Showing papers by "Luigi Stella published in 1997"
TL;DR: The relativistic dragging of inertial frames around fast rotating collapsed stars is substantial and can give rise to observable effects as mentioned in this paper, which can be applied to the kHz quasi periodic oscillations (QPOs) sources, low mass X-ray binaries (LMXRBs) containing an accreting neutron star.
Abstract: 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.
513 citations
TL;DR: In this paper, strong ~13 s X-ray pulsations in the ROSAT Position Sensitive Proportional Counter light curve of HD 49798, a 1.55 day single-component spectroscopic binary containing a hydrogen-depleted subdwarf O6 star was detected.
Abstract: We have discovered strong ~13 s X-ray pulsations in the ROSAT Position Sensitive Proportional Counter light curve of HD 49798, a 1.55 day single-component spectroscopic binary containing a hydrogen-depleted subdwarf O6 star. We find no evidence for period changes during the ~4 hr ROSAT pointing. The source X-ray spectrum is extremely soft, with an unabsorbed 0.1-2 keV luminosity of a few times 1032 ergs s-1 (distance of 650 pc). A higher luminosity might be hidden in the EUV. Our results imply that the unseen companion is an accreting degenerate star, a white dwarf, or, more likely, a neutron star. In any case, HD 49798 corresponds to a previously unobserved evolutionary stage of a massive binary system after the common-envelope phase and spiraling in.
67 citations
TL;DR: In this article, the RossiXTE observations of the anomalous X-ray pulsars 1E 1048.1-5937 and 1E 2259+586 were used to derive stringent limits on the projected semi-axis.
Abstract: 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.
64 citations
TL;DR: In this paper, a shot noise model was developed to test the presence of a coupling between the periodic and aperiodic variability of X-ray pulsars, and the model power spectrum was fitted to the power spectra of three x-ray pulses, Vela X-1, 4U 1145-62, and Cen X-3, observed with EXOSAT.
Abstract: Besides the narrow peaks originating from the periodic signal, the power spectra of accreting X-ray pulsars display continuum components usually increasing toward low frequencies; these arise from the source aperiodic variability. Most studies up to the present have adopted the view that the periodic and aperiodic variations are independent. However, any aperiodic variability in the emission from the accretion column(s) toward the magnetic neutron star should be modulated at the X-ray pulsar period, by virtue of the same rotation-induced geometric effects that 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 fitted 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 evidenced by a substantial broadening in the wings of the power spectrum peaks which are the result of the periodic modulation. We find also that these wings can mimic the presence of a knee in the continuum power spectrum components around the fundamental of the periodic modulation, therefore bringing into question the correlation reported by Takeshima between the X-ray pulsar frequency and the knee frequency, beyond which the continuum power spectral component steepens.
33 citations
TL;DR: In this article, a systematic search for periodic signals in a sample of ROSAT PSPC light curves was conducted, and the authors discovered 8.9 s X-ray pulsations in 2E 0050.1-7247, a variable Xray source in the Small Magellanic Cloud.
Abstract: During a systematic search for periodic signals in a sample of ROSAT PSPC light curves, we discovered ~8.9 s X-ray pulsations in 2E 0050.1-7247, 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 ~5 × 1034 ergs s-1 up to 1.4 × 1036 ergs s-1 with both the Einstein IPC and ROSAT PSPC. The X-ray energy spectrum is consistent with a power-law spectrum that steepens as the source luminosity decreases. We revealed a pronounced Hα activity from at least two B stars in the X-ray error circles. These results strongly suggest that the X-ray pulsar 2E 0050.1-7247 is in a Be-type massive binary.
32 citations
TL;DR: In this paper, the authors discovered 8.9s X-ray pulsations in 2E0050.1-7247 in the Small Magellanic Cloud (SMC) and revealed a pronounced Halpha activity from at least two B stars in the error circles.
Abstract: 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.
23 citations
Journal Article•
TL;DR: In this paper, a ROSAT-HRI observation of the soft X-ray transient Cen X-4 during quiescence was reported, and the authors found a variation in the flux by a factor of 3 in less than four days.
Abstract: We report on a ROSAT-HRI observation of the soft X-ray transient Cen X-4 during quiescence. We discover a variation in the flux by a factor of 3 in less than four days. This relatively fast variation, the first observed from a quiescent soft X-ray transient, rules out some of the emission mechanisms that have been proposed for the quiescent flux. Accretion either onto the neutron star surface or onto the magnetospheric boundary is clearly favored.
3 citations
Posted Content•
TL;DR: In this article, a ROSAT-HRI observation of the soft X-ray transient Cen X-4 during quiescence was reported, and the authors found a variation in the flux by a factor of 3 in less than four days.
Abstract: We report on a ROSAT-HRI observation of the soft X-ray transient Cen X-4 during quiescence. We discover a variation in the flux by a factor of 3 in less than four days. This relatively fast variation, the first observed from a quiescent soft X-ray transient, rules out some of the emission mechanisms that have been proposed for the quiescent flux. Accretion either onto the neutron star surface or onto the magnetospheric boundary is clearly favored.
2 citations
TL;DR: Anomalous X-ray Pulsars (AXPs) as discussed by the authors are a class of pulsars characterized by periods in the 5-10 s range and by the absence of massive companion stars.
Abstract: 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.
1 citations
Journal Article•
TL;DR: In this article, the positions of gamma ray bursts were not sufficiently well known within a short timescale to localize and identify them with known celestial sources, and the authors reported the detection of the same optical transient, in images obtained only 16 hours after the burst.
Abstract: 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.
1 citations
Posted Content•
TL;DR: In this article, the positions of gamma ray bursts were not sufficiently well known within a short timescale to localize and identify them with known celestial sources, and the authors reported the detection of the same optical transient, in images obtained only 16 hours after the burst.
Abstract: 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.
TL;DR: In this article, the authors discovered a periodicity at ∼8.7 s from the X-ray sources 4U142+61, previously considered a possible black hole candidate on the basis of its ultrasoft spectrum.