We present spectral and temporal results from INTEGRAL long-term monitoring of the unidentified X-ray source IGR J17354−3255. We show that it is a weak persistent hard X-ray source spending a major ...fraction of the time in an out-of-outburst state with an average of 18-60 keV X-ray flux of ∼1.1 milliCrab, occasionally interspersed with fast X-ray flares (duration from a few hours to a few days) with a dynamic range as high as ∼200. From archival Swift/X-ray telescope observations, we also show that the dynamic range from non-detection to highest level of measured X-ray activity is >300. Our imager on board the lntegral satellite (IBIS) timing analysis strongly confirms the ∼8.4 d orbital period previously detected with Swift/burst alert telescope (BAT); in addition we show that the shape of the orbital profile is rather smooth and appears to be dominated by low-level X-ray emission rather than by bright outbursts; the measured degree of outburst recurrence is ∼25 per cent. The spectral and temporal characteristics of IGR J17354−3255 are highly indicative of a supergiant high-mass X-ray binary (SGXB) nature. However, our inferred dynamic ranges in both soft and hard X-rays are significantly greater than those of classical SGXB systems, but instead are typical of intermediate supergiant fast X-ray transients (SFXTs). Finally, we note for the first time that the observed fast flaring X-ray behaviour of IGR J17354−3255 is very similar to that detected with AGILE from the spatially associated MeV source AGL J1734−3310, suggesting a possible physical link between the two objects.
Supergiant high-mass X-ray binaries (SGXBs) are believed to be rare objects, as stars in the supergiant phase have a very short lifetime and to date only about a dozen of them have been discovered. ...They are known to be persistent and bright X-ray sources. INTEGRAL is changing this classical picture, as its observations are revealing the presence of a new subclass of SGXBs that have been labeled supergiant fast X-ray transients (SFXTs), since they are strongly characterized by fast X-ray outbursts lasting less than a day, typically a few hours. We report on IBIS detections of newly discovered fast X-ray outbursts from 10 sources, four of which have been recently optically identified as supergiant high-mass X-ray binaries. For one of them in particular, IGR J11215-5952, we observe fast X-ray transient behavior for the first time. The remaining six sources (IGR J16479-4514, IGR J16418-4532, IGR J16195-4945=AX J161929-4945, XTE J1743-363, AX J1749.1-2733, and IGR J17407-2808) are still unclassified; however, they can be considered candidate SFXTs because of their similarity to the known SFXTs.
We report on combined INTEGRAL and XMM-Newton observations of the supergiant fast X-ray transient (SFXT) IGR J16418−4532. The observations targeted the X-ray eclipse region of IGR J16418−4532's orbit ...with continuous INTEGRAL observations across ∼25 per cent of orbital phase and two quasi-simultaneous XMM-Newton observations of length 20 and 14 ks, occurring during and just after the eclipse, respectively. An enhanced INTEGRAL emission history is provided with 19 previously unreported outbursts identified in the archival 18-60 keV data set. The XMM-Newton eclipse observation showed prominent Fe emission and a flux of 2.8 × 10−13 erg cm−2 s−1 (0.5-10 keV). Through the comparison of the detected eclipse and post-eclipse flux, the supergiant mass-loss rate through the stellar wind was determined as Ṁ
w = 2.3-3.8 × 10−7 M yr−1. The post-eclipse XMM-Newton observation showed a dynamic flux evolution with signatures of the X-ray pulsation, a period of flaring activity, structured n
H variations and the first ever detection of an X-ray intensity dip, or 'off-state', in a pulsating SFXT. Consideration is given to the origin of the X-ray dip, and we conclude that the most applicable of the current theories of X-ray dip generation is that of a transition between Compton-cooling-dominated and radiative-cooling-dominated subsonic accretion regimes within the 'quasi-spherical' model of wind accretion. Under this interpretation, which requires additional confirmation, the neutron star in IGR J16418−4532 possesses a magnetic field of ∼1014 G, providing tentative observational evidence of a highly magnetized neutron star in a SFXT for the first time. The implications of these results on the nature of IGR J16418−4532 itself and the wider SFXT class are discussed.
We present the results of a 30 ks XMM-Newton observation of the supergiant fast X-ray transient (SFXT) SAX J1818.6-1703 -- the first in-depth soft X-ray study of this source around periastron. ...INTEGRAL observations shortly before and after the XMM-Newton observation show the source to be in an atypically active state. Over the course of the XMM-Newton observation, the source shows a dynamic range of -100 with a luminosity greater than 1 x 10 super( 35) erg s super( -1) for the majority of the observation. After an ~6 ks period of low-luminosity (~10 super( 34) erg s super( -1)) emission, SAX J1818.6-1703 enters a phase of fast flaring activity, with flares ~250 s long, separated by ~2 ks. The source then enters a larger flare event of higher luminosity and ~8 ks duration. Spectral analysis revealed evidence for a significant change in spectral shape during the observation with a photon index varying from ... ~ 2.5 during the initial low-luminosity emission phase, to ... ~1.9 through the fast flaring activity, and a significant change to ... ~ 0.3 during the main flare. The intrinsic absorbing column density throughout the observation (n sub( H) ~ 5 x 10 super( 23) cm super( -2)) is among the highest measured from an SFXT, and together with the XMM-Newton and INTEGRAL luminosities, consistent with the neutron star encountering an unusually dense wind environment around periastron. Although other mechanisms cannot be ruled out, we note that the onset of the brighter flares occurs at 3 x 10 super( 35) erg s super( -1), a luminosity consistent with the threshold for the switch from a radiative-dominated to Compton cooling regime in the quasi-spherical settling accretion model. (ProQuest: ... denotes formulae/symbols omitted.)
About 30 per cent of the sources in the fourth INTEGRAL/IBIS catalogue are unidentified in that they lack an optical counterpart. To be able to classify them, X-ray observations are of crucial ...importance as they can place tighter constraints on the high-energy error box, which is usually of the order of a few arcminutes, and allow their broad-band spectrum to be studied. To this aim we have cross-correlated the list of all unidentified IBIS sources in the fourth catalogue with the archive of all XMM–Newton pointings, finding a set of six objects with archival data. For one of them, IGR J17331−2406, no X-ray source is detected by XMM inside the IBIS error box, most likely due to the fact that it is a transient object. In the case of IGR J17445−2747 two possible X-ray counterparts are found inside the IBIS error box: one is very weak while the other is bright, but only detected once. In each of the remaining four cases: IGR J155359−5750, AX J1739.3−2923, AX J1740.2−2903 and IGR J18538−0102, we find instead a convincing association, for which we provide an improved X-ray position and information on the optical/infrared counterpart. We also performed a detailed analysis of their XMM/IBIS spectra, and on the basis of all information acquired we suggest that IGR J155359−5750 is an active galactic nucleus (AGN) of intermediate type, AX J1739.3−2923 and AX J1740.2−2903 are high-mass X-ray binary systems, IGR J17331−2406 and IGR J17445−2747 are Galactic transient sources and IGR J18538−0102 could be a background AGN.
ABSTRACT
The accretion mechanism producing the short flares observed from the Supergiant Fast X-ray Transients (SFXT) is still highly debated and forms a major part in our attempts to place these ...X-ray binaries in the wider context of the High Mass X-ray Binaries. We report on a 216 ks
INTEGRAL
observation of the SFXT IGR J16328-4726 (2014 August 24–27) simultaneous with two fixed-time observations with
XMM
-
Newton
(33 and 20 ks) performed around the putative periastron passage, in order to investigate the accretion regime and the wind properties during this orbital phase. During these observations, the source has shown luminosity variations, from
∼
4
×
10
34
to
∼
10
36
erg
s
−
1
, linked to spectral properties changes. The soft X-ray continuum is well modeled by a power law with a photon index varying from ∼1.2 up to ∼1.7 and with high values of the column density in the range of
∼
2
–
4
×
10
23
cm
−
2
. We report on the presence of iron lines at ∼6.8–7.1 keV, suggesting that the X-ray flux is produced by the accretion of matter from the companion wind characterized by density and temperature inhomogeneities.
The optical spectroscopic followup of 27 sources belonging to a sample of 30 high-energy objects selected by positionally cross correlating the first Fermi/LAT Catalog and the ROSAT All-Sky Survey ...Bright Source Catalog is presented here. It has been found or confirmed that 25 of them are BL Lacertae objects (BL Lacs), while the remaining two are Galactic cataclysmic variables (CVs). This strongly suggests that the sources in the first group are responsible for the GeV emission detected with Fermi, while the two CVs most likely represent spurious associations. We thus find an 80% a posteriori probability that the sources selected by matching GeV and X-ray catalogs belong to the BL Lac class. We also show suggestions that the BL Lacs selected with this approach are probably high-synchrotron-peaked sources and in turn good candidates for the detection of ultra-high-energy (TeV) photons from them.
Temporal analysis of INTEGRAL/IBIS data has revealed a 5.7195 ± 0.0007 d periodicity in the supergiant fast X-ray transient source AX J1845.0−0433, which we interpret as the orbital period of the ...system. The new-found knowledge of the orbital period is utilized to investigate the geometry of the system by means of estimating an upper limit for the size of the supergiant (<27 R) as well as the eccentricity of the orbit ( < 0.37).
We report on a 250 ks long X-ray observation of the supergiant fast X-ray transient IGR J16479−4514 performed with Suzaku in 2012 February. During this observation, about 80 per cent of the short ...orbital period (P
orb ∼ 3.32 d) was covered as continuously as possible for the first time. The source light curve displays variability of more than two orders of magnitude, starting with a very low emission state (10−13 erg cm−2 s−1; 1-10 keV) lasting the first 46 ks, consistent with being due to the X-ray eclipse by the supergiant companion. The transition to the uneclipsed X-ray emission is energy dependent. Outside the eclipse, the source spends most of the time at a level of 6-7 × 10−12 erg cm−2 s−1 punctuated by two structured faint flares with a duration of about 10 and 15 ks, respectively, reaching a peak flux of 3-4 × 10−11 erg cm−2 s−1, separated by about 0.2 in orbital phase. Remarkably, the first faint flare occurs at a similar orbital phase of the bright flares previously observed in the system. This indicates the presence of a phase-locked large-scale structure in the supergiant wind, driving a higher accretion rate on to the compact object. The average X-ray spectrum is hard and highly absorbed, with a column density, N
H, of 1023 cm−2, clearly in excess of the interstellar absorption. There is no evidence for variability of the absorbing column density, except that during the eclipse, where a less absorbed X-ray spectrum is observed. A narrow Fe Kα emission line at 6.4 keV is viewed along the whole orbit, with an intensity which correlates with the continuum emission above 7 keV. The scattered component visible during the X-ray eclipse allowed us to directly probe the wind density at the orbital separation, resulting in ρw = 7 × 10−14 g cm−3. Assuming a spherical geometry for the supergiant wind, the derived wind density translates into a ratio
M km−1 which, assuming terminal velocities in a large range 500-3000 km s−1, implies an accretion luminosity two orders of magnitude higher than that observed. As a consequence, a mechanism should be at work reducing the mass accretion rate. Different possibilities are discussed.
We report on a broad-band X-ray study (0.5–60 keV) of the poorly known candidate supergiant fast X-ray transient (SFXT) IGR J18462 − 0223, and on optical and near-infrared (NIR) followup observations ...of field objects. The out-of-outburst X-ray state has been investigated for the first time with archival INTEGRAL/IBIS, ASCA, Chandra, and Swift/XRT observations. This allowed us to place stringent 3σ upper limits on the soft (0.5–10 keV) and hard (18–60 keV) X-ray emission of 2.9 × 10-13 erg cm-2 s-1 and 8 × 10-12 erg cm-2 s-1, respectively. The source was also detected during an intermediate soft X-ray state with flux equal to 1.6 × 10-11 erg cm-2 s-1 (0.5–10 keV). In addition, we report on the INTEGRAL/IBIS discovery of three fast hard X-ray flares (18–60 keV) having a duration in the range 1–12 h and the flaring behavior was investigated in soft X-rays (3–10 keV) with archival INTEGRAL/JEM–X observations. The duty cycle (1.2%) and the dynamic ranges ( > 1380 and > 190 in the energy bands 0.5–10 keV and 18–60 keV, respectively) were measured for the first time. Archival UKIDSS JHK NIR data, together with our deep R-band imaging of the field, unveiled a single, very red object inside the intersection of the Swift/XRT and XMM-Newton error circles. This source has optical/NIR photometric properties compatible with a very heavily absorbed blue supergiant located at ~11 kpc, so is a strong candidate for a counterpart to IGR J18462 − 0223. NIR spectroscopy is advised to confirm the association. Finally, a hint of a possible orbital period was found at ~2.13 days. If confirmed by further studies, this would make IGR J18462 − 0223 the SFXT with the shortest orbital period among the currently known systems.
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