arXiv subjects

L. Stella

Publications and source records attributed to L. Stella.

At least 271 records · Page 15Linked to original sources

arXiv1999

BeppoSAX spectroscopy of the globular cluster X-ray source X1746-371 (NGC6441)

During a BeppoSAX observation of the X-ray source X1746-371 located in the globular cluster NGC6441 a type I X-ray burst, parts of 4 intensity dips, and energy dependent flaring were detected. The dips repeat every 5.8 +0.3 -0.9 hr and show no obvious energy dependence. If the dips are due to electron scattering this energy independence implies an abundance >130 times less than solar, confirming an earlier measurement. Since the overall cluster abundance is close to solar this low abundance is unexpected. Photoionization of the absorbing material, obscuration of an extended source, and variations in multiple components that combine to produce an apparent energy independence are all excluded. Thus, the nature of the dips remains uncertain. The best-fit model to the overall 0.3-30 keV spectrum is a disk-blackbody with a temperature of 2.82 +/- 0.04 keV, together with a cutoff power-law with a photon index of -0.32 +/- 0.80 and a cutoff energy of 0.90 +/- 0.26 keV. Absorption, consistent with the optical extinction to NGC6441 of (0.28 +/- 0.04) 10^22 atom/cm2 is required. All previous best-fit spectral models for this source are excluded at high confidence. The spectrum is dominated by the blackbody-like component, with the cutoff power-law only contributing an average of 12% of the 1-10 keV flux. During flaring intervals the contribution of this component decreases to \~6% with variations in the intensity of the blackbody-like component being responsible for most of the flaring activity.

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arXiv1999

A BeppoSAX study of the pulsating transient X0115+63: the first X-ray spectrum with four cyclotron harmonic features

The recurrent hard pulsating X-ray transient §was observed with \B on March 19, when the source was at a 2--10 keV flux level of $\sim$310~mCrab. We report on the high energy spectrum of the source, concentrating on cyclotron resonant scattering features. The spectrum is strongly pulse phase dependent and absorption features are detected at virtually all phases. In particular, four absorption-like features at 12.74 keV, 24.16 keV, 35.74 keV, and 49.5 keV are observed in the descending edge of the main peak of the pulse profile. The ratios between the centroid energies of the lines with respect to the first are 1:(1.9):(2.8):(3.9). These values are close to the harmonic relation expected from cyclotron resonant scattering in a strong magnetic field, when relativistic effects are taken into account. The equivalent widths of the second, third, and fourth harmonics are found to be larger than that of the first harmonic, confirming the key role of two-photon processes in the spectral formation. These results provide the first evidence for {\it four harmonically spaced lines} in the spectrum of an accreting X-ray pulsar, yielding the clearest confirmation to date of their magnetic origin.

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arXiv1999

ESO deep observations of the optical afterglow of GRB990510

We present the results of optical observations of the GRB990510 field carried out at different epochs from European Southern Observatory (ESO) telescopes. Deep observations, down to a limiting magnitude of about 27 and 24 in the Bessel-R and Gunn-I band, respectively, were obtained between May 16 and 18 from the ESO NTT-SUSI2 telescope and on May 20 from the ESO 3.6 m (EFOSC2) telescope. These observations, together with other published photometric data, allowed to monitor the faint tail of the decaying Optical Transient (OT) associated to the GRB990510. We discuss the light curves in the different filters (V, R and I) in the light of the recently proposed decay laws. No obvious host associated to the GRB990510 optical afterglow was found in the R and I band images. By comparing the light curves with respect to the theoretical colors of galaxies with different morphology we derived a lower limit of R about 26.6 for the host galaxy.

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arXiv1999

GRB 990510: linearly polarized radiation from a fireball

Models for gamma-ray burst afterglows envisage an hyper-relativistic fireball that is decelerated in the ambient medium around the explosion site. This interaction produces a shock wave which amplifies the magnetic field and accelerates electrons to relativistic energies, setting the conditions for an efficient production of synchrotron photons. If produced in a region of large-scale ordered magnetic field, synchrotron radiation can be highly polarized. The optical transient associated with GRB 990510 was observed ~18.5 hr after the event and linear polarization in the R band was measured at a level of 1.7 +/- 0.2 %. This is the first detection of linear polarization in the optical afterglow of a gamma-ray burst. We exclude that this polarization is due to dust in the interstellar material, either in our Galaxy or in the host galaxy of the gamma-ray burst. These results provide important new evidence in favor of the synchrotron origin of the afterglow emission, and constrains the geometry of the fireball and/or magnetic field lines.

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arXiv1999

On the anomalous X-ray afterglows of GRB 970508 and GRB 970828

Recently, BeppoSAX and ASCA have reported an unusual resurgence of soft X-ray emission during the afterglows of GRB 970508 and GRB 970828, together with marginal evidence for the existence of Fe-lines in both objects. We consider the implications of the existence of a torus of iron-rich material surrounding the sites of gamma ray bursts as would be expected in the SupraNova model; in particular, we show that the fireball will quickly hit this torus, and bring it to a temperature ~3x10^7 K. Bremsstrahlung emission from the heated up torus will cause a resurgence of the soft X-ray emission with all expected characteristics (flux level, duration and spectral hardening with time) identical to those observed during the reburst. Also, thermal emission from the torus will account for the observed iron line flux. These events are also observable, for instance by new missions such as SWIFT, when beaming away from our line sight makes us miss the main burst, as Fast (soft) X-ray Transients, with durations ~10^3 s, and fluences ~10^-7-10^-4 erg cm^-2. This model provides evidence in favor of the SupraNova model for Gamma Ray Bursts.

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arXiv1999

BeppoSAX monitoring of the ``anomalous'' X-ray pulsar 4U 0142+61

The 8.7s X-ray pulsar 4U 0142+61 was monitored by BeppoSAX between January 1997 and February 1998. This source belongs to the rapidly growing class of "anomalous" X-ray pulsars (AXPs) which have pulse periods in the 6-12s range and no plausible optical, infrared or radio counterparts. The BeppoSAX periods measurements show that 4U 0142+61 continues its spin-down at a nearly constant rate of about 2 times 10^{-12}s s^{-1}. The 0.5-10 keV pulse shape is double peaked. The phase-averaged spectrum can be described by a steep absorbed power-law (Gamma about 4) plus a blackbody with kT = 0.4 keV. 4U 0142+61 was also detected serendipitously in a March 1996 RossiXTE observation pointed towards the nearby 1455s X-ray pulsar RX J0146.9+6121. The timing analysis results are reported for this observation and the spin history of 4U 0142+61 since 1979 is discussed.

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arXiv1999

BeppoSAX observations of the black hole candidates LMC X-1 and LMC X-3

We describe BeppoSAX observations of the black hole candidates LMC X--1 and LMC X--3 performed in Oct. 1997. Both sources can be modelled by a multicolor accretion disk spectrum, with temperature $\sim 1$ keV. However, there is some evidence that a thin emitting component coexists with the thick disk at these temperatures. In the direction of LMC X--1, we detected a significant emission above 10 keV, which we suspect originates from the nearby source PSR 0540-69. For LMC X--1, we estimate an absorbing column density of $\simeq 6\times 10^{21}$ cm$^{-2}$, which is almost ten times larger than that found for LMC X--3. In both sources, we find no indication of emission or absorption features whatsoever.

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arXiv1999

The identification of the long-period X-ray pulsar 1WGA J1958.2+323 with a Be-star/X-ray binary

We present the results of optical observations performed between May and September 1998 of the stars within the position error circle of the recently discovered 12min pulsating X-ray source 1WGA J1958.2+3232. Based on photometry and slitless spectroscopy, we selected a likely optical counterpart, which was subsequently determined to be a m_V=15.7 B0Ve star, the spectral properties of which are in good agreement with the X-ray results. The proposed optical counterpart shows several H, He and Fe emission-lines, while the interstellar absorption spectral features place the star at a distance of about 800 pc.The inferred X-ray luminosity for this distance is about 10^33 erg s^-1 in the 2--10 keV band. We conclude that 1WGA J1958.2+3232 is a likely new long-period low-luminosity accreting neutron star in a Be-star/X-ray binary system.

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arXiv1998

On gravitomagnetic precession around black holes

We compute exactly the Lense-Thirring precession frequency for point masses in the Kerr metric, for arbitrary black hole mass and specific angular momentum. We show that this frequency, for point masses at or close to the innermost stable orbit, and for holes with moderate to extreme rotation, is less than, but comparable to the rotation frequency. Thus, if the quasi periodic oscillations (QPOs) observed in the modulation of the X-ray flux from some black holes candidates are due to Lense-Thirring precession of orbiting material, we predict that a separate, distinct QPO ought to be observed in each object.

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arXiv1998

The Comptonized X-ray source X1724-308 in the globular cluster Terzan 2

We report on the BeppoSAX observation of the X-ray source X1724-308 in the globular cluster Terzan 2. The broadband spectrum can be described as the superposition of a power-law with photon spectral index 1.6-1.9 and a thermal component with typical temperature ~1 keV. X1724-308is detected in the PDS up to 150 keV with a S/N ratio >3, notwithstanding a sharp exponential cutoff at \~90 keV. The broadband spectrum can be best interpreted as due to the Comptonization of a Wien photon distribution with kT ~1 keV by a spherically-symmetric hot plasma with electron temperature ~30 keV and optical depth ~3. A comparison with the fainter and steeper state, which was observed by EXOSAT, suggests a correlation between the temperature of the Comptonizing electron distribution (or the plasma optical depth) and the X-ray intensity. The X-ray thermal component could originate from a boundary layer between the accretion disk and the compact object, or from the accretion disk itself.

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arXiv1998

The Neutron Stars of Soft X-Ray Transients

Soft X-ray Transients (SXRTs) have long been suspected to contain old, weakly magnetic neutron stars that have been spun up by accretion torques. After reviewing their observational properties, we analyse the different regimes that likely characterise the neutron stars in these systems across the very large range of mass inflow rates, from the peak of the outbursts to the quiescent emission. While it is clear that close to the outburst maxima accretion onto the neutron star surface takes place, as the mass inflow rate decreases, accretion might stop at the magnetospheric boundary because of the centrifugal barrier provided by the neutron star. For low enough mass inflow rates (and sufficiently short rotation periods), the radio pulsar mechanism might turn on and sweep the inflowing matter away. The origin of the quiescent emission, observed in a number of SXRTs at a level of ~10^(32)-10^(33) erg/s, plays a crucial role in constraining the neutron star magnetic field and spin period. Accretion onto the neutron star surface is an unlikely mechanism for the quiescent emission of SXRTs, as it requires very low magnetic fields and/or long spin periods. Thermal radiation from a cooling neutron star surface in between the outbursts can be ruled out as the only cause of the quiescent emission. We find that accretion onto the neutron star magnetosphere and shock emission powered by an enshrouded radio pulsar provide far more plausible models. In the latter case the range of allowed neutron star spin periods and magnetic fields is consistent with the values recently inferred from the properties of kHz QPO in LMXRBs. If quiescent SXRTs contain enshrouded radio pulsars, they provide a missing link between X-ray binaries and millisecond pulsars.

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arXiv1998

ROSAT archival observations of 1SAX J0054.9-7226 = 2E 0053.2-7242, a newly discovered X-ray pulsar in the SMC

We analysed 13 archival ROSAT PSPC and HRI observations which included the position of a newly discovered 59s X-ray pulsar in the Small Magellanic Cloud, 1SAX J0054.9-7226 = 2E 0053.2-7242. The source was detected three times between 1991 and 1996 at a luminosity level ranging from about 8 x 10^{34} - 4 x 10^{35} erg s^{-1} (0.1-2.4 keV). Highly significant pulsations at 59.072s were detected during the 1991 October 8-9 observation. The ROSAT period, together with those measured by RXTE and BeppoSAX yields a period derivative of Pdot = - 0.016 s yr^{-1}. A much more accurate source position (10 arcsec uncertainty) was obtained through the ROSAT HRI detection on 1996 April restricting to three m_V > 15.5 stars the likely counterpart of 1SAX J0054.9-7226 = 2E 0053.2-7242.

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arXiv1998

Aquila X-1 from outburst to quiescence: the onset of the propeller effect and signs of a turned-on rotation-powered pulsar

We report on the March-April 1997 BeppoSAX observations of Aql X-1, the first to monitor the evolution of the spectral and time variability properties of a neutron star soft X-ray transient from the outburst decay to quiescence. We observed a fast X-ray flux decay, which brought the source luminosity from ~10^36 to ~10^{33} erg/s in less than 10 days. The X-ray spectrum showed a power law high energy tail with photon index Gamma~2 which hardened to Gamma~1-1.5 as the source reached quiescence. These observations, together with the detection by RossiXTE of a periodicity of a few milliseconds during an X-ray burst, likely indicate that the rapid flux decay is caused by the onset of the propeller effect arising from the very fast rotation of the neutron star magnetosphere. The X-ray luminosity and hard spectrum that characterise the quiescent emission can be consistently interpreted as shock emission by a turned-on rotation-powered pulsar.

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arXiv1998

The discovery of 12min X-ray pulsations from 1WGA J1958.2+3232

During a systematic search for periodic signals in a sample of ROSAT PSPC (0.1-2.4 keV) light curves, we discovered 12min large amplitude X-ray pulsations in 1WGA J1958.2+3232, an X-ray source which lies close to the galactic plane. The energy spectrum is well fit by a power law with a photon index of 0.8, corresponding to an X-ray flux of about 10E-12 ergs cmE-2 sE-1. The source is probably a long period, low luminosity X-ray pulsar, similar to X Per, or an intermediate polar.

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arXiv1997

Recent Results on the Anomalous X-ray Pulsars

The ''Anomalous X-ray Pulsars'' (AXPs) are a small group of X-ray pulsars characterized by periods in the 5-10 s range and by the absence of massive companion stars. There are now 7 possible members of this class of objects. We review recent observational results on their X-ray spectra, spin period evolution, and searches for orbital motion and discuss the implications for possible models.

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arXiv1997

New limits on the orbital parameters of 1E1048.1-5937 and 1E2259+586 from RossiXTE observations

We report on two RossiXTE observations of the anomalous X-ray pulsars 1E 1048.1-5937 and 1E 2259+586. Both sources have continued their almost constant spin-down during 1995/96. We carried out a search for orbital Doppler shifts, in their observed spin frequencies, deriving stringent limits on the projected semi-axis. Unless these systems have unlikely small inclinations, main sequence companions can be excluded. If 1E 1048.1-5937 and 1E 2259+586 are indeed binary systems, their companion stars must be either white dwarfs, or helium-burning stars with M < 0.8 Msolar, possibly underfilling their Roche lobe.

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arXiv1997

The Optical Transient of GRB970228, 16 hours after the burst

Until recently the positions of gamma ray bursts were not sufficiently well known within a short timescale to localize and identify them with known celestial sources. Following the historical detection of the X-ray afterglow of the burst GRB970228, extending from 30 s to 3 days after the main peak, by the Beppo-SAX satellite and that of an optical transient 21 hr after the burst, we report here the detection of the same optical transient, in images obtained only 16 hours after the burst.

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arXiv1997

The discovery of 8.9s pulsations from the variable X-ray source 2E0050.1-7247 in the small Magellanic cloud

During a systematic search for periodic signals in a sample of ROSAT PSPC light curves, we discovered ~ 8.9s X-ray pulsations in 2E0050.1-7247, a variable X-ray source in the Small Magellanic Cloud. The source was detected several times between 1979 and 1993 at luminosity levels ranging from about 5 times 10^{34} erg/s up to 1.4 times 10^{36} erg/s with both the Einstein IPC and ROSAT PSPC. The X-ray energy spectrum is consistent with a power law spectrum which steepens as the source luminosity decreases. We revealed a pronounced Halpha activity from at least two B stars in the X-ray error circles. These results strongly suggest that the X-ray pulsar 2E0050.1-7247 is in a Be-type massive binary.

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