L. Sidoli

An alternative hypothesis for the outburst mechanism in Supergiant Fast X-ray Transients: the case of IGR J11215-5952

Context. The physical mechanism responsible for the short outbursts in a recently recognised class of high mass X-ray binaries, the supergiant fast X-ray transients (SFXTs), is still unknown. Two main hypotheses have been proposed to date: the sudden accretion by the compact object of small ejections originating in a clumpy wind from the supergiant donor, or outbursts produced at (or near) the periastron passage in wide and eccentric orbits, to explain the low (∼10 32 erg s −1 ) quiescent emission. Neither proposed mechanisms explain the whole phenomenology of these sources. IGR J11215-5952, discovered in April 2005 by the INTEGRAL satellite, is a SFXT which undergoes an outburst every 329 days, a periodicity likely associated with the orbital period of the binary system. Aims. We propose a new explanation for the outburst mechanism, based on the X-ray observations of the unique SFXT known to display periodic outbursts, IGR J11215-5952. Methods. We performed three Target of Opportunity (ToO) observations with Swift, XMM-Newton and INTEGRAL at the time of the fifth outburst, expected on 2007 February 9. Swift observations of the February 2007 outburst have been reported elsewhere. Another ToO with Swift was performed in July 2007, to monitor the supposed “apastron” passage. Results. XMM-Newton observed the source on 2007 February 9, for 23 ks, at the peak of the outburst, while INTEGRAL started the observation two days later, failing to detect the source, which had already undergone the decaying phase of the fast outburst. XMM-Newton data show large variability, with a bright flare at the beginning of the observation (lasting about 1 h), followed by a lower intensity phase (about one order of magnitude fainter) with a large variability as well as low level flaring activity. The spin periodicity discovered by RXTE is confirmed, and a spin-phase spectral variability is observed and studied in detail. The Swift campaign performed in July 2007 reveals a second outburst on 2007 July 24, as bright as that observed about 165 days before. Conclusions. The new X-ray observations allow us to propose an alternative hypothesis for the outburst mechanism in SFXTs, linked to the possible presence of a second wind component, in the form of an equatorial disc from the supergiant donor. We discuss the applicability of the model to the short outburst durations of all other SFXTs, where a clear periodicity in the outbursts has not been found yet. The new outburst from IGR J11215–5952 observed in July suggests that the true orbital period is ∼165 days, instead of 329 days, as previously thought.

The structure of blue supergiant winds and the accretion in supergiant High Mass X-ray Binaries

We have developed a stellar wind model for OB supergiants to investigate the effects of accretion from a clumpy wind on the luminosity and variability properties of High Mass X‐ray Binaries. Assuming that the clumps are confined by ram pressu re of the ambient gas and exploring different distributions for their mass and radii, w e computed the expected X‐ray light curves in the framework of the Bondi-Hoyle accretion theory, modified to take into account the presence of clumps. The resulting variability properti es are found to depend not only on the assumed orbital parameters but also on the wind characteristics. We have then applied this model to reproduce the X-ray light curves of three representative High Mass X-ray Binaries: two persistent supergiant systems (Vela X-1 and 4U 1700‐377) and the Supergiant Fast X-ray Transient IGR J11215 5952. The model can reproduce well the observed light curves, but requiring in all cases an overall mass loss from the supergiant about a factor 3 10 smaller than the values inferred from UV lines studies that assume a homogeneous wind.

Multiple flaring activity in the supergiant fast X‐ray transient IGR J08408−4503 observed with Swift

IGR J08408-4503 is a supergiant fast X-ray transient discovered in 2006 with a confirmed association with a 08.5Ib(f) supergiant star, HD 74194. We report on the analysis of two outbursts caught by Swift/Burst Alert Telescope (BAT) on 2006 October 4 and 2008 July 5, and followed up at softer energies with Swift/X-ray Telescope (XRT). The 2008 XRT light curve shows a multiple-peaked structure with an initial bright flare that reached a flux of ∼10 -9 erg cm -2 s -1 (2-10 keV), followed by two equally bright flares within 75 ks. The spectral characteristics of the flares differ dramatically, with most of the difference, as derived via time-resolved spectroscopy, being due to absorbing column variations. We observe a gradual decrease in the N H , derived with a fit using absorbed power-law model, as time passes. We interpret these N H variations as due to an ionization effect produced by the first flare, resulting in a significant decrease in the measured column density towards the source. The durations of the flares as well as the times of the outbursts suggest that the orbital period is ∼35 d, if the flaring activity is interpreted within the framework of the Sidoli et al. model with the outbursts triggered by the neutron star passage inside an equatorial wind inclined with respect to the orbital plane.

Swift/XRT observes the fifth outburst of the periodic supergiant fast X-ray transient IGR j11215-5952

Context. IGR J11215-5952 is a hard X‐ray transient source discovered in April 2005 with INTEGRAL and a confirmed member of the new class of High Mass X‐ray Binaries, the Supergiant Fast X‐ray Transients (SFXTs). Archival INTEGRAL data and RXTE observations showed that the outbursts occur with a periodicity of∼330 days. Thus, IGR J11215‐5952 is the first SFXT displaying periodic outbursts, possibly related to the orbital period . Aims. We performed a Target of Opportunity observation with Swift with the main aim of monitoring the source behaviour around the time of the fifth outburst, expected on 2007 Feb 9. Methods. The source field was observed with Swift twice a day (2ks/day) starting from 4th February, 2007, until the fifth outbur st, and then for∼ 5 ks a day afterwards, during a monitoring campaign that lasted 23 days for a total on-source exposure of∼ 73 ks. This is the most complete monitoring campaign of an outburst from a SFXT. Results. The spectrum during the brightest flares is well described by an absorbed power law with a photon index of 1 and NH ∼ 1× 10 22 cm −2 . A 1‐10 keV peak luminosity of∼10 36 erg s −1 was derived (assuming 6.2 kpc, the distance of the optical counterpart). Conclusions. These Swift observations are a unique data-set for an outburst of a SFXT, thanks to the combination of sensitivity and time coverage, and they allowed a study of IGR J11215‐5952 from outburst onset to almost quiescence. We find that the accre tion phase lasts longer than previously thought on the basis of lower sensitivity instruments observing only the brightest fl ares. The observed phenomenology is consistent with a smoothly increasing flux triggered at the periastron passage in a wide eccen tric orbit with many flares superimposed, possibly due to episodic or in homogeneous accretion.

Two years of monitoring supergiant fast X-ray transients with Swift

We present results based on 2 yr of intense Swift monitoring of three supergiant fast X-ray transients (SFXTs), IGR J16479-4514, XTE J1739-302 and IGR J17544-2619, which we started in 2007 October. Our out-of-outburst intensity-based X-ray (0.3-10 keV) spectroscopy yields absorbed power laws characterized by hard photon indices (Γ ~ 1-2). The broad-band (0.3-150 keV) spectra of these sources, obtained while they were undergoing new outbursts observed during the second year of monitoring, can be fitted well with models typically used to describe the X-ray emission from accreting neutron stars in high-mass X-ray binaries. We obtain an assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument. By considering our monitoring as a casual sampling of the X-ray light curves, we can infer that the time these sources spend in bright outbursts is between 3 and 5 per cent of the total. The most probable X-ray flux for these sources is ~(1-2) × 10 -11 erg cm -2 s -1 (2-10 keV, unabsorbed), corresponding to luminosities of the order of a few 10 33 to a few 10 34 erg s -1 (two orders of magnitude lower than the bright outbursts). In particular, the duty-cycle of inactivity is ~19, 39 and 55 per cent (~5 per cent uncertainty) for IGR J16479-4514, XTE J1739-302 and IGR J17544-2619, respectively. We present a complete list of BAT onboard detections, which further confirm the continued activity of these sources. This demonstrates that true quiescence is a rare state and that these transients accrete matter throughout their life at different rates. Variability in the X-ray flux is observed at all time-scales and intensity ranges we can probe. Superimposed on the day-to-day variability is intraday flaring, which involves flux variations up to one order of magnitude that can occur down to time-scales as short as ~1 ks, and which can be naturally explained by the accretion of single clumps composing the donor wind with masses M cl ~ (0.3-2) × 10 19 g. Thanks to the Swift observations, the general picture we obtain is that, despite individual differences, common X-ray characteristics of this class are now well defined, such as outburst lengths well in excess of hours, with a multiple peaked structure, and a high dynamic range (including bright outbursts), up to approximately four orders of magnitude.

XMM-Newton observation of a spectral state transition in the peculiar radio/X-ray/gamma-ray source LS I +61 303

We report the results of XMM-Newton and BeppoSAX observations of the radio and X-ray emitting star LS I+61 303, likely associated with the gamma-ray source 2CG 135+01 and recently detected also at TeV energies. The data include a long XMM-Newton pointing carried out in January 2005, which provides the deepest look ever obtained for this object in the 0.3-12 keV range. During this observation the source flux decreased from a high level of ∼13 x 10 -12 erg cm -2 s -1 to 4 x 10 -12 erg cm -2 s -1 within 2-3 h. This flux range is the same seen in shorter and less sensitive observations carried out in the past, but the new data show for the first time that transitions between the two levels can occur on short time scales. The flux decrease was accompanied by a significant softening of the spectrum, which is well described by a power law with photon index changing from 1.62 ± 0.01 to 1.83 ± 0.01. A correlation between hardness and intensity is also found when comparing different short observations spanning almost 10 years and covering various orbital phases. LS I +61 303 was detected in the 15-70 keV range with the PDS instrument in one of the BeppoSAX observations, providing evidence for variability also in the hard X-ray range. The X-ray spectra, discussed in the context of multiwavelength observations, place some interesting constraints on the properties and location of the high-energy emitting region.

INTEGRAL results on supergiant fast X-ray transients and accretion mechanism interpretation: ionization effect and formation of transient accretion discs

We performed a systematic analysis of all INTEGRAL observations from 2003 to 2009 of 14 supergiant fast X-ray transients (SFXTs), implying a net exposure time of about 30 Ms. For each source we obtained light curves and spectra (3–100 keV), discovering several new outbursts. We discuss the X-ray behaviour of SFXTs emerging from our analysis in the framework of the clumpy wind accretion mechanism we proposed. We discuss the effect of X-ray photoionization on accretion in close binary systems such as IGR J16479−4514 and IGR J17544−2619. We show that, because of X-ray photoionization, there is a high probability of an accretion disc forming from the capture of angular momentum in IGR J16479−4514, and we suggest that the formation of transient accretion discs could be partly responsible for the flaring activity in SFXTs with narrow orbits. We also propose an alternative way to explain the origin of flares with peculiar shapes observed in our analysis applying the model of Lamb et al., which is based on accretion via the Rayleigh–Taylor instability and was originally proposed to explain Type II bursts.

Monitoring supergiant fast X-ray transients with Swift: results from the first year

The advent of Swift has allowed, for the first time, the possibility to give Super giant Fast X‐ray Transients (SFXTs), the new class of High Mass X‐ray Binaries discovered by INTEGRAL, non serendipitous attention throughout most phases of their life. In this paper we present our results based on the first year of intense Swift monitoring of four SFXTs, IGR J16479 4514, XTE J1739 302, IGR J17544 2619, and AX J1841.0 0536. We obtain the first assessment of how long each source spends i n each state using a systematic monitoring with a sensitive instrument. The duty-cycle of inactivity is � 17, 28, 39, 55 % (�5 % uncertainty), for IGR J16479 4514, AX J1841.0 0536, XTE J1739‐ 302, and IGR J17544 2619, respectively, so that true quiescence, which is below our detection ability even with the exposures we collected in one year, is a rare state, when compared with estimates from less sensitive instruments. This demonstrates that these transients accrete matter throughout their lifetime at different rates. AX J1841.0 0536 is the only source which has not undergone a bright outburst during our monitoring campaign. Although individual sources behave somewhat differently, common X‐ray characteristics of this class are emerging such as outburst lengths well in excess of hours, with a multiple peaked structure. A high dynamic range (including bright outbursts) of � 4 orders of magnitude have been observed in IGR J17544 2619 and XTE J1739 302, of �3 in IGR J16479 4514, and of about 2 in AX J1841.0 0536 (this lowest range is due to the lack of bright flares). We also present a complete list of B AT on-board detections, which complements our previous work, and further confirms the cont inuous activity of these sources. We performed out-of-outburst intensity-based spectroscopy. In particular, spectral fits with an absorbed blackbody always result in blackbody radii of a few hundred meters, consistent with being emitted from a small portion of the neutron star surface, very likely the neutron star polar caps. We used the whole BAT dataset, since the beginning of the mission, to search for periodicities due to orbital motion and found Porb = 3.32 d for IGR J16479 4514, confirming previous findings. We also present the UVOT data of these sour ces; we show the UVOT light curves of AX J1841.0 0536 and the ones of XTE J1739 302 before, during, and after the outbursts.

Bright flares in supergiant fast X-ray transients

At steady low-luminosity states, Supergiant Fast X-ray Transients (SFXTs) can be at the stage of quasi-spherical settling accretion onto slowly rotatin g magnetized neutron stars from the OB-companion winds. At this stage, a hot quasi-static shell is formed above the magnetosphere, the plasma entry rate into magnetosphere is controlled by (ineffi cient) radiative plasma cooling, and the accretion rate onto the neutron star is supp ressed by a factor of∼ 30 relative to the Bondi-Hoyle-Littleton value. Changes in the local wind velocity and density due to, e.g., clumps, can only slightly increase the mass accretion rate (a factor of∼ 10) bringing the system into the Compton cooling dominated regime and led to the production of moderately bright flares ( Lx . 10 36 erg/s). To interpret the brightest flares ( Lx > 10 36 erg/s) displayed by the SFXTs within the quasi-spherical settling accretion regimes, we propose that a larger increase in the mass accretion rate can be produced by sporadic capture of magnetized stellar wind plasma. At suffi cently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical e nergy released in an SFXT bright flare corresponds to the mass of the shell. This view is consis tent with the energy released in SFXT bright flares (∼ 10 38 − 10 40 ergs), their typical dynamic range (∼ 100), and with the observed dependence of these characteristics on the average unflaring X-ray luminosity of SFXTs. Thus the flaring behaviour of SFXTs, as opposed to stea dy HMXBs, may be primarily related to their low X-ray luminosity allowing sporadic magnetic reconnection to occur during magnetized plasma entry into the magnetosphere.

Supergiant Fast X-ray Transients in outburst: new Swift observations of XTE J1739−302, IGR J17544−2619 and IGR J08408−4503

We report on new X‐ray outbursts observed with Swift from three Supergiant Fast X‐ ray Transients (SFXTs): XTE J1739 302, IGR J17544 2619, and IGR J08408 4503. XTE J1739 302 underwent a new outburst on 2008, August 13, IGR J17544 2619 on 2008, September 4, while IGR J08408 4503 on 2008, September 21. While the XTE J1739 302 and IGR J08408 4503 bright emission triggered the Swift/Burst Alert Telescope, IGR J17544 2619 did not, thus we could perform a spectral investigation only of the spectrum below 10 keV. The broad band spectra from XTE J1739 302 and IGR J08408 4503 were compatible with the X‐ray spectral shape displayed during the previous flares. A variable absorbing column density during the flare was observed in XTE J1739 302 for the first time. The broad-band spectrum of IGR J08408 4503 requires the presence of two distinct photon populations, a cold one (�0.3 keV) most likely from a thermal halo around the neutron star and a hotter one (1.4‐1.8 keV) from the accreting column. The outburst from XTE J1739 302 could be monitored with a very good sampling, thus revealing a shape which can be explained with a second wind component in this SFXT, in analogy to what we have suggested in the periodic SFXT IGR J11215‐5952. The outburst recurrence timescale in IGR J17544 2619 during our monitoring campaign with Swift suggests a long orbital period of �150 days (in an highly eccentric orbit), compatible with what previously observed with INTEGRAL.