arXiv subjects

Luigi Stella

Publications and source records attributed to Luigi Stella.

At least 91 records · Page 5Linked to original sources

arXiv2003

Unveiling the nature of the 321s Orbital Period X-ray source RX J0806.3+1527

A nearly simultaneous X-ray/optical (Chandra and VLT) observational campaign of RX J0806.3+1527 has been carried out during November 2001. These observations allowed us to phase the X-ray and optical light curves for the first time. We measured a phase-shift of about 0.5, in good agreement with the presence of two distinct emission regions and with the X-ray irradiation process predictions. The Chandra data allowed us also to study in details the X-ray spectrum of RX J0806.3+1527, which is consistent with a soft (kT<70 eV) and small (R_BB<20 km) blackbody component. We discuss the present findings on the light of the models proposed so far to account for the X-ray emission detected from RX J0806.3+1527, and its twin source RX J1914.4+2456.

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arXiv2003

Evidence of a cyclotron feature in the spectrum of the anomalous X-ray pulsar 1RXS J170849-400910

We report the results of a long observation of the anomalous X-ray pulsar 1RXS J170849-400910 obtained with the BeppoSAX satellite in August 2001. The best-fit phase-averaged spectrum was an absorbed power law plus blackbody model, with photon index Gamma~2.4 and a blackbody temperature of kT_bb~0.4 keV. We confirm the presence of significant spectral variations with the rotational phase of the pulsar. In the spectrum corresponding to the rising part of the pulse we found an absorption-like feature at ~8.1 keV (a significance of 4 sigma), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9x10^11 G and 1.6x10^15 G in the case of electrons and protons, respectively. If confirmed, this would be the first detection of a cyclotron feature in the spectrum of an anomalous X-ray pulsar.

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arXiv2001

The discovery and study of the optical counterparts of the transient X-ray pulsars RXJ0052.1-7319 and XTEJ0111.2-7317 in the SMC

We report on the discovery and confirmation of the optical counterparts of the two transient X-ray pulsars, RXJ0052.1-7319 and XTEJ0111.2-7317. In the narrow (~3" radius) X-ray error circle of RXJ0052.1-7319 we found a V=14.6 (B-V=0.1) 09.5IIIe (a classification as a B0Ve star is also possible, since the luminosity class depends on the uncertainty on the adopted reddening). Medium resolution spectra for this object show Balmer lines in emission with an equivalent width of Halpha=-12/-16A. In the X-ray error box of XTEJ0111.2-7317 we found a relatively bright object (V=15.4, B-V=0.06) which has been classified as a B0.5-B1Ve star and that was later confirmed by Coe et al. (2000) as the most plausible counterpart for XTEJ0111.2-7317. Also in this case we easily detect Balmer emission lines with an Halpha equivalent width of about -21A. There is also evidence for the presence of a surrounding nebula, possibly a supernova remnant. A further bright B0Ve star was found just outside the X-ray error circle of XTEJ0111.2-7317. We discuss the implication of these results in the light of the current knowledge of Be/X-ray binary systems in the Magellanic Clouds and within our Galaxy.

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arXiv2001

The identification of the optical/IR counterpart of the 29.5-s transient X-ray pulsar GS 1843+009

We report on the identification of the optical/IR counterpart of the 29.5-s transient X-ray pulsar GS 1843+009. We re-analysed an archival ROSAT HRI observation of GS 1843+009, obtaining a new refined position. The optical and IR follow-up observations carried out for the new error circle allowed us to find a relatively faint (V=20.9) and variable early type reddened star (V-R=2.1). The optical spectra show the Balmer and Paschen series in emission, while the IR observations confirm the presence of a flux excess (H=13.2, J-H=0.54), suggesting that the star is surrounded by a circumstellar envelope. Spectroscopic and photometric data together indicate a B0-B2 IV-Ve spectral-type star located at a distance of > 10kpc confirming the Be-star/X-ray binary nature of GS 1843+009.

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arXiv2001

Illuminated, and enlightened, by GRB 991216

We consider models for the generation of the emission line recently discovered in the X-ray afterglow spectrum of several bursts, and especially of GRB 991216 observed by Chandra. These observations suggest the presence of 0.1-1 solar masses of iron in the vicinity of the bursts. We show that there are strong geometrical and kinematical constraints on the line emitting material. We discuss several classes of models, favoring one where the line photons are produced by reflection off the walls of a wide funnel of semi-opening angle 45 degrees, excavated in a young (a few months old) supernova remnant made of ~10 solar masses with radius 6x10^15 cm, and ~1 solar mass of iron, providing strong support for the SupraNova model.

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arXiv2000

On the bolometric quiescent luminosity and luminosity swing of black hole candidate and neutron star low mass X-ray transients

Low mass X-ray transients (LMXRTs) hosting black hole candidates (BHCs) display on average a factor of ~100 larger swing in the minimum (quiescent) to maximum (outburst) X-ray luminosity than neutron stars (NSs), despite the fact that the swing in the mass inflow rate is likely in the same range. Advection dominated accretion flows (ADAFs) were proposed to interpret such a difference. The residual optical/UV emission of quiescent LMXRTs, after subtraction of the companion star spectrum, is produced by synchrotron radiation in the (latest version) of ADAF and therefore is part of the ADAF's luminosity budget. We demonstrate that, once the residual optical/UV emission is taken into account, the bolometric luminosity swing of BHCs is consistent with that of NSs. We explore here an alternative scenario to ADAFs in which very little mass accretion onto the collapsed star takes place in the quiescence intervals. The residual optical/UV emission of BHCs are expected to derive from the energy released by the matter transferred from the companion star at radii comparable to the circularisation radius. The quiescent X-ray luminosity originates either from accretion onto the BH at very low rates and/or from coronal activity in the companion star or in the outer disk. For comparably small mass inflow rates, the NSs in these systems are likely in the radio pulsar regime. In the interaction of the radio pulsar relativistic wind with matter transferred from the companion star, a shock forms, the power law-like emission of which powers both the harder X-ray emission and most of the residual optical/UV. The soft, thermal-like X-ray component may arise from the cooling of the NS surface. This scenario matches well both the X-ray and bolometric luminosity swing of LMXRTs. (ABRIDGED).

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arXiv2000

BeppoSAX observation of Cen X-4 in quiescence

We report on a 61 ks BeppoSAX observation of the soft X-ray transient Cen X-4 during quiescence which allowed to study the source X-ray spectrum from ~ 0.3 keV to ~ 8 keV. A two-component spectral model was required, consisting of a black body with temperature of ~ 0.1 keV and a power law with photon index ~ 2. These values are compatible with earlier ASCA results indicating that Cen X-4 may be stable, within a factor of a few, over a 5 year baseline.

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arXiv1999

Evidence for an ~80 day periodicity in the X-ray transient pulsar XTE J1946+274

We report evidence for an ~ 80 d periodicity in the X-ray flux of the hard transient pulsar XTE J1946+274. The 1.3-12 keV light curve obtained with the RossiXTE All Sky Monitor shows five regularly spaced flares over a ~ 1 year baseline starting from the outburst onset in Sept. 1998. The first and strongest flare is somewhat longer than the subsequent four flares, which recur in a fairly periodic fashion. This suggests that the profile of the first flare is dominated by the time variability of the Be star ejection episode, while the following four flares are primarily caused by the neutron star motion along an eccentric orbit.

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arXiv1999

SupraNova Events from Spun-up Neutron Stars: an Explosion in Search of an Observation

We consider a formation scenario for supramassive neutron stars (SMNSs) taking place through mass and angular momentum transfer from a close companion during a Low Mass X-ray Binary (LMXB) phase, with the ensuing suppression of the magnetic field. We show that this formation channel is likely to work for all equations of state except the stiffest ones. After the end of the mass transfer phase, SMNSs will loose through magnetic dipole radiation most of their angular momentum, triggering the star's collapse to a black hole. We discuss the rate of occurrence of these collapses, and propose that these stars, because of the baryon-clear environment in which the implosion/explosion takes place, are the originators of gamma ray bursts.

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arXiv1999

Correlations in the QPO Frequencies of Low Mass X-Ray Binaries and the Relativistic Precession Model

A remarkable correlation between the centroid frequencies of quasi periodic oscillations, QPOs, (or peaked noise components) from low mass X-ray binaries, has been recently discovered by Psaltis, Belloni and van der Klis (1999). This correlation extends over nearly 3 decades in frequency and encompasses both neutron star and black hole candidate systems. We discuss this result in the light of the relativistic precession model, which has been proposed to interpret the kHz QPOs as well as some of the lower frequency QPOs of neutron star low mass X-ray binaries of the Atoll and Z classes. Unlike other models the relativistic precession model does not require the compact object to be a neutron star and can be applied to black hole candidates as well. We show that the predictions of the relativistic precession model match both the value and dependence of the correlation to a very good accuracy without resorting to additional assumptions.

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arXiv1999

Broad Band X-ray Spectra of M31 Sources with BeppoSAX

We present the first spectral study of the X-ray emitting stellar sources in M31 in the energy band from 0.1 to 10 keV. We find that the globular cluster sources have spectral characteristics consistent with those of the Milky Way object, namely that the spectrum can be described by a thermal model with 6-20 keV from 2 to 10 keV. Evidence of high absorption in some of these sources is most likely an indication that they lie in or behind the HI ring in the disk of the galaxy. We also find one peculiar globular cluster source, with spectral characteristics more typically associated with either High Mass X-ray Binaries or black hole candidates. We therefore suggest that either the source has been wrongly identified with a globular cluster or that the system contains a Black Hole. We confirm earlier report that the spectrum of the bulge of M31 is consistent with the superposition of many LMXB spectra. It is likely that a large fraction of the 15-30 keV detection obtained from the PDS instrument is associated with the bulge, thus extending the spectral data for this complex of sources up to 30 keV. The high energy part of the spectrum can be parameterized with typical LMXB spectra, while at low energies an additional component is required.

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arXiv1999

Further evidence that 1RXS J170849.0-400910 is an Anomalous X-ray pulsar

We report the results of two ROSAT HRI observations of the recently discovered 11s X-ray pulsar 1RXS J170849.0-400910. A refined position with a smaller error radius (10" uncertainty) and a new spin period measurement were obtained. These results allowed to derive a period derivative of about 7 times 10^-4 s yr^-1 and to perform a photometric and spectroscopic study of the possible optical counterparts of the source. The limits derived from the optical to X-ray flux ratio exclude the presence of a massive OB companion. These findings, together with the nearly constant X-ray flux, the stability of the pulse shape and pulsed fraction across observations spanning three years, strongly support the inclusion of this 11s pulsar in the class of Anomalous X-ray Pulsars (AXPs).

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arXiv1998

kHz Quasi Periodic Oscillations in Low Mass X-ray Binaries as Probes of General Relativity in the Strong Field Regime

We consider the interpretation of a pair of kHz Quasi Periodic Oscillations (QPOs) in the Fourier spectra of two Low Mass X-Ray Binaries, Sco X-1 and 4U1608-52, hosting an old accreting neutron star. The observed frequency difference of these QPOs decreaseas as their frequency increases, contrary to simple beat frequency models, which predict a constant frequency difference. We show that the behaviour of these QPOs is instead well matched in terms of the fundamental frequencies (in the radial and azimuthal directions) for test particle motion in the gravitational field of the neutron star, for reasonable star masses, and nearly independent of the star spin. The radial frequency must be much smaller than the azimuthal one, testifying that kHz QPOs are produced close to the innermost stable orbit. These results are not reproduced through the post--Newtonian (PN) approximation of General Relativity (GR). kHz QPOs from X-ray binaries likely provide an accurate laboratory for strong field GR.

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arXiv1998

A gamma ray burst with small contamination

We present a scenario (SupraNova) for the formation of GRBs occurring when a supramassive neutron star (SMNS) loses so much angular momentum that centrifugal support against self--gravity becomes impossible, and the star implodes to a black hole. This may be the baryon--cleanest environment proposed so far, because the SN explosion in which the SMNS formed swept the medium surrounding the remnant, and the quickly spinning remnant loses energy through magnetic dipole radiation at a rate exceeding its Eddington luminosity by some four orders of magnitude. The implosion is adiabatic because neutrinos have short mean free paths, and silent, given the prompt collapse of the polar caps. However, a mass ~ 0.1 M_solar in the equatorial belt can easily reach centrifugal equilibrium. The mechanism of energy extraction is via the conversion of the Poynting flux (due to the large--scale magnetic field locked into the minitorus) into a magnetized relativistic wind. Occasionally this model will produce quickly decaying, or non--detectable afterglows.

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arXiv1998

Relativistic precession around rotating neutron stars: Effects due to frame-dragging and stellar oblateness

General relativity predicts that a rotating body produces a frame-dragging (or Lense-Thirring) effect: the orbital plane of a test particle in a non-equatorial orbit precesses about the body's symmetry axis. In this paper we compute the precession frequencies of circular orbits around rapidly rotating neutron stars for a variety of masses and equations of state. The precession frequencies computed are expressed as numerical functions of the orbital frequency observed at infinity. The post-Newtonian expansion of the exact precession formula is examined to identify the relative magnitudes of the precession caused by the Lense-Thirring effect, the usual Newtonian quadrupole effect and relativistic corrections. The first post-Newtonian correction to the Newtonian quadrupole precession is derived in the limit of slow rotation. We show that the post-Newtonian precession formula is a good approximation to the exact precession close to the neutron star in the slow rotation limit (up to \sim 400 Hz in the present context). The results are applied to recent RXTE observations of neutron star low-mass X-ray binaries, which display kHz quasi-periodic oscillations and, within the framework of beat frequency models, allow the measurement of both the neutron star spin frequency and the Keplerian frequency of the innermost ring of matter in the accretion disk around it. For a wide range of realistic equations of state, we find that the predicted precession frequency of this ring is close to one half of the low-frequency (\sim 20 - 35 Hz) quasi-periodic oscillations seen in several Atoll sources.

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arXiv1998

A new instability of accretion disks around compact magnetic stars

Aperiodic variability and Quasi Periodic Oscillations (QPOs) are observed from accretion disks orbiting white dwarfs, neutron stars, and black holes, suggesting that the flow is universally broken up into discrete blobs. We consider the interaction of these blobs with the magnetic field of a compact, accreting star, where diamagnetic blobs suffer a drag. We show that when the magnetic moment is not aligned with the spin axis, the resulting force is pulsed, and this can lead to resonance with the oscillation of the blobs around the equatorial plane; a resonance condition where energy is effectively pumped into non--equatorial motions is then derived. We show that the same resonance condition applies for the quadrupolar component of the magnetic field. We discuss the conditions of applicability of this result, showing that they are quite wide. We also show that realistic complications, such as chaotic magnetic fields, buoyancy, radiation pressure, evaporation, Kelvin--Helmholtz instability, and shear stresses due to differential rotation do not affect our results. In accreting neutron stars with millisecond periods, we show that this instability leads to Lense-Thirring precession of the blobs, and that damping by viscosity can be neglected.

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arXiv1997

Lense-Thirring Precession and QPOs in Low Mass X-Ray Binaries

We show in this Letter that relativistic dragging of inertial frames around fast rotating collapsed stars is substantial and can give rise to observable effects. We apply this to the kHz quasi periodic oscillations (QPOs) sources, low mass X-ray binaries (LMXRBs) containing an accreting neutron star. Within the beat frequency model, both the Keplerian frequency of the innermost region of the accretion disk (~0.3-1.2 kHz) and the neutron star spin frequency (~0.3-0.4 kHz) are directly observed. From these the Lense-Thirring precession frequency (tens of Hz) of the same material in the innermost disk regions which gives rise to the kHz QPOs is determined within a factor of ~4, depending on the neutron star equation of state. The classical contribution from neutron star oblateness decreases the precession frequency slightly. The broad peaks at frequencies ~20-40 Hz in the power spectra of the Atoll-sources 4U1728-34, 4U0614+091 and KS1731-260 and their variations with the higher kHz QPO frequency are well matched by Lense-Thirring precession of material in the innermost disk region. We also suggest that the ~15-50 Hz horizontal branch QPOs of GX5-1 and GX17+2 (and likewise other Z-type low mass X-ray binaries) arise from the same mechanism.

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arXiv1996

On the relationship between the periodic and aperiodic variability of accreting X-ray pulsars

Besides the narrow peaks originating from the periodic signal, the power spectra of accreting X-ray pulsars display continuum components usually increasing towards low frequencies; these arise from the source aperiodic variability. Most studies so far adopted the view that the periodic and aperiodic variations are independent. However any aperiodic variability in the emission from the accretion column(s) towards the magnetic neutron star should be modulated at the X-ray pulsar period, by virtue of the same rotation-induced geometric effects which give rise to the periodic signal. We develop here a simple shot noise model to test the presence of a coupling between the periodic and aperiodic variability of X-ray pulsars. The model power spectrum is fit to the power spectra of three X-ray pulsars, Vela X-1, 4U 1145-62 and Cen X-3 observed with EXOSAT. In the first two cases we find that a highly significant coupling is present, as testified by a substantialbroadening in the wings of the power spectrum peaks due to the periodic modulation. We find also that these wings can mimick the presence of a knee in the continuum power spectrum components around the fundamental of the periodic modulation, therefore questioning the correlation reported by Takeshima (1992) between the X-ray pulsar frequency and the knee frequency, beyond which the continuum power spectral component steepens.

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