Following the report of enhanced X-ray activity towards the direction of Terzan 1 by MAXI from May 19--30 (Atel #5096), we performed an observation with Swift/XRT on June 4th (Obs. ID: 00032852001) and found no evidence of X-ray activity in the vicinity of Terzan 1. The exposure of this observation after standard processing was 35 seconds, though the unfiltered data's exposure time is 897 seconds.
We summarize here the main results of the timing analysis performed on the pulsed X‐ray emission of XTE J1751—305 and XTE J1814—338. These two sources encompass some of the main issues regarding the rotational evolution of Accreting Millisecond Pulsars: they have an opposite rotational reaction to the accretion of mass and only the phases of XTE J1751—305 evolve smoothly, while in XTE J1814—338 a modulation around an average trend appears in anti correlation with variations of the emitted X‐ray flux. We explore the possibility that this phenomenon is related to variations of the mass accretion rate on each spot, but only the behavior of the fundamental Fourier component could be fully reproduced according to the considered model. The most favorable explanation of this effect therefore appears to be still in terms of motions of the accretion path from the disk to the Neutron Star.
We report here on the orbital evolution of the accreting millisecond pulsar SAX J1808.4—3658. In particular, we find for this source an estimate of the orbital period derivative, Ṗorb = (3.40±0.12)×10−12 s/s. This derivative is positive and is more than one order of magnitude higher than what is expected from secular evolution driven by angular momentum losses caused by gravitational radiation under conservative mass transfer. In the hypothesis that the measured derivative of the orbital period reflects the secular evolution of the system, we propose a simple explanation of this puzzling result assuming that during X‐ray quiescence the source is ejecting matter (and angular momentum) from the inner Lagrangian point. The proposed orbital evolution of the system suggests a degenerate or fully convective companion star and indicates that this kind of sources are capable to efficiently ablate the companion star, and therefore are black widows visible in X‐rays during transient mass accretion episodes.
We have observed the accreting millisecond X-ray pulsar SAX J1808.4-3658 in its current outburst (ATel #1728, #1732, #1733) in the radio band with the Australia Telecope Compact Array (ATCA) on 2008 September 26th for about 5h (03:39 - 08:35 UT). The radio-counterpart was not detected. The 3-sigma upper-limits are 0.45 mJy/beam at 4.8 GHz and 0.72 mJy/beam at 8.6 GHz. The Australia Telescope National Facility is funded by the Commonwealth of Australia for operation as a National Facility managed by the CSIRO.
The transient IGR J17191-2821 (Atels #1021,#1022,#1025) has reached a state of increase activity demonstrating a bright outburst. During RXTE/PCA bulge scan observations taken on April 29 (14:39:50 UTC) and May 2 (16:30:43 UTC) the source was detected at a level of ~30 and ~70 mCrab (2-10 keV), respectively. During a Swift/XRT TOO observation (1 ksec) on May 1 (13:03:20 UTC) we detected the source at a high level of ~1.2E-9 erg/sec/cm^2, corresponding to ~ 46 mCrab (absorbed 2-10 keV flux).
We report on intermittent X-ray pulsations with a frequency of 442.36 Hz from the neutron star X-ray binary SAX J1748.9–2021 in the globular cluster NGC 6440. The pulsations were seen during both 2001 and 2005 outbursts of the source, but only intermittently, appearing and disappearing on timescales of hundreds of seconds. We find a suggestive relation between the occurrence of type I X-ray bursts and the appearance of the pulsations, but the relation is not strict. This behavior is very similar to that of the intermittent accreting millisecond X-ray pulsar HETE J1900.1–2455. The reason for the intermittence of the pulsations remains unclear. However, it is now evident that a strict division between pulsating and nonpulsating neutron star systems does not exist. By studying the Doppler shift of the pulsation frequency we determine an orbit with a period of 8.7 hr and a projected semimajor axis of 0.39 lt-s. The companion star might be a main-sequence or a slightly evolved star with a mass of ~1 M☉. Therefore, SAX J1748.9–2021 has a longer period and may have a more massive companion star than all the other accreting millisecond X-ray pulsars except for Aql X-1.
Three types of quasi-periodic oscillations (QPOs) have been discovered so far in the persistent emission of the most luminous neutron star low-mass X-ray binaries, the Z sources: ~10-60 Hz horizontal-branch and ~6-20 Hz normal/flaring-branch oscillations and ~200-1200 Hz kilohertz QPOs, which usually occur in pairs. Here we study the horizontal-branch oscillations and the two simultaneous kilohertz QPOs, which were discovered using the Rossi X-Ray Timing Explorer, comparing their properties in five Z sources with the predictions of the magnetospheric beat-frequency and Lense-Thirring precession models. We find that the variation of the horizontal-branch oscillation frequency with accretion rate predicted by the magnetospheric beat-frequency model for a purely dipolar stellar magnetic field and a radiation-pressure-dominated inner accretion disk is consistent with the observed variation. The model predicts a universal relation between the horizontal-branch oscillation, stellar spin, and upper kilohertz QPO frequencies that agrees with the data on five Z sources. The model implies that the neutron stars in the Z sources are near magnetic spin equilibrium, that their magnetic field strengths are ~109-1010 G, and that the critical fastness parameter for these sources is ≳0.8. If the frequency of the upper kilohertz QPO is an orbital frequency in the accretion disk, the magnetospheric beat-frequency model requires that a small fraction of the gas in the disk does not couple strongly to the stellar magnetic field at 3-4 stellar radii but instead drifts slowly inward in nearly circular orbits until it is within a few kilometers of the neutron star surface. The Lense-Thirring precession model is consistent with the observed magnitudes of the horizontal-branch oscillation frequencies only if the moments of inertia of the neutron stars in the Z sources are ~4-5 times larger than the largest values predicted by realistic neutron star equations of state. If instead the moments of inertia of neutron stars have the size expected and their spin frequencies in the Z sources are approximately equal to the frequency separation of the kilohertz QPOs, Lense-Thirring precession can account for the magnitudes of the horizontal-branch oscillation frequencies only if the fundamental frequency of the horizontal-branch oscillation is at least 4 times the precession frequency. We argue that the change in the slope of the correlation between the frequency of the horizontal-branch oscillation and the frequency of the upper kilohertz QPO, when the latter is greater than 850 Hz, is directly related to the varying frequency separation of the kilohertz QPOs.
Aql X-1 is the most prolific low mass X-ray binary transient hosting a neutron star. In this paper we focus on the return to quiescence following the 2010 outburst of the source. This decay was monitored thanks to 11 pointed observations taken with XMM-Newton, Chandra and Swift. The decay from outburst to quiescence is very fast, with an exponential decay characteristic time scale of ~2 d. Once in quiescence the X-ray flux of Aql X-1 remained constant, with no further signs of variability or decay. The comparison with the only other well-monitored outburst from Aql X-1 (1997) is tail-telling. The luminosities at which the fast decay starts are fully compatible for the two outbursts, hinting at a mechanism intrinsic to the system and possibly related to the neutron star rotation and magnetic field (i.e., the propeller effect). In addition, for both outbursts, the decay profiles are also very similar, likely resulting from the shut-off of the accretion process onto the neutron star surface. Finally, the quiescent neutron star temperatures at the end of the outbursts are well consistent with one another, suggesting a hot neutron star core dominating the thermal balance. Small differences in the quiescent X-ray luminosity among the two outbursts can be attributed to a different level of the power law component.
We report on a series of Swift/X-ray telescope observations, performed between 2012 February and 22 March, during the quiescent state of the neutron-star X-ray binary SAX J1750.8−2900. In these observations, the source was either just detected or undetected, depending on the exposure length (which ranged from ∼0.3 to ∼3.8 ks). The upper limits for the non-detections were consistent with the detected luminosities (when fitting a thermal model to the spectrum) of ∼1034 erg s−1 (0.5–10 keV). This level is consistent with what has been measured previously for this source in quiescence. However, on March 17 the source was found to have an order of magnitude larger count rate. When fitting the flare spectrum with an absorbed power-law model, we obtained a flare luminosity of (3–4) × 1034 erg s−1 (0.5–10 keV). Follow-up Swift observations showed that this flare lasted <16 d. This event was very likely due to a brief episode of low-level accretion on to the neutron star and provides further evidence that the quiescent state of neutron-star X-ray transients might not be as quiet as is generally assumed. The detection of this low-level accretion flare raises the question whether the quiescent emission of the source (outside the flare) could also be due to residual accretion, albeit continuous instead of episodic. However, we provide arguments which would suggest that the lowest intensity level might instead represent the cooling of the accretion-heated neutron star.
