4,341 research outputs found
Spectral Changes in the Hyperluminous Pulsar in NGC 5907 as a Function of Super-Orbital Phase
We present broad-band, multi-epoch X-ray spectroscopy of the pulsating
ultra-luminous X-ray source (ULX) in NGC 5907. Simultaneous XMM-Newton and
NuSTAR data from 2014 are best described by a multi-color black-body model with
a temperature gradient as a function of accretion disk radius significantly
flatter than expected for a standard thin accretion disk (T(r) ~ r^{-p}, with
p=0.608^{+0.014}_{-0.012}). Additionally, we detect a hard power-law tail at
energies above 10 keV, which we interpret as being due to Comptonization. We
compare this observation to archival XMM-Newton, Chandra, and NuSTAR data from
2003, 2012, and 2013, and investigate possible spectral changes as a function
of phase over the 78d super-orbital period of this source. We find that
observations taken around phases 0.3-0.4 show very similar temperature
profiles, even though the observed flux varies significantly, while one
observation taken around phase 0 has a significantly steeper profile. We
discuss these findings in light of the recent discovery that the compact object
is a neutron star and show that precession of the accretion disk or the neutron
star can self-consistently explain most observed phenomena.Comment: 7 pages, 5 figures, submitted to ApJ; comments welcom
Morphological analysis on the coherence of kHz QPOs
We take the recently published data of twin kHz quasi-period oscillations
(QPOs) in neutron star (NS) lowmass X-ray binaries (LMXBs) as the samples, and
investigate the morphology of the samples, which focuses on the quality factor,
peak frequency of kHz QPOs, and try to infer their physical mechanism. We
notice that: (1) The quality factors of upper kHz QPOs are low (2 ~ 20 in
general) and increase with the kHz QPO peak frequencies for both Z and Atoll
sources. (2) The distribution of quality factor versus frequency for the lower
kHz QPOs are quite different between Z and Atoll sources. For most Z source
samples, the quality factors of lower kHz QPOs are low (usually lower than 15)
and rise steadily with the peak frequencies except for Sco X-1, which drop
abruptly at the frequency of about 750 Hz. While for most Atoll sources, the
quality factors of lower kHz QPOs are very high (from 2 to 200) and usually
have a rising part, a maximum and an abrupt drop. (3) There are three Atoll
sources (4U 1728-34, 4U 1636-53 and 4U 1608-52) of displaying very high quality
factors for lower kHz QPOs. These three sources have been detected with the
spin frequencies and sidebands, in which the source with higher spin frequency
presents higher quality factor of lower kHz QPOs and lower difference between
sideband frequency and lower kHz QPO frequency.Comment: 8 pages, 8 figures, publishe
Simultaneous BeppoSAX and RXTE observations of the X-ray burst sources GX 3+1 and Ser X-1
We have obtained spectral and timing data on GX 3+1 and Ser X-1. Both sources
were observed simultaneously with BeppoSAX and RXTE. The RXTE data is used to
provide power spectra and colour-colour diagrams in order to constrain the
state (and thus track ) the sources are in. The BeppoSAX data provide
the broad-band spectra. The spectra of both sources are reasonably well-fit
using a model consisting of a disk-blackbody, a comptonized component and a Fe
line, absorbed by interstellar absorption. The electron temperature (kT) of the Comptonizing plasma is in both cases 2.5 keV. This implies
that no strong high-energy tail from the Comptonized component is present in
either of the sources. We discuss the similarities between these burst sources
and the luminous X-ray sources located in globular clusters. We find that the
spectral parameters of the comptonized component provide information about the
mass-accretion rate, which agrees well with estimates from the timing and
spectral variations.Comment: 8 pages, accepted by A&
RXTE Studies of X-ray Spectral Variations with Accretion Rate in 4U 1915-05
We present the results of detailed spectral studies of the ultra-compact low
mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-ray Timing
Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to
exhibit a ~199-day modulation in its 2--12 keV flux. Observations were
performed with the PCA and HEXTE instruments on RXTE at roughly one-month
intervals to sample this long-term period and study accretion rate-related
spectral changes. We obtain good fits with a model consisting of a blackbody
and an exponentially cut-off power law. The spectral parameters are strongly
correlated with both the broad-band (2--50 keV) luminosity and the position in
the color-color diagram, with the source moving from a low hard state to a high
soft state as the accretion rate increases. The blackbody component appears to
drive the spectral evolution. Our results are consistent with a geometry in
which the soft component arises from an optically thick boundary layer and the
hard component from an extended Comptonizing corona. Comparing our results with
those of a similar study of the brighter source 4U 1820-30 (Bloser et al.
2000), we find that the two ultra-compact LMXBs occupy similar spectral states
even though the transitions occur at very different total luminosities.Comment: 27 pages LaTeX, 8 figures, accepted to the Astrophysical Journa
Further Observations of the Intermediate Mass Black Hole Candidate ESO 243-49 HLX-1
The brightest Ultra-Luminous X-ray source HLX-1 in the galaxy ESO 243-49
currently provides strong evidence for the existence of intermediate mass black
holes. Here we present the latest multi-wavelength results on this intriguing
source in X-ray, UV and radio bands. We have refined the X-ray position to
sub-arcsecond accuracy. We also report the detection of UV emission that could
indicate ongoing star formation in the region around HLX-1. The lack of
detectable radio emission at the X-ray position strengthens the argument
against a background AGN.Comment: 4 pages, 2 figures. Accepted 11th of Feb 2010. Contributed talk to
appear in Proceedings of "X-ray Astronomy 2009: Present Status,
Multi-Wavelength Approach and Future Perspectives", Bologna, Italy, September
7-11, 2009, AIP, eds. A. Comastri, M. Cappi, and L. Angelin
Optical variability of the accretion disk around the intermediate mass black hole ESO 243-49 HLX-1 during the 2012 outburst
We present dedicated quasi-simultaneous X-ray (Swift) and optical (Very Large
Telescope (VLT), V- and R-band) observations of the intermediate mass black
hole candidate ESO 243-49 HLX-1 before and during the 2012 outburst. We show
that the V-band magnitudes vary with time, thus proving that a portion of the
observed emission originates in the accretion disk. Using the first quiescent
optical observations of HLX-1, we show that the stellar population surrounding
HLX-1 is fainter than V~25.1 and R~24.2. We show that the optical emission may
increase before the X-ray emission consistent with the scenario proposed by
Lasota et al. (2011) in which the regular outbursts could be related to the
passage at periastron of a star circling the intermediate mass black hole in an
eccentric orbit, which triggers mass transfer into a quasi-permanent accretion
disk around the black hole. Further, if there is indeed a delay in the X-ray
emission we estimate the mass-transfer delivery radius to be ~1e11 cm.Comment: 9 pages, 2 figures, accepted for publication in ApJ
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