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L. Sidoli

Publications and source records attributed to L. Sidoli.

At least 109 records · Page 6Linked to original sources
arXiv2008

Supergiant Fast X-ray Transients: interpretation of archival INTEGRAL data

INTEGRAL monitoring of the Galactic Plane in the last 5 years revealed a new subclass of High Mass X-ray Binaries (HMXBs), the Supergiant Fast X-ray Transients (SFXTs). They display flares lasting from minutes to hours, with peak luminosity of 1E36-1E37 erg/s and a frequent long term flaring activity reaching an X-ray luminosity of 1E33-1E34 erg/s, as recently detected by the Swift satellite. The quiescent level is around 1E32 erg/s. We performed a systematic re-analysis of archival INTEGRAL data of four SFXTs: IGRJ16479-4514, XTEJ1739-302, IGRJ17544-2619, IGRJ18410-0535. This led to the discovery of previously unnoticed outbursts from IGRJ16479-4514 and IGRJ17544-2619. We discuss these results in the framework of the different structure of the supergiant wind proposed to explain the outburst from this new class of sources.

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arXiv2008

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 O8.5Ib(f) supergiant star, HD 74194. We report on the analysis of two outbursts caught by Swift/BAT on 2006 October 4 and 2008 July 5, and followed up at softer energies with Swift/XRT. The 2008 XRT light curve shows a multiple-peaked structure with an initial bright flare that reached a flux of ~1E-9 erg/cm2/s (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 of the NH, derived with a fit using absorbed power law model, as time passes. We interpret these NH 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 days, if the flaring activity is interpreted within the framework of the Sidoli et al 2007 model with the outbursts triggered by the neutron star passage inside an equatorial wind inclined with respect to the orbital plane.

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arXiv2008

Monitoring Supergiant Fast X-ray Transients with Swift. III. Outbursts of the prototypical SFXTs IGR J17544-2619 and XTE J1739-302

IGR J17544-2619 and XTE J1739-302 are considered the prototypical sources of the new class of High Mass X-ray Binaries, the Supergiant Fast X-ray Transients (SFXTs).These sources were observed during bright outbursts on 2008 March 31 and 2008 April 8, respectively, thanks to an on-going monitoring campaign we are performing with Swift, started in October 2007. Simultaneous observations with XRT and BAT allowed us to perform for the first time a broad band spectroscopy of their outbursts. The X-ray emission is well reproduced with absorbed cutoff powerlaws, similar to the typical spectral shape from accreting pulsars. IGR J17544-2619 shows a significantly harder spectrum during the bright flare (where a luminosity in excess of 1E36 erg/s is reached) than during the long-term low level flaring activity (1E33-1E34 erg/s), while XTE J1739-302 displayed the same spectral shape, within the uncertainties, and a higher column density during the flare than in the low level activity. The light curves of these two SFXTs during the bright flare look similar to those observed during recent flares from other two SFXTs, IGRJ11215-5952 and IGRJ16479-4514, reinforcing the connection among the members of this class of X-ray sources.

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arXiv2008

XMM-Newton observations of the low-mass X-ray binary XB1832-330 in the galactic globular cluster NGC 6652

We report on two XMM-Newton observations performed in 2006 of the luminous low-mass X-ray binary XB1832-330 which is located in the galactic globular cluster NGC 6652 and is probably an ultracompact binary (orbital period less than 1 hour). The aim of these observations is to investigate the low-energy absorption towards NGC6652 and in particular to search for Ne-rich material local to the binary, which has been suggested as a possible spectral signature of Neon rich degenerate companions. XMM-Newton observed the source twice, in 2006 September and October. High resolution spectroscopy with the RGS was used to estimate the ratio of the neutral neon to oxygen column densities to search for an anomalous Ne abundance in this X-ray binary. We find no evidence for anomalous Ne/O ratios, finding Ne/O=0.18+/-0.06 and Ne/O=0.17+/-0.03 (1 sigma uncertainties), in the two observations, respectively. These values are consistent with that in the interstellar medium. Timing analysis of EPIC data suggests possible periodicities at 9170+/-235 s and 18616 +/- 531 s, which need confirmation. A by-product of these observations consists of the spatial analysis of the source field, which resulted in the detection of 46 faint sources within the EPIC field of view, which are not present in the Second XMM-Newton Serendipitous Source Catalogue. All these faint sources are likely foreground objects. We performed the first high spectral resolution observations of XB1832-33, a probable ultracompact binary, without finding any evidence for an anomalous Ne abundance.

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arXiv2008

Monitoring Supergiant Fast X-ray Transients with Swift. Rise to the outburst in IGR J16479-4514

IGR J16479-4514 is a Supergiant Fast X-ray Transient (SFXT), a new class of High Mass X-ray Binaries, whose number is rapidly growing thanks to the observations of the Galactic plane performed with the INTEGRAL satellite. IGR J16479-4514 has been regularly monitored with Swift/XRT since November 2007, to study the quiescent emission, the outburst properties and their recurrence. A new bright outburst, reaching fluxes above 10$^{-9}$ erg cm$^{-2}$ s$^{-1}$, was caught by the Swift/BAT. Swift immediately re-pointed at the target with the narrow-field instruments so that, for the first time, an outburst from a SFXT where a periodicity in the outburst recurrence is unknown could be observed simultaneously in the 0.2--150 keV energy band. The X-ray emission is highly variable and spans almost four orders of magnitude in count rate during the Swift/XRT observations covering a few days before and after the bright peak. The X-ray spectrum in outburst is hard and highly absorbed. The power-law fit resulted in a photon index of 0.98$\pm{0.07}$, and in an absorbing column density of $\sim5\times10^{22}$ cm$^{-2}$. These observations demonstrate that in this source (similarly to what was observed during the 2007 outburst from the periodic SFXT IGR J11215-5952), the accretion phase lasts much longer than a few hours.

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arXiv2008

Monitoring Supergiant Fast X-ray Transients with Swift. I. Behavior outside outbursts

Supergiant Fast X-ray Transients (SFXTs) are a new class of HMXBs discovered thanks to the monitoring of the Galactic plane performed with the INTEGRAL satellite in the last 5 years. These sources display short outbursts (significantly shorter than typical Be/X-ray binaries) with a peak luminosity of a few 1E36 erg/s. The quiescent level, measured only in a few sources, is around 1E32 erg/s. We are performing a monitoring campaign with Swift of four SFXTs (IGRJ16479-4514, XTEJ1739-302, IGRJ17544-2619 and AXJ1841.0-0536/IGRJ18410-0535). We report on the first four months of Swift observations, started on 2007 October 26. We detect a low level X-ray activity in all four SFXTs which demonstrates that these transient sources accrete matter even outside their outbursts. This fainter X-ray activity is composed of many flares with a large flux variability, on timescales of thousands of seconds. The lightcurve variability is also evident on larger timescales of days, weeks and months, with a dynamic range of more than one order of magnitude in all four SFXTs. The X-ray spectra are typically hard, with an average 2-10 keV luminosity during this monitoring of about 1E33-1E34 erg/s. We detected pulsations from the pulsar AXJ1841.0-0536, with a period of 4.7008+/-0.0004 s. This monitoring demonstrates that these transients spend most of the time accreting matter, although at a much lower level (~100-1000 times lower than during the bright outbusts), and that the true quiescence, characterized by a soft spectrum and a luminosity of a few 1E32 erg/s, observed in the past only in a couple of members of this class, is probably a very rare state.

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arXiv2008

Sub-luminous X-ray Bursters Unveiled with INTEGRAL

In 2005 March 22nd, the INTEGRAL satellite caught a type-I X-ray burst from the unidentified source XMMU J174716.1-281048, serendipitously discovered with XMM-Newton in 2003. Based on the type-I X-ray burst properties, we derived the distance of the object and suggested that the system is undergoing a prolonged accretion episode of many years. We present new data from a Swift/XRT campaign which strengthen this suggestion. AX J1754.2-2754 was an unclassified source reported in the ASCA catalogue of the Galactic Centre survey. INTEGRAL observed a type-I burst from it in 2005, April 16th. Recently, a Swift ToO allowed us to refine the source position and establish its persistent nature.

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arXiv2008

On the nature of the X-ray pulsar SAX J1324-6200

We present recent observations of the X-ray pulsar SAX J1324-6200 obtained in December 2007 with the Swift satellite yielding a significant improvement in the source localization with respect to previous data and a new measurement of the spin period P=172.84s. A single object consistent in colors with a highly reddened early type star is visible in the X-ray error box. The period is significantly longer than that obtained in 1997, indicating that SAX J1324-6200 has been spinning down at an average rate of ~6x10^-9 s s^-1. We discuss the possible nature of the source showing that it most likely belongs to the class of low luminosity, persistent Be/neutron star binaries.

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arXiv2007

A new explanation for the SFXTs outbursts

The physical mechanism responsible for the short outbursts in a recently recognized class of High Mass X-ray Binaries, the Supergiant Fast X-ray Transients (SFXTs), is still unknown. Recent observations performed with Swift/XRT, XMM-Newton and INTEGRAL of the 2007 outburst from IGRJ11215-5952, the only SFXT known to exhibit periodic outbursts, suggest a new explanation for the outburst mechanism in this class of transients, linked to the possible presence of a second wind component in the supergiant companion, in the form of an equatorial wind. 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, is discussed. The scenario we are proposing also includes the persistently accreting supergiant High Mass X-ray Binaries.

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arXiv2007

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

(ABRIDGED)- The physical mechanism responsible for the short outbursts in a recently recognized 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, in order to explain the low (1E32 erg/s) quiescent emission.Neither proposed mechanisms seem to explain the whole phenomenology of these sources. Here we propose a new explanation for the outburst mechanism, based on new X-ray observations of the unique SFXT known to display periodic outbursts, IGRJ11215-5952. We performed three Target of Opportunity 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, in order to monitor the supposed ``apastron'' passage. A second unexpected outburst was discovered on 2007 July 24, after about 165 days from the February 2007 outburst. 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 disk 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 IGRJ11215-5952 observed in July suggests that the true orbital period is ~165days, instead of 329days, as previously thought.

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arXiv2007

XMMU J174716.1-281048: a "quasi-persistent" very faint X-ray transient?

The X-ray transient XMMU J174716.1-281048 was serendipitously discovered with XMM-Newton in 2003. It lies about 0.9 degrees off the Galactic Centre and its spectrum shows a high absorption (~8 x 10E22 cm^(-2)). Previous X-ray observations of the source field performed in 2000 and 2001 did not detect the source, indicative of a quiescent emission at least two orders of magnitude fainter. The low luminosity during the outburst (~5 x 10E34 erg/s at 8 kpc) indicates that the source is a member of the ``very faint X-ray transients'' class. On 2005 March 22nd the INTEGRAL satellite caught a possible type-I X-ray burst from the new INTEGRAL source IGR J17464-2811, classified as fast X-ray transient. This source was soon found to be positionally coincident, within the uncertainties, with XMMU J174716.1-281048. Here we report data analysis of the X-ray burst observed with the IBIS and JEM-X telescopes and confirm the type-I burst nature. We also re-analysed XMM-Newton and Chandra archival observations of the source field. We discuss the implications of these new findings, particularly related to the source distance as well as the source classification.

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arXiv2007

Swift/XRT observes the fifth outburst of the periodic Supergiant Fast X-ray Transient IGR J11215-5952

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. 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. The source field was observed with Swift twice a day (2ks/day) starting from 4th February, 2007, until the fifth outburst, 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. The spectrum during the brightest flares is well described by an absorbed power law with a photon index of 1 and N_H~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). 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 accretion phase lasts longer than previously thought on the basis of lower sensitivity instruments observing only the brightest flares. The observed phenomenology is consistent with a smoothly increasing flux triggered at the periastron passage in a wide eccentric orbit with many flares superimposed, possibly due to episodic or inhomogeneous accretion.

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arXiv2006

Long term spectral variability in the Soft Gamma-ray Repeater SGR 1900+14

We present a systematic analysis of all the BeppoSAX data of SGR1900+14. The observations spanning five years show that the source was brighter than usual on two occasions: ~20 days after the August 1998 giant flare and during the 10^5 s long X-ray afterglow following the April 2001 intermediate flare. In the latter case, we explore the possibility of describing the observed short term spectral evolution only with a change of the temperature of the blackbody component. In the only BeppoSAX observation performed before the giant flare, the spectrum of the SGR1900+14 persistent emission was significantly harder and detected also above 10 keV with the PDS instrument. In the last BeppoSAX observation (April 2002) the flux was at least a factor 1.2 below the historical level, suggesting that the source was entering a quiescent period.

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arXiv2006

The ultra-compact binary 4U1850-087 observed with INTEGRAL: hard X-ray emission from an X-ray burster

The X-ray burster 4U1850-087, located in the Galactic globular cluster NGC6712, is an ultracompact binary (orbital period~21 min),likely harbouring a degenerate companion.The source has been observed at soft gamma-rays several times with the INTEGRAL satellite,during the monitoring of the Galactic plane, with an unprecedented exposure time.We analysed all available INTEGRAL observations, with the main aim of studying the long-term behaviour of this Galactic bulge X-ray burster.The spectral results are based on the systematic analysis of all INTEGRAL observations covering the source position performed between March 2003 and November 2005.The source X-ray emission is hard and is observed, for the first time, up to 100keV.A broad-band spectrum obtained combining the INTEGRAL spectrum together with a quasi-simultaneous XMM-Newton observation performed in September 2003 is well modeled with a disk-blackbody emission (with kTin=0.8keV) together with a power-law (photon index=2.1).The 2-100keV luminosity is 1.5E36 erg/s (assuming a distance of 6.8kpc).

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arXiv2006

Five years of SGR 1900+14 observations with BeppoSAX

We present a systematic analysis of all the BeppoSAX data of the soft gamma-ray repeater SGR 1900+14: these observations allowed us to study the long term properties of the source quiescent emission. In the observation carried out before the 1998 giant flare the spectrum in the 0.8-10 keV energy range was harder and there was evidence for a 20-150 keV emission, possibly associated with SGR 1900+14. This possible hard tail, if compared with the recent INTEGRAL detection of SGR 1900+14, has a harder spectrum (power-law photon index ~1.6 versus ~3) and a 20-100 keV flux ~4 times larger. In the last BeppoSAX observation (April 2002), while the source was entering the long quiescent period that lasted until 2006, the 2-10 keV flux was ~25% below the historical level. We also studied in detail the spectral evolution during the 2001 flare afterglow. This was characterized by a softening that can be interpreted in terms of a cooling blackbody-like component.

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arXiv2006

First broad band study of the mysterious source 1E 1743.1-2843

In the last years, the persistent source 1E 1743.1-2843 has been observed in the X-rays, but never above 20 keV. In previous works, it was stressed that a possible high energy emission could give further indications on the accreting object nature which remains still unknown. We present here more than two years of 1E 1743.1-2843 monitoring with INTEGRAL/IBIS as well as public XMM-Newton and Chandra X-ray observations. The temporal study in the 20-40 keV band shows a rather constant flux on few months time scale. Based on this result we have performed the broad-band spectral analysis using EPIC/IBIS non simultaneous data and ACIS-I/IBIS data collected during 2004. In ~2 Ms, we report a detection of 6 sigma in the energy range 35-70 keV. The first broad-band study (2-70 keV) shows a steep slope (~3) and a black body temperature of 1.7 keV. Combining spectral parameters and discussion about the luminosity evaluations for different possible distances, our conclusions are in favour of a LMXB system with a neutron star at distance higher than the Galactic Centre, even though a firm conclusion can not be stated.

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arXiv2006

Unveiling the nature of the highly absorbed X-ray source SAXJ1748.2-2808 with XMM-Newton

We report on the results of an EPIC XMM-Newton observation of the faint source SAXJ1748.2-2808 and the surrounding field. This source was discovered during the BeppoSAX Galactic center survey performed in 1997-1998. A spatial analysis resulted in the detection of 31 sources within the EPIC field of view. SAXJ1748.2-2808 is clearly resolved into 2 sources in EPIC images with the brighter contributing almost 80% of the 2-10keV flux. Spectral fits to this main source are consistent with an absorbed power-law with a photon index of 1.4+/-0.5 and absorption equivalent to 14E22cm-2 together with an iron line at 6.6+/-0.2 keV with an equivalent width of ~780eV. The significantly better statistics of the XMM-Newton observation, compared with BeppoSAX,allows to exclude a thermal nature for the X-ray emission. A comparison with other observations of SAXJ1748.2-2808 does not reveal any evidence for spectral or intensity long-term variability. Based on these properties we propose that the source is a low-luminosity high-mass X-ray binary located in the Galactic center region.

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