendipitous positions, they can best be detected and discovered by ...instruments having a sufficiently wide field of view and high sensitivity. The IBIS/ISGRI instrument onboard INTEGRAL is particularly suited to detect new or already known fast X-ray transient sources. We report on IBIS/ISGRI detection of newly discovered outbursts of three fast transient sources located at low Galactic latitude: SAX J1818.6-1703; IGR J16479-4514; IGR J17391-302/XTE J1739-302. The reported results confirm and strengthen the very fast transient nature of these sources, given that all their newly detected outbursts have a duration less than ~3 h. Additionally, they provide the first evidence for a possible recurrent fast transient behaviour as all three sources were detected in outburst by ISGRI more than once during the last 2 years.
Abstract
We present the results from INTEGRAL and Swift/XRT observations of the hitherto poorly studied unidentified X-ray transient AX J1949.8+2534, and on archival multiwavelength observations of ...field objects. Bright hard X-ray outbursts have been discovered above 20 keV for the first time, the measured duty cycle and dynamic range are of the order of ∼4 per cent and ≥ 630, respectively. The source was also detected during a low soft X-ray state (∼2 × 10−12 erg cm−2 s−1) thanks to a Swift/XRT followup, which allowed for the first time to perform a soft X-ray spectral analysis as well as significantly improve the source positional uncertainty from arcminute to arcsecond size. From archival near-infrared data, we pinpointed two bright objects as most likely counterparts whose photometric properties are compatible with an early-type spectral nature. This strongly supports a high-mass X-ray binary (HMXB) scenario for AX J1949.8+2534, specifically a Supergiant Fast X-ray Transient (more likely) or alternatively a Be HMXB.
We report on a 40 ks long, uninterrupted X-ray observation of the candidate supergiant fast X-ray transient (SFXT) IGR J16418−4532 performed with XMM-Newton on 2011 February 23. This high-mass X-ray ...binary lies in the direction of the Norma arm, at an estimated distance of 13 kpc. During the observation, the source showed strong variability exceeding two orders of magnitudes, never observed before from this source. Its X-ray flux varied in the range from ∼0.1 to 15 counts s−1, with several bright flares of different durations (from a few hundred to a few thousand seconds) and sometimes with a quasi-periodic behaviour. This finding supports the previous suggestion that IGR J16418−4532 is a member of the SFXT class. In our new observation we measured a pulse period of 1212 ± 6 s, thus confirming that this binary contains a slowly rotating neutron star. During the periods of low luminosity the source spectrum is softer and more absorbed than during the flares. A soft excess is present below 2 keV in the cumulative flare spectrum, possibly due to ionized wind material at a distance similar to the neutron star accretion radius. The kind of X-ray variability displayed by IGR J16418−4532, its dynamic range and time-scale, together with the sporadic presence of quasi-periodic flaring, are all suggestive of a transitional accretion regime between pure wind accretion and full Roche lobe overflow. We discuss here for the first time this hypothesis to explain the behaviour of IGR J16418−4532 and, possibly, of other SFXTs with short orbital periods.
We report on the discovery of two fast X-ray transients (FXTs) from analysis of archival INTEGRAL data. Both are characterized by a remarkable hard X-ray activity above 20 keV, in terms of duration ...(~15 and 30 min, respectively), peak flux (~10 super( -9) erg cm super( -2) s super( -1)) and dynamic range (~2400 and 1360, respectively). Swift/X-ray telescope follow-up observations failed to detect any quiescent or low-level soft X-ray emission from either of the two FXTs, providing an upper limit of the order of a few times 10 super( -12) erg cm super( -2) s super( -1). The main spectral and temporal Imager on Board INTEGRAL Satellite (IBIS)/INTEGRAL Soft Gamma-Ray Imager (ISGRI) characteristics are presented and discussed with the aim of inferring possible hints on their nature.
In this paper we report the second soft gamma-ray source catalog obtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRAL satellite. The scientific data set is based on more than 10 Ms of ...high-quality observations performed during the first 2 years of Core Program and public IBIS/ISGRI observations, and covers ~50% of the whole sky. The main aim of the first survey was to scan systematically, for the first time at energies above 20 keV, the whole Galactic plane to achieve a limiting sensitivity of ~1 mcrab in the central radian. The target of the second year of the INTEGRAL mission lifetime was to expand as much as possible our knowledge of the soft gamma-ray sky, with the same limiting sensitivity, to at least 50% of the whole sky, mainly by including a substantial coverage of extragalactic fields. This catalog comprises more than 200 high-energy sources detected in the energy range 20-100 keV, including new transients not active during the first year of operation, faint persistent objects revealed with longer exposure time, and several Galactic and extragalactic sources in sky regions not observed in the first survey. The mean position error for all the sources detected with significance above 10 ? is ~40", enough to identify most of them with a known X-ray counterpart and to unveil the nature of most of the strongly absorbed ones, even though they are very difficult to detect in X-rays. Based on observations with INTEGRAL, an ESA project with instruments and science data center funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic, and Poland, and with the participation of Russia and the USA.
XMM-Newton observations of the supergiant fast X-ray transient IGR J17544−2619 are reported and placed in the context of an analysis of archival INTEGRAL/IBIS data that provide a refined estimate of ...the orbital period at 4.9272 ± 0.0004 d. A complete outburst history across the INTEGRAL mission is reported. Although the new XMM-Newton observations (each lasting ∼15 ks) targeted the peak flux in the phase-folded hard X-ray light curve of IGR J17544−2619, no bright outbursts were observed, the source spending the majority of the exposure at intermediate luminosities of the order of several 1033 erg s−1 (0.5-10 keV) and displaying only low level flickering activity. For the final portion of the exposure, the luminosity of IGR J17544−2619 dropped to ∼4 × 1032 erg s−1 (0.5-10 keV), comparable with the lowest luminosities ever detected from this source, despite the observations being taken near to periastron. We consider the possible orbital geometry of IGR J17544−2619 and the implications for the nature of the mass transfer and accretion mechanisms for both IGR J17544−2619 and the supergiant fast X-ray transients (SFXTs) population. We conclude that accretion under the 'quasi-spherical accretion' model provides a good description of the behaviour of IGR J17544−2619 and suggests an additional mechanism for generating outbursts based upon the mass accumulation rate in the hot shell (atmosphere) that forms around the neutron star under the quasi-spherical formulation. Hence, we hope to aid in explaining the varied outburst behaviours observed across the SFXT population with a consistent underlying physical model.
Analysis of INTEGRAL/IBIS survey observations has revealed that the rare intermediate polar and asynchronous polar cataclysmic variables (CVs) are consistently found to emit in the 20–100 keV energy ...band, whereas synchronous polars and the common non-magnetic CVs rarely do so. From the correlation of a candidate INTEGRAL/IBIS survey source list with a CV catalogue, 15 CV detections by IBIS have been established including a new INTEGRAL source IGR J06253+7334. The properties of these sources and four additional CV candidates are discussed in the context of their 20–100 keV emission characteristics, and we conclude that the INTEGRAL mission is an important tool in the detection of new magnetic CV systems. Furthermore, analysis of the time-averaged spectra of CVs detected by INTEGRAL indicates that although there is little difference between the spectral slopes of the different subtypes, intermediate polars may be considerably more luminous than polars in the soft gamma-ray regime. We also present the detection of an unusual high-energy burst from V1223 Sgr discovered by the inspection of the IBIS light curve. Additionally, we have compared the IBIS and optical American Association of Variable Star Observers (AAVSO) light curves of SS Cyg and extracted IBIS spectra during single periods of optical outburst and quiescence. We find that the 20–100 keV flux is an order of magnitude greater during optical quiescence. This is in agreement with previous studies which show that the hard X-ray component of SS Cyg is suppressed during high accretion states.
We report on a broad-band X-ray study (0.5–250 keV) of the Supergiant Fast X-ray Transient IGR J18483−0311 using archival INTEGRAL data and a new targeted XMM–Newton observation. Our INTEGRAL ...investigation discovered for the first time an unusually long X-ray activity (3–60 keV) which continuously lasted for at least ∼11 d, i.e. a significant fraction (∼60 per cent) of the entire orbital period, and spanned orbital phases corresponding to both periastron and apastron passages. This prolongated X-ray activity is at odds with the much shorter durations marking outbursts from classical SFXTs especially above 20 keV, as such it represents a departure from their nominal behaviour and it adds a further extreme characteristic to the already extreme SFXT IGR J18483−0311. Our IBIS/ISGRI high energy investigation (100–250 keV) of archival outbursts activity from the source showed that the recently reported hint of a possible hard X-ray tail is not real and it is likely due to noisy background. The new XMM–Newton targeted observation did not detect any sign of strong X-ray outburst activity from the source despite being performed close to its periastron passage, on the contrary IGR J18483−0311 was caught during the common intermediate X-ray state with a low-luminosity value of ∼3 × 1033 erg s−1 (0.5–10 keV). We discuss all the reported results in the framework of both spherically symmetric clumpy wind scenario and quasi-spherical settling accretion model.
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