Neutron Star Soft X-ray Transients Research Articles

Effect of non-stationary accretion on spectral state transitions: An example of a persistent neutron star LMXB 4U1636–536

Observations of black hole and neutron star X-ray binaries show that the luminosity of the hard-to-soft state transition is usually higher than that of the soft-to-hard state transition, indicating additional parameters other than mass accretion rate are required to interpret spectral state transitions. It has been found in some individual black hole or neutron star soft X-ray transients that the luminosity corresponding to the hard-to-soft state transition is positively correlated with the peak luminosity of the following soft state. In this work, we report the discovery of the same correlation in the single persistent neutron star low mass X-ray binary (LMXB) 4U 1636–536 based on data from the All Sky Monitor (ASM) on board RXTE, the Gas Slit Camera (GSC) on board MAXI and the Burst Alert Telescope (BAT) on board Swift. We also found such a positive correlation holds in this persistent neutron star LMXB in a luminosity range spanning about a factor of four. Our results indicate that non-stationary accretion also plays an important role in driving X-ray spectral state transitions in persistent accreting systems with small accretion flares, which is much less dramatic compared with the bright outbursts seen in many Galactic LMXB transients.

Constraints on Thermal X‐Ray Radiation from SAX J1808.4−3658 and Implications for Neutron Star Neutrino Emission

Thermal X-ray radiation from neutron star soft X-ray transients in quiescence provides the strongest constraints on the cooling rates of neutron stars, and thus on the interior composition and properties of matter in the cores of neutron stars. We analyze new (2006) and archival (2001) XMM-Newton observations of the accreting millisecond pulsar SAX J1808.4-3658 in quiescence, which provide the most stringent constraints to date. The X-ray spectrum of SAX J1808.4-3658 in the 2006 observation is consistent with a power-law of photon index 1.83\pm0.17, without requiring the presence of a blackbody-like component from a neutron star atmosphere. Our 2006 observation shows a slightly lower 0.5-10 keV X-ray luminosity, at a level of 68^{+15}_{-13}% that inferred from the 2001 observation. Simultaneous fitting of all available XMM data allows a constraint on the quiescent neutron star (0.01-10 keV) luminosity of L_{NS}<1.1*10^{31} erg/s. This limit excludes some current models of neutrino emission mediated by pion condensates, and provides further evidence for additional cooling processes, such as neutrino emission via direct Urca processes involving nucleons and/or hyperons, in the cores of massive neutron stars.

Chandra observations of the millisecond X-ray pulsar IGR J00291+5934 in quiescence

In this Paper we report on our analysis of three Chandra observations of the accretion-powered millisecond X-ray pulsar IGR J00291+5934 obtained during the late stages of the 2004 outburst. We also report the serendipitous detection of the source in quiescence by ROSAT during MJD 48830-48839. The detected 0.3-10 keV source count rates varied significantly between the Chandra observations from (7.2+-1.2)x10^-3, (6.8+-0.9)x10^-3, and (1.4+-0.1)x10^-2 counts per second for the 1st, 2nd, and 3rd Chandra observation, on MJD 53371.88, 53383.99, and 53407.57, respectively. The count rate for the 3rd observation is 2.0+-0.4 times as high as that of the average of the first two observations. The unabsorbed 0.5-10 keV source flux for the best-fit power-law model to the source spectrum was (7.9+-2.5)x10^-14, (7.3+-2.0)x10^-14, and (1.17+-0.22)x10^-13 erg cm^-2 s^-1 for the 1st, 2nd, and 3rd Chandra observation, respectively. We find that this source flux is consistent with that found by ROSAT [~(5.4+-2.4)x10^-14 erg cm^-2 s^-1]. Under the assumption that the interstellar extinction, N_H, does not vary between the observations, we find that the blackbody temperature during the 2nd Chandra observation is significantly higher than that during the 1st and 3rd observation. Furthermore, the effective temperature of the neutron star derived from fitting an absorbed blackbody or neutron star atmosphere model to the data is rather high in comparison with many other neutron star soft X-ray transients in quiescence, even during the 1st and 3rd observation. If we assume that the source quiescent luminosity is similar to that measured for two other accretion powered millisecond pulsars in quiescence, the distance to IGR J00291+5934 is 2.6-3.6 kpc.

Long-term activity of the Rapid Burster

The recurrence time , but no correlation with the outburst duration. The observed behaviour is discussed in terms of the thermal instability of the accretion disk. A comparison of this prototype with other neutron star soft X-ray transients, like Aql X-1 and 4U 1608–52, helps us find the common links in the disk behaviour in such systems.

The Correlation between Hard X-Ray Peak Flux and Soft X-Ray Peak Flux in the Outburst Rise of Low-Mass X-Ray Binaries

We have analyzed Rossi X-Ray Timing Explorer pointed observations of the outbursts of black hole and neutron star soft X-ray transients in which an initial low/hard state, or island state, followed by a transition to a softer state was observed. In three sources—the black hole transient XTE J1550-564, the neutron star transient Aquila X-1, and the quasi-persistent neutron star low-mass X-ray binary 4U 1705-44—two such outbursts were found. We find that the flux of the soft X-ray peak, which lags the hard X-ray peak by a few days to several weeks, scales with the flux of the hard X-ray peak. We conclude that we are able to predict the soft X-ray outburst peak flux based on the preceding hard X-ray peak flux, implying an early setup of the outbursts. We also find that the X-ray luminosity corresponding to the peak of the hard X-ray flux, which corresponds to the X-ray luminosity of the start of the hard-to-soft state transition, varies by a factor of about 2. If the accretion geometry early in the outburst rise is composed of two flows (e.g., a hot sub-Keplerian halo flow and a Keplerian disk flow or an outflow and a Keplerian disk flow), the correlation indicates that the two flows are initially related, probably owing to processes in the outer part of the accretion disk. We discuss constraints on a single-flow model and a disk-jet model from these observations.

Black hole and neutron star soft X-ray transients: a hard X-ray view of their outbursts

Abstract The RXTE public observations of the outbursts of black hole soft X-ray transients XTE J1550-564, XTE J1859+226, 4U 1630-47, XTE J1118+480, XTE J1650-500, and the neutron star soft X-ray transients 4U 1608-52, Aquila X-1, including a variable ‘persistent’ neutron star low mass X-ray binary 4U 1705-44, are summarized in this paper. The hard X-ray view of those outbursts, which is quite different from that of the soft X-ray band, suggests that there are several types of outbursts which result in different hard X-ray outburst profile – the outburst profiles are energy dependent. One type is the low/hard state outbursts, the other type is the outburst showing transitions from the low/hard state to the high/soft state, or to the intermediate or to the very high state. The later has an initial low/hard state, introducing the phenomena that the hard X-ray precedes the soft X-ray in the outburst rise. Such outbursts in XTE J1550-564, Aql X-1 and 4U 1705-44 support a two-accretion-flow model which involves one Keplerian disk flow and one sub-Keplerian flow for the initial outburst rise.

The properties of outbursts and long-term activity of the soft X-ray transient 4U 1608–52 (QX Nor)

An analysis of the long-term activity of 4U 1608–52, based on the data, is presented. Although the individual outbursts in 4U 1608–52 differ in both their duration and peak intensity, their rising and final decay branches display remarkable similarities. The variations of the soft X-ray color between 1.5–4 keV are found to follow the luminosity during the decay of outburst much better than the variations between 4–12 keV. The recurrence time T C of outbursts was analyzed by the method of the residuals. The mean value of T C is 330 days between the years 1996–2003 but a strong long-term trend of a decreasing TC can be resolved. A series of four or five consecutive outbursts displays a clear correlation between the decreasing TC and increasing peak intensity, but no correlation with the outburst duration. The observed behaviour is discussed in terms of the thermal instability model. We argue that the disk viscosity achieves similar values for the individual outbursts in 4U 1608–52. Only during the brightest outbursts does the luminosity appear to be high enough for the disk to be ionized out to its outer rim by the X-rays from the neutron star, according to the model by King & Ritter ([CITE]). We also argue that the disk is never considerably depleted in outburst. A comparison with the similar system, Aql X-1, helps us to find the common links between the disk behaviour in the individual neutron-star soft X-ray transients.

The Evolution of the High‐Energy Tail in the Quiescent Spectrum of the Soft X‐Ray Transient Aquila X‐1

A moderate level of variability has been detected in the quiescent luminosity of several neutron star soft X-ray transients. Spectral variability was first revealed by Chandra observations of Aql X-1 in the 4 months that followed the 2000 X-ray outburst. By adopting the canonical model for the quiescent spectrum of soft X-ray transients, i.e., an absorbed neutron star atmosphere model plus a power-law tail, in 2002 Rutledge et al. concluded that the observed spectral variations could be ascribed to temperature variations of the neutron star atmosphere. These results can hardly be reconciled with the neutron star cooling that is expected to take place in between outbursts (after deep crustal heating in the accretion phase). Here we reanalyze the Chandra spectra of Aql X-1, together with a long BeppoSAX observation in the same period, and propose a different interpretation of the spectral variability: that it is due to correlated variations of the power-law component and the column density (>5, a part of which might be intrinsic to the source), while the temperature and flux of the neutron star atmospheric component remain unchanged. This lends support to the idea that the power-law component arises from emission at the shock between the radio pulsar wind and inflowing matter from the companion star.

Lithium Production in Hot Advection‐dominated Accretion Flows in Soft X‐Ray Transients

High Li abundances have been reported in the late-type secondaries of five soft X-ray transients (SXTs): V404 Cyg, A0620-00, GS 2000+25, Nova Mus 1991, and Cen X-4. Since Li is likely to be depleted in stars of this type, the origin of the Li is puzzling. Li has not been seen in similar secondaries of cataclysmic variables, which suggests that the high Li abundance is not due to an anomalous suppression of Li depletion in close binaries. SXTs in the quiescent state have been modeled in terms of hot advection-dominated accretion flows (ADAFs), in which the ions are essentially at virial temperature. At such temperatures, Li production via α-α spallation is possible. We show that ADAFs can produce sufficient Li via spallation to explain the observations in the black hole SXTs V404 Cyg, A0620-00, GS 2000+25, and Nova Mus 1991. If this explanation is correct, it is the first independent confirmation that accretion flows in quiescent SXTs have a two-temperature plasma with ion temperature much higher than electron temperature. Depending on the Li depletion timescale in the secondary, which may range between 107-109 yr, the model requires ~10-4 to 10-6 of the accreted mass to be intercepted by the secondary after undergoing Li production and being ejected. In the case of the neutron star SXT, Cen X-4, we can explain the observed Li only if the mass-accretion rate is ~10-3 times the Eddington rate and if there is enhanced ejection due to a propeller effect. We discuss possible observational tests of this proposal. Li production during outbursts could be quite important and may even dominate over the production during quiescence, but the estimate of the Li yield is uncertain. We calculate the expected luminosity in gamma-ray lines due to the production of excited Li and Be nuclei, but conclude that the line cannot be detected with current instruments.

Black hole X-ray transients

Irradiation models for soft X-ray transients (SXTs) are considered. These require the presence of hard X-rays for the outburst to be triggered. If the accreting object is a neutron star this flux is likely to arise in a boundary layer. For black-hole transients recent work by White and Lightman (1989) shows that two-temperature behavior is likely in the inner disk at accretion rates higher than a critical value M(dot)TT, thus providing the required hard X-rays. M(dot)TT is a function of the disk viscosity parameter alpha and the mass of the accreting object. The quiescent accretion rate of the black-hole candidate SXT A0620 - 00 is probably slightly lower than M(dot)TT and its disk is in a one-temperature state. The slow expansion of the secondary increases the mass transfer rate and when a two-temperature structure arises the system goes into outburst. This model requires alpha at about 0.1. Compared with neutron-star SXTs having similar binary parameters, black hole SXTs are likely to be brighter both in quiescence and in outburst. Their outbursts are likely to be shorter, but the quiescent intervals are expected to be similar to those in the neutron-star case.