arXiv subjects

Sandro Mereghetti

Publications and source records attributed to Sandro Mereghetti.

At least 91 records · Page 5Linked to original sources

arXiv2013

RX J0648.0--4418: the fastest-spinning white dwarf

RX J0648.0-4418 is a post common-envelope X-ray binary composed of a hot subdwarf and one of the most massive white dwarfs with a dynamical mass measurement (1.28+/-0.05 M_sun). This white dwarf, with a spin period of 13.2 s, rotates more than twice faster than the white dwarf in the cataclysmic variable AE Aqr. The current properties of these two binaries, as well as their future evolution, are quite different, despite both contain a fast-spinning white dwarf. RX J0648.0-4418 could be the progenitor of either a Type Ia supernova or of a non-recycled millisecond pulsars.

astro-ph.HE↗

arXiv2012

X-ray emission from hot subdwarfs with compact companions

We review the X-ray observations of hot subdwarf stars. While no X-ray emission has been detected yet from binaries containing B-type subdwarfs, interesting results have been obtained in the case of the two luminous O-type subdwarfs HD 49798 and BD +37 442. Both of them are members of binary systems in which the X-ray luminosity is powered by accretion onto a compact object: a rapidly spinning (13.2 s) and massive (1.28 M_sun) white dwarf in the case of HD 49798 and most likely a neutron star, spinning at 19.2 s, in the case of BD +37 442. Their study can shed light on the poorly known processes taking place during common envelope evolutionary phases and on the properties of wind mass loss from hot subdwarfs.

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arXiv2011

Search for X-ray emission from subdwarf B stars with compact companion candidates

Stellar evolutionary models predict that most of the early type subdwarf stars in close binary systems have white dwarf companions. More massive companions, such as neutron stars or black holes, are also expected in some cases. The presence of compact stars in these systems can be revealed by the detection of X-rays powered by accretion of the subdwarf's stellar wind or by surface thermal emission. Using the Swift satellite, we carried out a systematic search for X-ray emission from a sample of twelve subdwarf B stars which, based on optical studies, have been suggested to have degenerate companions. None of our targets was detected, but the derived upper limits provide one of the few observational constraints on the stellar winds of early type subdwarfs. If the presence of neutron star companions is confirmed, our results constrain the mass loss rates of some of these subdwarf B stars to values <10^{-13}-10^{-12} Msun/yr.

astro-ph.SR↗

arXiv2011

INTEGRAL results on gamma-ray bursts and soft gamma-ray repeaters

Despite not being specifically designed for the study of gamma-ray bursts (GRBs), the INTEGRAL satellite is giving a relevant contribution to this field with the detection of about 10 GRB per year in the IBIS field of view. The ground-based INTEGRAL Burst Alert System (IBAS) has provided real-time localizations with 2-3 arcmin accuracy for most of these events. The INTEGRAL sample now comprises 81 bursts, including 4 of the short class. IBAS has also revealed other transient events from various classes of Galactic sources, most notably short bursts and flares from magnetar candidates. Here I present a summary of the IBAS performances in the eight years since the INTEGRAL launch and review some of the INTEGRAL results for GRBs and Soft Gamma-ray Repeaters.

astro-ph.HE↗

arXiv2010

X-ray emission from isolated neutron stars

X-ray emission is a common feature of all varieties of isolated neutron stars (INS) and, thanks to the advent of sensitive instruments with good spectroscopic, timing, and imaging capabilities, X-ray observations have become an essential tool in the study of these objects. Non-thermal X-rays from young, energetic radio pulsars have been detected since the beginning of X-ray astronomy, and the long-sought thermal emission from cooling neutron star's surfaces can now be studied in detail in many pulsars spanning different ages, magnetic fields, and, possibly, surface compositions. In addition, other different manifestations of INS have been discovered with X-ray observations. These new classes of high-energy sources, comprising the nearby X-ray Dim Isolated Neutron Stars, the Central Compact Objects in supernova remnants, the Anomalous X-ray Pulsars, and the Soft Gamma-ray Repeaters, now add up to several tens of confirmed members, plus many candidates, and allow us to study a variety of phenomena unobservable in "standard'' radio pulsars.

astro-ph.HE↗

arXiv2010

Unveiling the hard X-ray spectrum from the "burst-only" source SAX J1753.5-2349 in outburst

Discovered in 1996 by BeppoSAX during a single type-I burst event, SAX J1753.5-2349 was classified as "burst-only" source. Its persistent emission, either in outburst or in quiescence, had never been observed before October 2008, when SAX J1753.5-2349 was observed for the first time in outburst. Based on INTEGRAL observations,we present here the first high-energy emission study (above 10 keV) of a so-called "burst-only". During the outburst the SAX J1753.5-2349 flux decreased from 10 to 4 mCrab in 18-40 keV, while it was found being in a constant low/hard spectral state. The broad-band (0.3-100 keV) averaged spectrum obtained by combining INTEGRAL/IBIS and Swift/XRT data has been fitted with a thermal Comptonisation model and an electron temperature >24 keV inferred. However, the observed high column density does not allow the detection of the emission from the neutron star surface. Based on the whole set of observations of SAX J1753.5-2349, we are able to provide a rough estimate of the duty cycle of the system and the time-averaged mass-accretion rate. We conclude that the low to very low luminosity of SAX J1753.5-2349 during outburst may make it a good candidate to harbor a very compact binary system.

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arXiv2009

A low luminosity state in the massive X-ray binary SAX J0635+0533

The X-ray pulsar SAX J0635+0533 was repeatedly observed with the XMM-Newton satellite in 2003-2004. The precise localization provided by these observations confirms the association of SAX J0635+0533 with a Be star. The source was found, for the first time, in a low intensity state, a factor ~30 lower than that seen in all previous observations. The spectrum, well fitted by an absorbed power law with photon index ~1.7 and N_H = 1.2x10^22 cm^-2, was compatible with that of the high state. The low flux did not allow the detection of the pulsations at 33.8 ms seen BeppoSAX and RXTE data. In view of the small luminosity observed in 2003-2004, we reconsider the peculiarities of this source in both the accretion and rotation powered scenarios.

astro-ph.HE↗

arXiv2009

Recent results on magnetars

Several observations obtained in the last few years indicate that Soft Gamma-ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are basically a single class of isolated neutron stars. Their properties are well explained by the magnetar model, based on neutron stars powered by magnetic fields as high as 10^14 -10^15 G. Here I report some recent results obtained for the transient Soft Gamma-ray Repeater SGR 1627-41, that started a new outburst after about 10 years from the previous one, and for the Anomalous X-ray Pulsar 1E 1547.0-5408. The latter source recently showed a remarkable bursting activity, that reinforces the similarity between AXPs and SGRs.

astro-ph.HE↗

arXiv2009

The first outburst of the new magnetar candidate SGR 0501+4516

We report here on the outburst onset and evolution of the new Soft Gamma Repeater SGR 0501+4516. We monitored the new SGR with XMM-Newton starting on 2008 August 23, one day after the source became burst-active, and continuing with 4 more observations, with the last one on 2008 September 30. Combining the data with the Swift-XRT and Suzaku data, we modelled the outburst decay over 160 days, and we found that the source flux decreased exponentially with a timescale of t_c=23.8 days. In the first XMM-Newton observation a large number of short X-ray bursts were observed, the rate of which decayed drastically in the following observations. We found large changes in the spectral and timing behavior of the source during the outburst, with softening emission as the flux decayed, and the non-thermal soft X-ray spectral component fading faster than the thermal one. Almost simultaneously to our XMM-Newton observations (on 2008 August 29 and September 2), we observed the source in the hard X-ray range with INTEGRAL, which clearly detected the source up to ~100keV in the first pointing, while giving only upper limits during the second pointing, discovering a variable hard X-ray component fading in less than 10 days after the bursting activation. We performed a phase-coherent X-ray timing analysis over about 160 days starting with the burst activation and found evidence of a strong second derivative period component (\ddot{P} = -1.6(4)x10^{-19} s/s^{-2}). Thanks to the phase-connection, we were able to study the the phase-resolved spectral evolution of SGR 0501+4516 in great detail. We also report on the ROSAT quiescent source data, taken back in 1992 when the source exhibits a flux ~80 times lower than that measured during the outburst, and a rather soft, thermal spectrum.

astro-ph.HE↗

arXiv2008

The strongest cosmic magnets: Soft Gamma-ray Repeaters and Anomalous X-ray Pulsars

Two classes of X-ray pulsars, the Anomalous X-ray Pulsars and the Soft Gamma-ray Repeaters, have been recognized in the last decade as the most promising candidates for being magnetars: isolated neutron stars powered by magnetic energy. I review the observational properties of these objects, focussing on the most recent results, and their interpretation in the magnetar model. Alternative explanations, in particular those based on accretion from residual disks, are also considered. The possible relations between these sources and other classes of neutron stars and astrophysical objects are also discussed.

astro-ph↗

arXiv2008

1E161348-5055 in the Supernova Remnant RCW 103: A Magnetar in a Young Low Mass Binary System?

We suggest that the unique X-ray source 1E161348-5055 at the centre of the supernova remnant RCW 103 consists of a neutron star in close orbit with a low mass main sequence star. The time signature of 6.67 hr is interpreted as the neutron star's spin period. This requires the neutron star to be endowed with a high surface magnetic field of~10^15 G. Magnetic or/and material (propeller) torques are able to spin rapidly the young neutron star down to an asymptotic, equilibrium spin period in close synchronism with the orbital period, similarly to what happens in the Polar Cataclysmic Variables. 1E161348-5055 could be the first case of a magnetar born in a young low mass binary system.

astro-ph↗

arXiv2007

XMM-Newton discovery of 7 s pulsations in the isolated neutron star RX J1856.5-3754

Thanks to the high counting statistics provided by a recent XMM-Newton observation of RX J1856.5-3754, we have discovered that this isolated neutron star pulsates at a period of 7.055 s. This confirms that RX J1856.5-3754 is similar in nature to the other six thermally emitting, nearby neutron stars discovered in soft X-rays with ROSAT. The pulsations are detected at consistent periods in several XMM-Newton observations spanning from April 2002 to October 2006, yielding an upper limit of Pdot<1.9x10^{-12} s/s (90% c.l.) on the period derivative. This implies a surface magnetic field smaller than 1.2x10^{14} G, under the usual assumption of vacuum dipole magnetic braking. The pulse profile is nearly sinusoidal with a pulsed fraction in the 0.15-1.2 keV range of only ~1.2%, the smallest ever seen in an isolated X-ray pulsar.

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arXiv2006

INTEGRAL observations of the X-ray burster 4U1850-087: first detection of hard X-ray emission extending above 50keV

The X-ray burster 4U1850-087, located in the globular cluster NGC6712, is an ultracompact binary likely harbouring a degenerate companion.The source has been observed with INTEGRAL several times, during the monitoring of the Galactic plane, with an unprecedented exposure time. The broad-band spectrum (2-100 keV; INTEGRAL together with a quasi-simultaneous XMM-Newton observation) is well described with a disk-blackbody emission (kTin=0.8keV) together with a power-law (photon index of 2). We report here the first detection of hard X-ray emission from this source above 50 keV. A lower limit on the presence of a high energy cut-off can be placed at Ec>100keV.

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arXiv2006

INTEGRAL observations of IGRJ11215-5952: the first Supergiant Fast X-ray Transient displaying periodic outbursts

The hard X-ray source IGRJ11215-5952, discovered with INTEGRAL during a brief outburst in 2005, has been proposed as a new member of the class of Supergiant Fast X-ray Transients. Analysing archival INTEGRAL observations of the source field,we have discovered two previously unnoticed outbursts (in July 2003 and in May 2004),spaced by intervals of ~330 days, suggesting a possible orbital period.The 5-100keV spectrum is well described by a cut-off power law, with a photon index of 0.5, and a cut-off energy 15-20keV, typical of High Mass X-ray Binaries containing a neutron star. The luminosity is ~3E36erg/s assuming 6.2kpc, the distance of the likely optical counterpart, the blue supergiant HD306414.A fourth outburst was discovered in 2006 with XTE/PCA, 329 days after the third one, confirming the periodic nature of the source outbursts.Follow-up observations with Swift/XRT refined the source position and confirmed the association with HD306414.The 5-100keV spectrum, the recurrent nature of the outbursts, the blue supergiant companion star HD 306414, support the hypothesis that IGRJ11215-5952 is a Supergiant Fast X-ray Transient, and it is the first object of this class of High Mass X-ray Binaries displaying periodic outbursts.

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arXiv2006

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 13E12 erg/cm2/s to 4E12 erg/cm2/s within 2-3 hours.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+/-0.1 to 1.83+/-0.1. 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.

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arXiv2006

XMM-Newton observation of the Be/neutron star system RX J0146.9+6121: a soft X-ray excess in a low luminosity accreting pulsar

We report on the XMM-Newton observation of the Be/neutron star X-ray binary system RX J0146.9+6121, a long period (~ 23 m) pulsar in the NGC 663 open cluster. The X-ray luminosity decreased by a factor two compared to the last observation carried out in 1998, reaching a level of ~ 10^34 erg/s, the lowest ever observed in this source. The spectral analysis reveals the presence of a significant excess at low energies over the main power-law spectral component. The soft excess can be described by a black-body spectrum with a temperature of about 1 keV and an emitting region with a radius of ~ 140 m. Although the current data do not permit to ascertain whether the soft excess is pulsed or not, its properties are consistent with emission from the neutron star polar cap. This is the third detection of a soft excess in a low luminosity (~ 10^34 erg/s) pulsar, the other being X Per and 3A 0535+262, suggesting that such spectral component, observed up to date in higher luminosity systems, is a rather common feature of accreting X-ray pulsars. The results on these three sources indicate that, in low luminosity systems, the soft excess tends to have a higher temperature and a smaller surface area than in the high luminosity ones.

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arXiv2005

Dust-scattered X-ray halos around gamma-ray bursts: GRB 031203 revisited and the new case of GRB 050713A

Scattering by interstellar dust grains can produce time variable X-ray halos around gamma-ray bursts (GRB). In particular, an X-ray expanding ring is expected when a short pulse of X-ray radiation is scattered by a narrow layer of dust in our Galaxy. We present a new method to detect and analyze dust scattering expanding rings around gamma-ray bursts, using as an example XMM-Newton data of GRB 031203. Consistent with previous reports (Vaughan et al. 2004), we find that the two expanding rings observed in this burst are due to dust unevenly distributed in two layers at distances of 870 and 1384 pc, with the more distant one responsible for 70% of the total optical depth. Our modelling of the rings indicates that the prompt X-ray emission of GRB 031203 was consistent with a power law spectrum with photon index $Γ$=2.1$\pm$0.2 and 1-2 keV fluence of (3.6$\pm$0.2)$\times10^{-7}$ erg cm$^{-2}$. Thanks to the sensitivity of our technique, we discovered an expanding ring around another burst recently observed with XMM-Newton, GRB 050713A. In this case the dust layer is located at a distance of 364 pc and we derive a GRB fluence of (1.2$\pm$0.3)$\times10^{-7}$ erg cm$^{-2}$ (1-2 keV). A search for similar halos in other twelve bursts observed with XMM-Newton gave negative results.

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arXiv2005

Discovery and Monitoring of the likely IR Counterpart of SGR 1806-20 during the 2004 gamma-ray burst-active state

The sky region including the Chandra position of SGR 1806-20 was monitored in the IR band during 2004, following its increased high energy bursting activity. Observations were performed using NAOS-CONICA, the adaptive optics IR camera mounted on Yepun VLT, which provided images of unprecedented quality (FWHM better than 0.1"). After the 2004 December 27th giant flare, the source position has been nailed by VLA observations of its radio counterpart, reducing the positional uncertainty to 0.04". Using IR data from our monitoring campaign, we discovered the likely IR counterpart to SGR 1806-20 based on positional coincidence with the Chandra and VLA uncertainty regions and flux variability of a factor of about 2 correlated with that at higher energies. We compare our findings with other isolated neutron star classes thought to be related, at some level, with SGRs.

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