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.
Abstract We present the discovery, with the Neutron Star Interior Composition Explorer (NICER), that SRGA J144459.2−604207 is a 447.9 Hz accreting millisecond X-ray pulsar (AMXP), which underwent a 4 week long outburst starting on 2024 February 15. The AMXP resides in a 5.22 hr binary, orbiting a low-mass companion donor with M d > 0.1 M ⊙ . We report on the temporal and spectral properties from NICER observations during the early days of the outburst, from 2024 February 21 through 2024 February 23, during which NICER also detected a type I X-ray burst that exhibited a plateau lasting ∼6 s. The spectra of the persistent emission were well described by an absorbed thermal blackbody and power-law model, with blackbody temperature kT ≈ 0.9 keV and power-law photon index Γ ≈ 1.9. Time-resolved burst spectroscopy confirmed the thermonuclear nature of the burst, where an additional blackbody component reached a maximum temperature of nearly kT ≈ 3 keV at the peak of the burst. We discuss the nature of the companion as well as the type I X-ray burst.
Abstract We present the first joint NuSTAR and NICER observations of the ultracompact X-ray binary 4U 0614+091. This source shows quasiperiodic flux variations on the timescale of ∼days. We use reflection modeling techniques to study various components of the accretion system as the flux varies. We find that the flux of the reflected emission and the thermal components representing the disk and the compact object trend closely with the overall flux. However, the flux of the power-law component representing the illuminating X-ray corona scales in the opposite direction, increasing as the total flux decreases. During the lowest flux observation, we see evidence of accretion disk truncation from roughly 6 gravitational radii to 11.5 gravitational radii. This is potentially analogous to the truncation seen in black hole low-mass X-ray binaries, which tends to occur during the low/hard state at sufficiently low Eddington ratios.
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.
Knitted electrodes provide numerous advantages over the gel electrodes typically used in clinical practice when designing wearable Electrocardiogram (ECG) systems. They typically have enhanced durability, better textile integration and do not dry out. However, the higher skin/electrode impedance makes them susceptible to noise from electrical interference. Adding a buffer amplifier circuit close to the electrode, creating an 'active' electrode, is one way to mitigate this. However, the choice of where to integrate these amplifiers in the garment remains. Therefore, this work measured the signal-to-noise ratio (SNR) of an ECG output when comparing the distance between the electrode and the operational amplifier (op-amp) buffer and found that when the buffer was placed directly under a knitted electrode, the noise was lower than that seen with gel electrodes. This also provided information on the impact of distance on the SNR.
We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of mHz X-ray brightness oscillations from the "clocked burster." NICER observed the source in the periods 2017 June 20 - 29, July 11 - 13, and September 9 - 15, for a total useful exposure of 34 ks. Two consecutive dwells obtained on 2017 September 9 revealed highly significant oscillations at a frequency of 8 mHz. The fractional, sinusoidal modulation amplitude increases from 0.7 % at 1 keV to approximately 2 % at 6 keV. Similar oscillations were also detected at lower significance in three additional dwells. The oscillation frequency and amplitude are consistent with those of mHz QPOs reported in other accreting neutron star systems. A thermonuclear X-ray burst was also observed on 2017 June 22. The burst properties and X-ray colors are both consistent with the source being in a soft spectral state during these observations, findings that are confirmed by ongoing monitoring with MAXI and SWIFT-BAT. Assuming that the mHz oscillations are associated with black body emission from the neutron star surface, modeling of the phase-resolved spectra shows that the oscillation is consistent with being produced by modulation of the temperature component of this emission. In this interpretation, the black body normalization, proportional to the emitting surface area, is consistent with being constant through the oscillation cycle. We place the observations in the context of the current theory of marginally stable burning and briefly discuss the potential for constraining neutron star properties using mHz oscillations.
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.
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