26 research outputs found
Diverse Long-Term Variability of Five Candidate High-Mass X-ray Binaries from Swift Burst Alert Telescope Observations
We present an investigation of long-term modulation in the X-ray light curves
of five little-studied candidate high-mass X-ray binaries using the Swift Burst
Alert Telescope. IGR J14488-5942 and AX J1700.2-4220 show strong modulation at
periods of 49.6 and 44 days, respectively, which are interpreted as orbital
periods of Be star systems. For IGR J14488-5942, observations with Swift X-ray
Telescope show a hint of pulsations at 33.4 s. For AX J1700.2-4220, 54 s
pulsations were previously found with XMM. Swift J1816.7-1613 exhibits
complicated behavior. The strongest peak in the power spectrum is at a period
near 150 days, but this conflicts with a determination of a period of 118.5
days by La Parola et al. (2014). AX J1820.5-1434 has been proposed to exhibit
modulation near 54 days, but the extended BAT observations suggest modulation
at slightly longer than double this at approximately 111 days. There appears to
be a long-term change in the shape of the modulation near 111 days, which may
explain the apparent discrepancy. The X-ray pulsar XTE J1906+090, which was
previously proposed to be a Be star system with an orbital period of ~30 days
from pulse timing, shows peaks in the power spectrum at 81 and 173 days. The
origins of these periods are unclear, although they might be the orbital period
and a superorbital period respectively. For all five sources, the long-term
variability, together with the combination of orbital and proposed pulse
periods, suggests that the sources contain Be star mass donors.Comment: Accepted for publication in The Astrophysical Journal. 15 pages, 27
figures. (v2 corrects citation
A Study of the 20 Day Superorbital Modulation in the High-Mass X-ray Binary IGR J16493-4348
We report on Nuclear Spectroscopic Telescope Array (NuSTAR), Neil Gehrels
Swift Observatory (Swift) X-ray Telescope (XRT) and Swift Burst Alert Telescope
(BAT) observations of IGR J16493-4348, a wind-fed Supergiant X-ray Binary
(SGXB) showing significant superorbital variability. From a discrete Fourier
transform of the BAT light curve, we refine its superorbital period to be
20.058 0.007 days. The BAT dynamic power spectrum and a fractional root
mean square analysis both show strong variations in the amplitude of the
superorbital modulation, but no observed changes in the period were found. The
superorbital modulation is significantly weaker between MJD 55,700 and MJD
56,300. The joint NuSTAR and XRT observations, which were performed near the
minimum and maximum of one cycle of the 20 day superorbital modulation, show
that the flux increases by more than a factor of two between superorbital
minimum and maximum. We find no significant changes in the 3-50 keV pulse
profiles between superorbital minimum and maximum, which suggests a similar
accretion regime. Modeling the pulse-phase averaged spectra we find a possible
Fe K emission line at 6.4 keV at superorbital maximum. The feature is
not significant at superorbital minimum. While we do not observe any
significant differences between the pulse-phase averaged spectral continua
apart from the overall flux change, we find that the hardness ratio near the
broad main peak of the pulse profile increases from superorbital minimum to
maximum. This suggests the spectral shape hardens with increasing luminosity.
We discuss different mechanisms that might drive the observed superorbital
modulation.Comment: 17 pages, 14 figures, 3 tables, accepted for publication in The
Astrophysical Journal on 2019 May 1
Constraining the evolution of the unstable accretion disk in SMC X-1 with NICER
Neutron star high mass X-ray binaries with superorbital modulations in
luminosity host warped inner accretion disks that occult the neutron star
during precession. In SMC X-1, the instability in the warped disk geometry
causes superorbital period "excursions:" times of instability when the
superorbital period decreases from its typical value of 55 days to 40
days. Disk instability makes SMC X-1 an ideal system in which to investigate
the effects of variable disk geometry on the inner accretion flow. Using the
high resolution spectral and timing capabilities of the Neutron Star Interior
Composition Explorer (NICER) we examined the high state of four different
superorbital cycles of SMC X-1 to search forchanges in spectral shape and
connections to the unstable disk geometry. We performed pulse phase-averaged
and phase-resolved spectroscopy to closely compare the changes in spectral
shape and any cycle-to-cycle variations. While some parameters including the
photon index and absorbing column density show slight variations with
superorbital phase, these changes are most evident during the intermediate
state of the supeorbital cycle. Few spectral changes are observed within the
high state of the superorbital cycle, possibly indicating the disk instability
does not significantly change SMC X-1's accretion process.Comment: 11 pages, 5 figures. Accepted to Ap
Investigating the superorbital modulations in 4U 1909+07, IGR J16418-4532 and IGR J16479-4514 with Swift XRT, BAT and NuSTAR observations
A puzzling variety of superorbital modulations have been discovered in
several supergiant High-Mass X-ray binaries (sgHMXBs). To investigate the
mechanisms driving these superorbital modulations, we have analyzed long-term
Neil Gehrels Swift Observatory (Swift) Burst Alert Telescope (BAT) observations
of three sgHMXBs: 4U 1909+07, IGR J16418-4532 and IGR J16479-4514 and
constructed their dynamic power spectra and superorbital intensity profiles.
These Swift BAT observations are complemented by pointed Swift X-ray Telescope
(XRT) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations performed
near the predicted maximum and minimum phase of a single superorbital cycle for
each of these sources. The BAT dynamic power spectra show changes in the
strength of the superorbital modulation on timescales of years, with either the
peak at the fundamental frequency and/or the second harmonic present at
different times for all three sources. The pointed Swift XRT and NuSTAR
observations show no significant differences between the pulse profiles and
spectral parameters at the superorbital maximum and minimum phase. This is
likely due to the fact the superorbital modulation had weakened significantly
during the times when the NuSTAR observations were carried out for all three
sources. The results from the Swift XRT, BAT and NuSTAR analysis indicate the
possible presence of multiple co-rotating interaction regions (CIRs) in the
stellar winds of the supergiant stars, although a structured stellar wind from
the supergiant star due to tidal oscillations cannot be ruled out.Comment: Accepted for publication in the Astrophysical Journa
Probing spectral and timing properties of the X-ray pulsar RX J0440.9+4431 in the giant outburst of 2022-2023
The X-ray pulsar RX J0440.9+4431 went through a giant outburst in 2022 and
reached a record-high flux of 2.3 Crab, as observed by Swift/BAT. We study the
evolution of different spectral and timing properties of the source using NICER
observations. The pulse period is found to decrease from 208 s to 205 s, and
the pulse profile evolves significantly with energy and luminosity. The
hardness ratio and hardness intensity diagram (HID) show remarkable evolution
during the outburst. The HID turns towards the diagonal branch from the
horizontal branch above a transition (critical) luminosity, suggesting the
presence of two accretion modes. Each NICER spectrum can be described using a
cutoff power law with a blackbody component and a Gaussian at 6.4 keV. At
higher luminosities, an additional Gaussian at 6.67 keV is used. The observed
photon index shows negative and positive correlations with X-ray flux below and
above the critical luminosity, respectively. The evolution of spectral and
timing parameters suggests a possible change in the emission mechanism and
beaming pattern of the pulsar depending on the spectral transition to sub- and
super-critical accretion regimes. Based on the critical luminosity, the
magnetic field of the neutron star can be estimated in the order of 10
or 10 G, assuming different theoretical models. Moreover, the observed
iron emission line evolves from a narrow to a broad feature with luminosity.
Two emission lines originating from neutral and highly ionized Fe atoms were
evident in the spectra around 6.4 keV and 6.67 keV (higher luminosities).Comment: Published in Monthly Notices of the Royal Astronomical Societ