Search for intermittent X-ray pulsations from neutron stars in low-mass X-ray binaries (original) (raw)
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Millisecond Pulsations from a Low‐Mass X‐Ray Binary in the Galactic Center Region
The Astrophysical Journal, 1997
We report the detection with the proportional counter array (PCA) onboard the Rossi X-Ray T iming Explorer (RXT E) of 589 Hz oscillations during three type I X-ray bursts from a low-mass X-ray binary located in the Galactic center region. The bursts, which occurred on 1996 May 15.814, June 4.612, and June 19.414 UT, were observed serendipitously during routine monitoring observations of GRO J1744[28 by RXT E. The strongest pulsations were observed from the May 15 burst, reaching an amplitude of 18% of the [8 keV Ñux in this burst. The pulsation amplitudes in each burst are a strong function of photon energy since no signals were detected in the 1È8 keV band, but there were strong detections above 8 keV. The evolution of the X-ray spectrum through each burst is consistent with modest radius expansion. The pulsations are conÐned to the postcontraction portion of each burst proÐle, consistent with the oscillations recently reported in a burst from KS 1731[26 and also some bursts from 4U 1728[34. The location of the burster in the 1¡ (FWHM) PCA Ðeld of view is constrained by comparing the count rates in the Ðve independent detectors of the PCA. This analysis strongly excludes GRO J1744[28 as the source of these bursts but does not yet allow a deÐnitive identi-Ðcation of the source with any known burster in the Ðeld of view. However, the derived position strongly overlaps that of MXB 1743[29, identifying this source as a strong candidate for the 589 Hz burster. We argue that the observed oscillations are consistent with rotational modulation of the X-ray brightness.
Evidence of 1122 Hz X-Ray Burst Oscillations from the Neutron Star X-Ray Transient XTE J1739-285
The Astrophysical Journal, 2007
We report on millisecond variability from the X-ray transient XTE J1739-285. We detected six X-ray type I bursts and found evidence for oscillations at 1122 ± 0.3 Hz in the brightest X-ray burst. Taking into consideration the power in the oscillations and the number of trials in the search, the detection is significant at the 99.96% confidence level. If the oscillations are confirmed, the oscillation frequency would suggest that XTE J1739-285 contains the fastest rotating neutron star yet found. We also found millisecond quasiperiodic oscillations in the persistent emission with frequencies ranging from 757 Hz to 862 Hz. Using the brightest burst, we derive an upper limit on the source distance of about 10.6 kpc.
Discovery of slow X-ray pulsations in the high-mass X-ray binary 4U 2206+54
Astronomy & Astrophysics, 2008
The source 4U 2206+54 is one of the most enigmatic high-mass X-ray binaries. In spite of intensive searches, X-ray pulsations have not been detected in the time range 0.001-1000 s. A cyclotron line at ~30 keV has been suggested by various authors but never detected with significance. The stellar wind of the optical companion is abnormally slow. The orbital period, initially reported to be 9.6 days, disappeared and a new periodicity of 19.25 days emerged. Our new long and uninterrupted RXTE observations allow us to search for long (~1 hr) pulsations for the first time. We have discovered 5560-s pulsations in the light curve of 4U 2206+54. Initially detected in RXTE data, these pulsations are also present in INTEGRAL and EXOSAT observations. The average X-ray luminosity in the energy range 2-10 keV is 1.5 x 10^{35} erg s^{-1} with a ratio Fmax/Fmin ~ 5. This ratio implies an eccentricity of ~0.4, somewhat higher than previously suggested. The source also shows a soft excess at low energies. If the soft excess is modelled with a blackbody component, then the size and temperature of the emitting region agrees with its interpretation in terms of a hot spot on the neutron star surface. The source displays variability on time scales of days, presumably due to changes in the mass accretion rate as the neutron star moves around the optical companion in a moderately eccentric orbit.
Astrophysical Journal, 2010
We report on observations of the unusual neutron-star binary system FIRST J102347.6+003841 carried out using the XMM-Newton satellite. This system consists of a radio millisecond pulsar (PSR J1023+0038) in a 0.198 day orbit with a ~0.2 M sun Roche-lobe-filling companion and appears to have had an accretion disk in 2001. We observe a hard power-law spectrum (Γ = 1.26(4)) with a possible thermal component, and orbital variability in X-ray flux and possibly hardness of the X-rays. We also detect probable pulsations at the pulsar period (single-trial significance ~4.5σ from an 11(2)% modulation), which would make this the first system in which both orbital and rotational X-ray pulsations are detected. We interpret the emission as a combination of X-rays from the pulsar itself and from a shock where material overflowing the companion meets the pulsar wind. The similarity of this X-ray emission to that seen from other millisecond pulsar binary systems, in particular 47 Tuc W (PSR J0024 - 7204W) and PSR J1740 - 5340, suggests that they may also undergo disk episodes similar to that seen in J1023 in 2001.
The Astrophysical Journal, 1997
We have detected quasi-periodic oscillations (QPOs) near 1 kHz from the low mass X-ray binary 4U 0614+091 in observations with XTE. The observations span several months and sample the source over a large range of X-ray luminosity. In every interval QPOs are present above 400 Hz with fractional RMS amplitudes from 3 to 12%. At high count rates, two high frequency QPOs are detected simultaneously. The difference of their frequency centroids is consistent with 323 Hz in all observations. During one interval a third signal is detected at 328 ± 2 Hz. This suggests the system has a stable 'clock' which is most likely the neutron star with spin period 3.1 msec. Thus, our observations and those of another neutron star system by provide the first evidence for millisecond pulsars within low-mass X-ray binary systems and reveal the 'missing-link' between millisecond radiopulsars and the late stages of binary evolution in low mass X-ray binaries . We suggest that the kinematics of the magnetospheric beat-frequency model applies to these QPOs. In this interpretation the high frequency signal is associated with the Keplerian frequency of the inner accretion disk and the lower frequency 'beat' signal arises from the differential rotation frequency of the inner disk and the spinning neutron star. Assuming the high frequency QPO is a Keplerian orbital frequency for the accretion disk, we find a maximum mass of 1.9M ⊙ and a maximum radius of 17 km for the neutron star.
Astronomy & Astrophysics, 2020
Context. Ultra-luminous X-ray sources (ULXs) are those X-ray sources located away from the centre of their host galaxy with luminosities exceeding the Eddington limit of a stellar-mass black hole (LX > 1039 erg s−1). Observed X-ray variability suggests that ULXs are X-ray binary systems. The discovery of X-ray pulsations in some of these objects (e.g. M82 X-2) suggests that a certain fraction of the ULX population may have a neutron star as the accretor. Aims. We present systematic modelling of low- and intermediate-mass X-ray binaries (LMXBs and IMXBs; donor-star mass range 0.92–8.0 M⊙ and neutron-star accretors) to explain the formation of this sub-population of ULXs. Methods. Using MESA, we explored the allowed initial parameter space of binary systems consisting of a neutron star and a low- or intermediate-mass donor star that could explain the observed properties of ULXs. These donors are transferring mass at super-Eddington rates while the accretion is limited locally in t...
X-ray bursts and burst oscillations from the slowly spinning X-ray pulsar IGR J17480−2446 (Terzan 5)
Monthly Notices of the Royal Astronomical Society, 2011
The newly discovered 11-Hz accreting pulsar, IGR J17480−2446, located in the globular cluster Terzan 5, has shown several bursts with a recurrence time as short as a few minutes. The source shows the shortest recurrence time ever observed from a neutron star. Here we present a study of the morphological, spectral and temporal properties of 107 bursts observed by the Rossi X-ray Timing Explorer. The recurrence time and the fluence of the bursts clearly anticorrelate with the increase in the persistent X-ray flux. The ratio between the energy generated by the accretion of mass and that liberated during bursts indicates that helium is ignited in a hydrogen-rich layer. Therefore, we conclude that all the bursts shown by IGR J17480−2446 are Type I X-ray bursts. Pulsations could be detected in all the brightest bursts and no drifts of the frequency are observed within 0.25 Hz of the spin frequency of the neutron star. These are also phase-locked with respect to the pulsations observed during the persistent emission and no rise in the rms associated to the pulse frequency is observed during the burst. This behaviour would favour a scenario where the Type I burst, possibly ignited at the polar caps, immediately propagates to the entire neutron star surface.
On the Amplitude of Burst Oscillations in 4U 1636-54: Evidence for Nuclear Powered Pulsars
Astrophysical Journal, 1998
We present a study of 581 Hz oscillations observed during a thermonuclear X-ray burst from the low mass X-ray binary (LMXB) 4U 1636-54 with the Rossi X-ray Timing Explorer (RXTE). We argue that the combination of large pulsed amplitudes near burst onset and the spectral evidence for localized emission during the rise strongly supports rotational modulation as the mechanism for the oscillations. We discuss how theoretical interpretation of spin modulation amplitudes, pulse profiles and pulse phase spectroscopy can provide constraints on the masses and radii of neutron stars. We also discuss the implication of these findings for the beat frequency models of kHz X-ray variability in LMXB.
Orbital evolution of X-ray binaries and quasi-periodic oscillations in X-ray pulsars
We have measured the orbital period derivatives in several high mass and low mass X-ray binaries. Mechanisms that could be responsible for the observed orbital evolution in these sources are briefly mentioned. We also describe studies of quasi-periodic oscillations in several X-ray pulsars and their relation with long term evolution of these systems and also the neutron star magnetic field strength.
Astronomy and Astrophysics, 2010
Context. It is commonly believed that millisecond radio pulsars have been spun up by transfer of matter and angular momentum from a low-mass companion during an X-ray active mass transfer phase. A subclass of low-mass X-ray binaries is that of the accreting millisecond X-ray pulsars, transient systems that show periods of X-ray quiescence during which radio emission could switch on. Aims. The aim of this work is to search for millisecond pulsations from three accreting millisecond X-ray pulsars, XTE J1751-305, XTE J1814-338, and SAX J1808.4-3658, observed during their quiescent X-ray phases at high radio frequencies (5 ÷ 8 GHz) in order to overcome the problem of the free-free absorption due to the matter engulfing the system. A positive result would provide definite proof of the recycling model, providing the direct link between the progenitors and their evolutionary products. Methods. The data analysis methodology has been chosen on the basis of the precise knowledge of orbital and spin parameters from X-ray observations. It is subdivided in three steps: we corrected the time series for the effects of (I) the dispersion due to interstellar medium and (II) of the orbital motions, and finally (III) folded modulo the spin period to increase the signal-to-noise ratio.