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1.
We report the discovery of type I X-ray bursts from the low-mass X-ray binary  4U 1708 − 40  during the 100-ks observation performed by BeppoSAX on 1999 August 15–16. Six X-ray bursts have been observed. The unabsorbed 2–10 keV fluxes of the bursts range from ∼3 to  9 × 10−10 erg cm−2 s−1  . A correlation between peak flux and fluence of the bursts is found, in agreement with the behaviour observed in other similar sources. There is a trend of the burst flux to increase with the time interval from the previous burst. From the value of the persistent flux we infer a mass accretion rate     , which may correspond to the mixed hydrogen/helium burning regime triggered by thermally unstable hydrogen. We have also analysed a BeppoSAX observation performed on 2001 August 22 and previous RXTE observations of  4U 1708 − 40  , where no bursts have been observed; we find persistent fluxes of more than a factor of 7 higher than the persistent flux observed during the BeppoSAX observation showing X-ray bursts.  相似文献   

2.
We have discovered a triple-peaked X-ray burst from the low-mass X-ray binary (LMXB) 4U 1636−53 with the Rossi X-ray Timing Explorer ( RXTE ). This is the first triple-peaked burst reported from any LMXB using RXTE , and it is only the second burst of this kind observed from any source. (The previous one was also from 4U 1636−53, and was observed with EXOSAT .) From fits to time-resolved spectra, we find that this is not a radius-expansion burst, and the same triple-peaked pattern seen in the X-ray light curve is also present in the bolometric light curve of the burst. Similar to what was previously observed in double-peaked bursts from this source, the radius of the emitting area increases steadily during the burst, with short periods in between during which the radius remains more or less constant. The temperature first increases steeply, and then decreases across the burst also showing three peaks. The first and last peak in the temperature profile occur, respectively, significantly before and after the first and last peaks in the X-ray and bolometric light curves. We found no significant oscillations during this burst. This triple-peaked burst, as well as the one observed with EXOSAT and the double-peak bursts in this source, all took place when 4U 1636−53 occupied a relatively narrow region in the colour–colour diagram, corresponding to a relatively high (inferred) mass-accretion rate. No model presently available is able to explain the multiple-peaked bursts.  相似文献   

3.
For the bright neutron star low-mass X-ray binary Sco X-1, we analyzed all updated frequencies of the twin kilohertz quasi-periodic oscillations (kHz QPOs), their correlations and distributions. We found that the frequency separation of the kHz QPO peaks appears not to be a constant, rather, it decreases with increasing inferred mass accretion rate. We show that the currently available data of Sco X-1 by Rossi X-ray Timing Explorer are inconsistent with the proposals of the beat model that the frequency separation is a constant. Our conclusions are consistent with those of some previous researchers and we discuss further implications for the kilohertz QPO models.  相似文献   

4.
In this paper we investigate the dynamical behaviour of radiation-driven winds, specifically winds that arise when Compton scattering transfers momentum from the radiation field to the gas flow. Such winds occur during strong X-ray bursts from slowly accreting neutron stars, and also may be driven from the inner regions of a black hole or neutron star accretion disc when the mass transfer rate is very high. By linearizing the radiation hydrodynamic equations around steady spherical outflow, we evaluate the time-dependent response of these winds to perturbations introduced at their inner boundaries. We find that although radiation-driven winds are generally stable, they act as mechanical filters that should produce quasi-periodic oscillations or peaked noise in their radiation output when perturbations force them stochastically. This behaviour may underlie the photospheric oscillations observed in some strong Type I X-ray bursts.  相似文献   

5.
Precision measurements of neutron star radii can provide a powerful probe of the properties of cold matter beyond nuclear density. Beginning in the late 1970s, it was proposed that the radius could be obtained from the apparent or inferred emitting area during the decay portions of thermonuclear (type I) X-ray bursts. However, this apparent area is generally not constant, preventing a reliable measurement of the source radius. Here, we report for the first time a correlation between the variation of the inferred area and the burst properties, measured in a sample of almost 900 bursts from 43 sources. We found that the rate of change of the inferred area during decay is anticorrelated with the burst decay duration. A Spearman rank correlation test shows that this relation is significant at the  <10−45  level for our entire sample, and at the  7 × 10−37  level for the 625 bursts without photospheric radius expansion. This anticorrelation is also highly significant for individual sources exhibiting a wide range of burst durations, such as 4U 1636–536 and Aql X-1. We suggest that variations in the colour factor, which relates the colour temperature resulted from the scattering in the neutron star atmosphere to the effective temperature of the burning layer, may explain the correlation. This in turn implies significant variations in the composition of the atmosphere between bursts with long and short durations.  相似文献   

6.
We present an analysis of the X-ray spectra of two strongly magnetic cataclysmic variables, DP Leo and WW Hor, made using XMM-Newton . Both systems were in intermediate levels of accretion. Hard optically thin X-ray emission from the shocked accreting gas was detected from both systems, while a soft blackbody X-ray component from the heated surface was detected only in DP Leo. We suggest that the lack of a soft X-ray component in WW Hor is owing to the fact that the accretion area is larger than in previous observations with a resulting lower temperature for the re-processed hard X-rays. Using a multi-temperature model of the post-shock flow, we estimate that the white dwarf in both systems has a mass greater than 1 M⊙. The implications of this result are discussed. We demonstrate that the 'soft X-ray excess' observed in many magnetic cataclysmic variables can be partially attributed to using an inappropriate model for the hard X-ray emission.  相似文献   

7.
We have observed the eclipsing low-mass X-ray binary MS 1603.6+2600 with Chandra for 7 ks. The X-ray spectrum is well fit with a single absorbed power law with an index of ∼2. We find a clear sinusoidal modulation in the X-ray light curve with a period of  1.7 ± 0.2 h  , consistent with the period of 1.85 h found before. However, no (partial) eclipses were found. We argue that if the X-ray flare observed in earlier X-ray observations was a type I X-ray burst, then the source can only be an accretion disc corona source at a distance of ∼11–24 kpc (implying a height above the Galactic disc of ∼8–17 kpc). It has also been proposed in the literature that MS 1603.76+2600 is a dipper at ∼75 kpc. We argue that, in this dipper scenario, the observed optical properties of MS 1603.6+2600 are difficult to reconcile with the optical properties one would expect on the basis of comparisons with other high-inclination, low-mass X-ray binaries, unless the X-ray flare was not a type I X-ray burst. In that case, the source can be a nearby soft X-ray transient accreting at a quiescent rate, as was proposed by Hakala et al., or a high-inclination source at ∼15–20 kpc.  相似文献   

8.
UW CrB (MS 1603+2600) is a peculiar short-period X-ray binary that exhibits extraordinary optical behaviour. The shape of the optical light curve of the system changes drastically from night to night, without any changes in overall brightness. Here we report X-ray observations of UW CrB obtained with XMM–Newton . We find evidence for several X-ray bursts, confirming a neutron star primary. This considerably strengthens the case that UW CrB is an accretion disc corona system located at a distance of at least 5–7 kpc (3–5 kpc above the Galactic plane). The X-ray and Optical Monitor (ultraviolet–optical) light curves show remarkable shape variation from one observing run to another, which we suggest are due to large-scale variations in the accretion disc shape resulting from a warp that periodically obscures the optical and soft X-ray emission. This is also supported by the changes in phase-resolved X-ray spectra.  相似文献   

9.
We examine the spectra of the persistent emission from anomalous X-ray pulsars (AXPs) and their variati on with the spin-down rate Ω. Based onan accretion-powered model, the influences of both the magnetic field and the mass accretion rate on the spectral properties of AXPs are addressed. We then investigate the relation between the spectral property of AXPs and mass accretion rate M. The result shows that there exists a linear correlation between the photon index and the mass accretion rate: the spectral hardness increases with increasing M. A possible emission mechanism for the explanation of the spectral properties of AXPs is also discussed.  相似文献   

10.
We report here results from a study of X-ray bursts from 3 magnetar candidates (SGR 1806-20, SGR 1900+14 and AXP 1E 2259+586). We have searched for a pulse phase dependence of the X-ray burst rate from these sources. X-ray light curves were obtained with the Proportional Counter Array on-board the Rossi X-ray Timing Explorer during the periods of intense burst activity in these sources. On detailed analysis of the three sources, we found a very significant burst rate for all pulsar phases. However, some locations appear to produce bursts slightly more often, rendering the non-isotropic distribution. Only in the case of SGR 1900+14, there is a clear pulse phase dependence of burst rate.  相似文献   

11.
Eddington-limited X-ray bursts from neutron stars can be used in conjunction with other spectroscopic observations to measure neutron star masses, radii and distances. In order to quantify some of the uncertainties in the determination of the Eddington limit, we analysed a large sample of photospheric radius-expansion thermonuclear bursts observed with the Rossi X-ray Timing Explorer . We identified the instant at which the expanded photosphere 'touches down' back on to the surface of the neutron star and compared the corresponding touchdown flux to the peak flux of each burst. We found that for the majority of sources, the ratio of these fluxes is smaller than ≃1.6, which is the maximum value expected from the changing gravitational redshift during the radius expansion episodes (for a  2 M⊙  neutron star). The only sources for which this ratio is larger than ≃1.6 are high-inclination sources that include dippers and Cyg X-2. We discuss two possible geometric interpretations of this effect and show that the inferred masses and radii of neutron stars are not affected by this bias. On the other hand, systematic uncertainties as large as ∼50 per cent may be introduced to the distance determination.  相似文献   

12.
The recent detection of a transient absorption feature in the X-ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma-ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor.  相似文献   

13.
With extensive monitoring data spanning over 30 years from Vela 5B , Ariel 5 , Ginga , Compton Gamma Ray Observatory , Rossi X-ray Timing Explorer and BeppoSAX , we find evidence for long-term X-ray variability on time-scales     from the black hole low-mass X-ray binary system     . Such variability resembles the outburst cycle of Z Cam-type dwarf novae, in which the standard disc instability model plays a crucial role. If such a model is applicable to     , then the observed variability might be due to the irradiation of an unstable accretion disc. We show that within the framework of the X-ray irradiation model, when the accretion rate exceeds a critical value,     enters a 'flat-topped' high/soft state, such as seen in 1998, which we suggest corresponds to the 'standstill' state of Z Cam systems.  相似文献   

14.
We present the broad-band noise structure of selected anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) in the 2–60 keV energy band. We have analysed Rossi X-Ray Timing Explorer Proportional Counter Array archival light curves for four AXPs and one SGR. We detect that the persistent emission of these sources shows band-limited noise at low frequencies in the range 0.005–0.05 Hz varying from 2.5 to 70 per cent integrated rms in times of prolonged quiescence and following outbursts. We discovered band-limited red noise in 1E 2259+586 only for ∼2 yr after its major 2002 outburst. The system shows no broad-band noise otherwise. Although this rise in noise in 1E 2259+586 occurred following an outburst which included a rotational glitch, the other glitching AXPs showed no obvious change in broad-band noise, thus it does not seem that this noise is correlated with glitches. The only source that showed significant variation in broad-band noise was 1E   1048.1−5937  , where the noise gradually rose for 1.95 yr at a rate of ∼3.6 per cent per year. For this source the increases in broad-band noise was not correlated with the large increases in persistent and pulsed flux, or its two short SGR-like bursts. This rise in noise did commence after a long burst, however, given the sparsity of this event, and the possibility that similar bursts went unnoticed the trigger for the rise is noise in 1E   1048.1−5937  is not as clear as for 1E 2259+586. The other three sources indicate a persistent band-limited noise at low levels in comparison.  相似文献   

15.
We describe the evolutionary progression of an outburst of the Rapid Burster. Four outbursts have been observed with the Rossi X-Ray Timing Explorer between 1996 February and 1998 May, and our observations are consistent with a standard evolution over the course of each. An outburst can be divided into two distinct phases. Phase I is dominated by type I bursts, with a strong persistent emission component; it lasts for 15–20 d. Phase II is characterized by type II bursts, which occur in a variety of patterns. The light curves of time-averaged luminosity for the outbursts show some evidence for reflares, similar to those seen in soft X-ray transients. The average recurrence time for Rapid Burster outbursts during this period was 218 d, in contrast to an average ∼180‐d recurrence period observed during 1976–1983.  相似文献   

16.
We present a general relativistic accretion disc model and its application to the soft-state X-ray spectra of black hole binaries. The model assumes a flat, optically thick disc around a rotating Kerr black hole. The disc locally radiates away the dissipated energy as a blackbody. Special and general relativistic effects influencing photons emitted by the disc are taken into account. The emerging spectrum, as seen by a distant observer, is parametrized by the black hole mass and spin, the accretion rate, the disc inclination angle and the inner disc radius.
We fit the ASCA soft-state X-ray spectra of LMC X-1 and GRO J1655-40 by this model. We find that, having additional limits on the black hole mass and inclination angle from optical/UV observations, we can constrain the black hole spin from X-ray data. In LMC X-1 the constraint is weak, and we can only rule out the maximally rotating black hole. In GRO J1655-40 we can limit the spin much better, and we find 0.68 a 0.88 . Accretion discs in both sources are radiation-pressure dominated. We do not find Compton reflection features in the spectra of any of these objects.  相似文献   

17.
We present an analysis of X-ray and ultraviolet (UV) data of the dwarf nova VW Hyi that were obtained with XMM–Newton during the quiescent state. The X-ray spectrum indicates the presence of an optically thin plasma in the boundary layer that cools as it settles on to the white dwarf. The plasma has a continuous temperature distribution that is well described by a power law or a cooling flow model with a maximum temperature of 6–8 keV. We estimate from the X-ray spectrum a boundary layer luminosity of  8 × 1030 erg s-1  , which is only 20 per cent of the disc luminosity. The rate of accretion on to the white dwarf is  5 × 10−12 M⊙ yr−1  , about half of the rate in the disc. From the high-resolution X-ray spectra, we estimate that the X-ray emitting part of the boundary layer is rotating with a velocity of 540 km s−1, which is close to the rotation velocity of the white dwarf but is significantly smaller than the Keplerian velocity. We detect a 60-s quasi-periodic oscillation of the X-ray flux, which is likely to be due to the rotation of the boundary layer. The X-ray and the UV flux show strong variability on a time-scale of ∼1500 s. We find that the variability in the two bands is correlated and that the X-ray fluctuations are delayed by ∼100 s. The correlation indicates that the variable UV flux is emitted near the transition region between the disc and the boundary layer and that accretion rate fluctuations in this region are propagated to the X-ray emitting part of the boundary layer within ∼100 s. An orbital modulation of the X-ray flux suggests that the inner accretion disc is tilted with respect to the orbital plane. The elemental abundances in the boundary layer are close to their solar values.  相似文献   

18.
This work presents a possible detection mechanism for close, detached, neutron star–red dwarf binaries, which are expected to be the evolutionary precursors of low-mass X-ray binaries (LMXBs). Although this pre-low-mass X-ray binary (pre-LMXB) phase of evolution is predicted theoretically, as yet no such systems have been identified observationally. The calculations presented here suggest that the X-ray luminosity of neutron star wind accretion in a pre-LMXB system can be expected to exceed the intrinsic X-ray luminosity of the red dwarf secondary star. Furthermore, the temperature of the radiation emitted from the neutron star wind accretion process is expected, within the confines of a reasonable set of conditions, to lie within the detection range of X-ray satellites. Sources with X-ray luminosities greater than that expected for a red dwarf star, but the positions of which coincide with that of a red dwarf star, are then candidate pre-LMXB systems. These candidate systems should be surveyed for the radial velocity shifts that would occur as a result of the orbital motion of a red dwarf star within a close binary system containing a high-mass compact object.  相似文献   

19.
The galactic black hole candidate Cygnus X-1 is observed to be in one of two X-ray spectral states: either the low/hard (low soft X-ray flux and a flat power-law tail) or high/soft (high blackbody-like soft X-ray flux and a steep power-law tail) state. The physical origin of these two states is unclear. We present here a model of an ionized accretion disc, the spectrum of which is blurred by relativistic effects, and fit it to the ASCA , Ginga and EXOSAT data of Cygnus X-1 in both spectral states. We confirm that relativistic blurring provides a much better fit to the low/hard state data and, contrary to some previous results, find the data of both states to be consistent with an ionized thin accretion disc with a reflected fraction of unity extending to the innermost stable circular orbit around the black hole. Our model is an alternative to those that, in the low/hard state, require the accretion disc to be truncated at a few tens of Schwarzschild radii, within which there is a Thomson-thin, hot accretion flow. We suggest a mechanism that may cause the changes in spectral state.  相似文献   

20.
We investigate the process of synchrotron radiation from thermal electrons at semirelativistic and relativistic temperatures. We find an analytic expression for the emission coefficient for random magnetic fields with an accuracy significantly higher than those derived previously. We also present analytic approximations to the synchrotron turnover frequency, treat Comptonization of self-absorbed synchrotron radiation, and give simple expressions for the spectral shape and the emitted power. We also consider modifications of the above results by bremsstrahlung.
We then study the importance of Comptonization of thermal synchrotron radiation in compact X-ray sources. We first consider emission from hot accretion flows and active coronae above optically thick accretion discs in black hole binaries and active galactic nuclei (AGNs). We find that for plausible values of the magnetic field strength, this radiative process is negligible in luminous sources, except for those with hardest X-ray spectra and stellar masses. Increasing the black hole mass results in a further reduction of the maximum Eddington ratio from this process. Then, X-ray spectra of intermediate-luminosity sources, e.g. low-luminosity AGNs, can be explained by synchrotron Comptonization only if they come from hot accretion flows, and X-ray spectra of very weak sources are always dominated by bremsstrahlung. On the other hand, synchrotron Comptonization can account for power-law X-ray spectra observed in the low states of sources around weakly magnetized neutron stars.  相似文献   

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