摘要:

The coupled evolution of the spin frequency, core temperature, and r‐mode amplitude of an accreting neutron star is calculated. We focus on those conditions that can produce persistent gravitational radiation from the r‐mode. During X‐ray quiescent phases of transient LMXBs, one may be able to identify the constant contribution of the gravitational wave emission to the spindown rate. Another signature is the r‐mode contribution to the heating. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1781031

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

By analyzing theRossi X‐ray Timing Explorerdata on SAX J1808.4‐3658, we show that the X‐ray emission in accretion powered millisecond pulsars can be produced by Comptonization in a hot slab (radiative shock) of Thomson optical depth &tgr;es∼ 1 at the neutron star surface. The escaping radiation consists of two components: a black body and a hard Comptonized tail. These components have very different angular distribution: the black body peaks along the slab normal (a “pencil‐like” emission pattern), while the tail has a broader angular distribution (a “fan”‐like pattern). This results in very different variability properties. We construct a detailed model of the X‐ray production accounting for the Doppler boosting, relativistic aberration and gravitational light bending. We are able to reproduce the pulse profiles at different energies, corresponding phase lags, as well as the time‐averaged spectrum. We obtain constraints on the neutron star radius:R∼ 11 km if its massM= 1.6M⊙, andR∼ 8.5 km ifM= 1.4M⊙. We present simple analytical formulae for computing the light curves and oscillation amplitudes expected from hot spots in X‐ray bursters and accretion powered millisecond pulsars. We also propose an analytical expression that can be used to determine the size of the black body emission region from the observed properties. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1781032

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

As part of a large‐scale search for coherent pulsations from LMXBs in theRXTEarchive, we have completed a detailed series of searches for coherent pulsations of 4U 1820‐30 — an ultracompact LMXB with a binary period of 11.4 minutes located in the globular cluster NGC 6624. The small binary period leads to a very high acceleration, so we used phase modulation searches as well as acceleration searches to give significant sensitivity to millisecond pulsations. We searched a total of 34 archivalRXTEobservations, 32 of which had an on‐source integration time longer than 10 ks, and some of which were made consecutively which allowed us to combine them. While we found no pulsations, we have been able to place the first stringent (95&percent; confidence) pulsed fraction limits of ≲0.8&percent; for all realistic spin frequencies (i.e. ≲1 kHz) and likely companion masses (0.02 M⊙⩽Mc⩽ 0.3 M⊙). By contrast all five LMXBs known to emit coherent pulsations have intrinsic pulsed fractions in the range 3&percent; to 7&percent; when pulsations are observed. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1781033

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

I review the basic phenomenology and theory of the millisecond brightness oscillations observed during thermonuclear X‐ray bursts from 12 of ≈ 65 accreting neutron stars in low‐mass X‐ray binaries. Compelling observations indicate that the oscillations are produced by surface brightness patterns on the rapidly rotating neutron stars. However, it remains to be understood (1) why the brightness patterns producing them persist for up to 15 s during an X‐ray burst, whereas the burning should cover the entire surface in less than 1 s, and (2) why the frequencies drift upward by ≈ 5 Hz during the course of the burst. These peculiarities can probably be explained by taking into account the expansion of the surface layers caused by the burning, zonal flows that form due to pressure gradients between the equator and poles, and Rossby‐Alfve´n modes that are excited in the surface ocean. Further progress toward understanding how burning progresses on the surface of the neutron star can be made with a next‐generation X‐ray timing mission, which would provide a larger sample of sources with oscillations, detect sideband signals produced by the spectrum of modes that should be excited in the neutron star ocean, and measure harmonic structure in the profiles of the oscillations. These observations would be crucial for measuring the distribution of the rotation rates of neutron stars, the progression of unstable nuclear burning in the accreted ocean, and the curvature of the space‐time around the neutron star. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1781034

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Observations with NASA’s Rossi X‐ray Timing Explorer (RXTE) have resulted in the discovery of fast (200 – 600 Hz), coherent X‐ray intensity oscillations (hereafter, “burst oscillations”) during thermonuclear X‐ray bursts from 12 low mass X‐ray binaries (LMXBs). Although many of their detailed properties remain to be fully understood, it is now beyond doubt that these oscillations result from spin modulation of the thermonuclear burst flux from the neutron star surface. Among the new timing phenomena revealed by RXTE the burst oscillations are perhaps the best understood, in the sense that many of their properties can be explained in the framework of this relatively simple model. Because of this, detailed modelling of burst oscillations can be an extremely powerful probe of neutron star structure, and thus the equation of state (EOS) of supranuclear density matter. Both the compactness parameter &bgr; =GM/c2R, and the surface velocity,vrot= &OHgr;spinR, are encoded in the energy‐dependent amplitude and shape of the modulation pulses. The new discoveries have spurred much new theoretical work on thermonuclear burning and propagation on neutron stars, so that in the near future it is not unreasonable to think that detailed physical models of the time dependent flux from burning neutron stars will be available for comparison with the observed pulse profiles from a future, large collecting area X‐ray timing observatory. In addition, recent high resolution burst spectroscopy with XMM/Newton suggests the presence of redshifted absorption lines from the neutron star surface during bursts. This leads to the possibility of using large area, high spectral resolution measurements of X‐ray bursts as a precise probe of neutron star structure. In this work I will explore the precision with which constraints on neutron star structure, and hence the dense matter EOS, can be made with the implementation of such programs. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1781035

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Presently seven superbursters have been identified representing 10&percent; of the total Galactic X‐ray burster population. Four superbursters were discovered with the Wide Field Cameras (WFCs) on BeppoSAX and three with the All‐Sky Monitor and Proportional Counter Array on RXTE. We discuss the properties of superbursters as derived from WFC observations. There are two interesting conclusions. First, theaveragerecurrence time of superbursts among X‐ray bursters that are more luminous than 10&percent; of the Eddington limit is 1.5 yr per object. Second, superbursters systematically have higher &agr; values and shorter ordinary bursts than most bursters that have not exhibited superbursts, indicating a higher level of stable thermonuclear helium burning. Theory predicts hitherto undetected superbursts from the most luminous neutron stars. We investigate the prospects for finding these in GX 17+2. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1781036

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Preliminary results are reported on the spectral and timing properties of the spectacular 2001 superburst of 4U 1636‐536 as seen by the RXTE/PCA. The (broad‐band) power‐spectral and hardness properties during the superburst are compared to those just before and after the superburst. Not all of the superburst emission can be fitted by pure black‐body radiation. We also gathered BeppoSAX/WFC and RXTE/ASM data, as well as other RXTE/PCA data, obtained days to months before and after the superburst to investigate the normal X‐ray burst behavior around the time of the superburst. The first normal X‐ray burst after the 2001 superburst was detected ≃23 days later. During inspection of all the RXTE/ASM data we found a third superburst. This superburst took place on June 26, 1999, which is ≃2.9 yrs after the 1996 superburst and ≃1.75 yrs before the 2001 superburst. The above findings are the strongest constraints observed so far on the duration of the cessation of normal X‐ray bursts after a superburst and the superburst recurrence times. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1781037

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We present the results of analysis of dynamic X‐ray burst spectra from a set of LMXBs sources observed with RXTE. The application of theory of spectral formation during decay stage of X‐ray burst allows to determine the fundamental parameters of a bursting compact star. The method implies the fitting of bolometric flux — color temperature dependence of burst radiation with the theoretical model which includes neutron star (NS) mass, radius, atmospheric chemical composition and the source distance as parameters. So far the technique was applied to Cyg X‐2, 4U 1728‐34, 4U 1820‐30. Our results generally support that burst atmosphere is usually helium‐dominated and neutron stars are more compact than standard NS. Superior statistics of the data for 4U 1728‐34 also allowed to detect the effect of a geometry evolution during X‐ray burst with radial expansion which is due to strong interaction of a burst with accretion disk. The effect is even more apparent for strange radial expansion burst from 4U 1820‐30. The application of new results for the nuclear matter equation of state to the compactness of a star in 4U 1728‐34 indicate that core of the star may contain quark condensate. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1781038

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Recently we have made measurements of thermonuclear burst energetics and recurrence times which are unprecedented in their precision, largely thanks to the sensitivity of theRossi X‐ray Timing Explorer(RXTE). In the “Clocked Burster”, GS 1826‐24, hydrogen burns during the burst via the rapid‐proton (rp) process, which has received particular attention in recent years through theoretical and modelling studies. The burst energies and the measured variation of alpha (the ratio of persistent to burst flux) with accretion rate strongly suggests solar metallicity in the neutron star atmosphere, although this is not consistent with the corresponding variation of the recurrence time. Possible explanations include extra heating between the bursts, or a change in the fraction of the neutron star over which accretion takes place. I also present results from 4U 1746‐37, which exhibits regular burst trains which are interrupted by “out of phase” bursts. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1781039

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Magnetars are a subclass of highly magnetized neutron stars characterized by their recurrent X‐ray bursts. While in an active (bursting) state, they are emitting hundreds of predominantly soft (kT= 30 keV), and short (0.1 – 100 ms long) events. Active states last anywhere between days and years. The magnetar quiescent source X‐ray light curves exhibit pulsations in the narrow range of 5 – 11 s; estimates of their spin‐down rates indicate that their magnetic fields are extremely high, of the order of 1014– 1015G. Such highB‐field objects, had been predicted to exist in 1992, and were confirmed observationally withRXTEin 1998. Two types of sources are magnetar candidates to date: Soft Gamma Repeaters and Anomalous X‐ray Pulsars. This review is focused on the history of Soft Gamma Repeaters and their spectral, timing and flux characteristics, both in their persistent and their burst emission. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1781040

出版商:AIP    

年代:1904

数据来源: AIP