Abstract: We investigate the thermonuclear bursting behavior of IGR J17473-2721, an X-ray transient that in 2008 underwent a six month long outburst, starting (unusually) with an X-ray burst. We detected a total of 57 thermonuclear bursts throughout the outburst with AGILE, Swift, RXTE, and INTEGRAL. The wide range of inferred accretion rates (between <1% and about 20% of the Eddington accretion rate m-dot_Edd) spanned during the outburst allows us to study changes in the nuclear burning processes and to identify up to seven different phases. The burst rate increased gradually with the accretion rate until it dropped (at a persistent flux corresponding to about 15% of m-dot_Edd) a few days before the outburst peak, after which bursts were not detected for a month. As the persistent emission subsequently decreased, the bursting activity resumed with a much lower rate than during the outburst rise. This hysteresis may arise from the thermal effect of the accretion on the surface nuclear burning processes, and the timescale is roughly consistent with that expected for the neutron star crust thermal response. On the other hand, an undetected superburst, occurring within a data gap near the outburst peak, could have produced a similar quenching of burst activity.
Abstract: The timing properties of the 4.45 s pulsar in the Be X-ray binary system GRO J1750-27 are examined using hard X-ray data from INTEGRAL and Swift during a type II outburst observed during 2008. The orbital parameters of the system are measured and agree well with those found during the last known outburst of the system in 1995. Correcting the effects of the Doppler shifting of the period, due to the orbital motion of the pulsar, leads to the detection of an intrinsic spin-up that is well described by a simple model including. P and P terms of - 7.5 x 10(-10) s s(-1) and 1 x 10(-16) s s(-2), respectively. The model is then used to compare the time-resolved variation of the X-ray flux and intrinsic spin-up against the accretion torque model of Ghosh & Lamb; this finds that GRO J1750-27 is likely located 12-22 kpc distant and that the surface magnetic field of the neutron star is similar to 2 x 10(12) G. The shape of the pulse and the pulsed fraction shows different behaviour above and below 20 keV, indicating that the observed pulsations are the convolution of many complex components.
Abstract: Aims. XTE J1701-407 is a newly discovered X-ray transient source. We investigate its flux variability and study the intermediate long-and short-bursts discovered by Swift on July 17, and 27, 2008, respectively. Methods. Only one intermediate long-burst, of duration approximate to 18 min and ten days later a short burst, have been recorded from XTE J1701-407. We analyzed the public available data from Swift and RXTE, and compared the observed properties of the intermediate long-burst with theoretical ignition condition and light curves to investigate the possible nuclear-burning processes. Results. The intermediate long-burst may have exhibited a photospheric radius expansion, allowing us to derive the source distance at 6.2 kpc, assuming the empirically derived Eddington luminosity for pure helium. The intermediate long-burst decay was described most accurately by using two exponential functions with e-folding times of tau(1) = 40 +/- 3 s and tau(2) = 221 +/- 9 s. The bursts occurred at a persistent luminosity of L-per = 8.3 x 10(36) erg s(-1) (approximate to 2.2% of the Eddington luminosity). For the intermediate long-burst, the mass accretion rate per unit area onto the neutron star was (m) over dot approximate to 4 x 10(3) g cm(-2) s(-1), and the total energy released was E-burst approximate to 3.5 x 10(40) erg. This corresponds to an ignition column depth of y(ign) approximate to 1.8 x 10(9) g cm(-2), for a pure helium burning. We find that the energetics of this burst can be modeled in different ways, as (i) pure helium ignition, as the result of either pure helium accretion or depletion of hydrogen by steady burning during accumulation; or (ii) as ignition of a thick layer of hydrogen-rich material in a source of low metallicity. However, comparison of the burst duration with model light curves suggests that hydrogen burning plays an important role during the burst, and therefore that this source is a low accretion-rate burster with a low metallicity in the accreted material.
Abstract: We report on the results of International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of the neutron star low-mass X-ray binary SAX J1810.8-2609 during its latest active phase in 2007 August. The current outburst is the first one since 1998 and the derived luminosity is 1.1-2.6 x 10(36) erg s(-1) in the 20-100 keV energy range. This low outburst luminosity and the long-term time-average accretion rate of similar to 5 x 10(-12) M-circle dot yr(-1) suggest that SAX J1810.8-2609 is a faint soft X-ray transient. During the flux increase, spectra are consistent with a thermal Comptonization model with a temperature plasma of kT(e) similar to 23-30 keV and an optical depth of tau similar to 1.2-1.5, independent of the luminosity of the system. This is a typical low hard spectral state for which the X-ray emission is attributed to the upscattering of soft seed photons by a hot, optically thin electron plasma. During the decay, spectra have a different shape, the high energy tail being compatible with a single power law. This confirm similar behavior observed by BeppoSAX during the previous outburst, with the absence of visible cutoff in the hard X-ray spectrum. INTEGRAL/JEM-X instrument observed four X-ray bursts in Fall 2007. The first one has the highest peak flux (approximate to 3.5 crab in 3-25 keV) giving an upper limit to the distance of the source of about 5.7 kpc, for a L-Edd approximate to 3.8 x 10(38) erg s(-1). The observed recurrence time of similar to 1.2 days and the ratio of the total energy emitted in the persistent flux to that emitted in the bursts (alpha similar to 73) allow us to conclude that the burst fuel was composed by mixed hydrogen and helium with X >= 0.4.
