1,122 research outputs found
The X-ray Position and Optical Counterpart of the Accretion-Powered Millisecond Pulsar XTE J1814-338
We report the precise optical and X-ray localization of the 3.2 ms
accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray
Observatory as well as optical observations conducted during the 2003 June
discovery outburst. Optical imaging of the field during the outburst of this
soft X-ray transient reveals an R = 18 star at the X-ray position. This star is
absent (R > 20) from an archival 1989 image of the field and brightened during
the 2003 outburst, and we therefore identify it as the optical counterpart of
XTE J1814-338. The best source position derived from optical astrometry is R.A.
= 18h13m39.s04, Dec.= -33d46m22.3s (J2000). The featureless X-ray spectrum of
the pulsar in outburst is best fit by an absorbed power-law (with photon index
= 1.41 +- 0.06) plus blackbody (with kT = 0.95 +- 0.13 keV) model, where the
blackbody component contributes approximately 10% of the source flux. The
optical broad-band spectrum shows evidence for an excess of infrared emission
with respect to an X-ray heated accretion disk model, suggesting a significant
contribution from the secondary or from a synchrotron-emitting region. A
follow-up observation performed when XTE J1814-338 was in quiescence reveals no
counterpart to a limiting magnitude of R = 23.3. This suggests that the
secondary is an M3 V or later-type star, and therefore very unlikely to be
responsible for the soft excess, making synchroton emission a more reasonable
candidate.Comment: Accepted for publication in ApJ. 6 pages; 3 figure
On the Origin of Radio Emission in the X-ray States of XTE J1650-500 during the 2001-2002 Outburst
We report on simultaneous radio and X-ray observations of the black hole
candidate XTE J1650-500 during the course of its 2001-2002 outburst. The
scheduling of the observations allowed us to sample the properties of XTE
J1650-50 in different X-ray spectral states, namely the hard state, the steep
power-law state and the thermal dominant state, according to the recent
spectral classification of McClintock & Remillard. The hard state is consistent
with a compact jet dominating the spectral energy distribution at radio
frequencies; however, the current data suggest that its contribution as direct
synchrotron emission at higher energies may not be significant. In that case,
XTE J1650-50 may be dominated by Compton processes (either inverse
Comptonization of thermal disk photons and/or SSC from the base of the compact
jet) in the X-ray regime. We, surprisingly, detect a faint level of radio
emission in the thermal dominant state that may be consistent with the emission
of previously ejected material interacting with the interstellar medium,
similar (but on a smaller angular scale) to what was observed in XTE J1550-564
by Corbel and co-workers. Based on the properties of radio emission in the
steep power-law state of XTE J1650-50, and taking into account the behavior of
other black hole candidates (namely GX 339-4, XTE J1550-564, and XTE J1859+226)
while in the intermediate and steep power-law states, we are able to present a
general pattern of behavior for the origin of radio emission in these two
states that could be important for understanding the accretion-ejection
coupling very close to the black hole event horizon.Comment: Accepted for publication in The Astrophysical Journal. 33 pages, 10
figure
A fast search strategy for gravitational waves from low-mass X-ray binaries
We present a new type of search strategy designed specifically to find
continuously emitting gravitational wave sources in known binary systems based
on the incoherent sum of frequency modulated binary signal sidebands. The
search pipeline can be divided into three stages: the first is a wide
bandwidth, F-statistic search demodulated for sky position. This is followed by
a fast second stage in which areas in frequency space are identified as signal
candidates through the frequency domain convolution of the F-statistic with an
approximate signal template. For this second stage only precise information on
the orbit period and approximate information on the orbital semi-major axis are
required apriori. For the final stage we propose a fully coherent Markov chain
monte carlo based follow up search on the frequency subspace defined by the
candidates identified by the second stage. This search is particularly suited
to the low-mass X-ray binaries, for which orbital period and sky position are
typically well known and additional orbital parameters and neutron star spin
frequency are not. We note that for the accreting X-ray millisecond pulsars,
for which spin frequency and orbital parameters are well known, the second
stage can be omitted and the fully coherent search stage can be performed. We
describe the search pipeline with respect to its application to a simplified
phase model and derive the corresponding sensitivity of the search.Comment: 13 pages, 3 figures, to appear in the GWDAW 11 conference proceeding
Monitoring and Discovering X-ray Pulsars in the Small Magellanic Cloud
Regular monitoring of the SMC with RXTE has revealed a huge number of X-ray
pulsars. Together with discoveries from other satellites at least 45 SMC
pulsars are now known. One of these sources, a pulsar with a period of
approximately 7.8 seconds, was first detected in early 2002 and since discovery
it has been found to be in outburst nine times. The outburst pattern clearly
shows a period of 45.1 +/- 0.4 d which is thought to be the orbital period of
this system. Candidate outburst periods have also been obtained for nine other
pulsars and continued monitoring will enable us to confirm these. This large
number of pulsars, all located at approximately the same distance, enables a
wealth of comparative studies. In addition, the large number of pulsars found
(which vastly exceeds the number expected simply by scaling the relative mass
of the SMC and the Galaxy) reveals the recent star formation history of the SMC
which has been influenced by encounters with both the LMC and the Galaxy.Comment: 5 pages, 4 figures, AIP conference proceedings format. Contribution
to "X-ray Timing 2003: Rossi and Beyond." meeting held in Cambridge, MA,
November, 200
Suzaku Observations of Four Heavily Absorbed HMXBs
We report on Suzaku observations of four unidentified sources from the
INTEGRAL and Swift BAT Galactic plane surveys. All the sources have a large
neutral hydrogen column density and are likely members of an emerging class of
heavily absorbed high mass X-ray binary (HMXB) first identified in INTEGRAL
observations. Two of the sources in our sample are approximately constant flux
sources, one source shows periodic variation and one source exhibited a short,
bright X-ray outburst. The periodicity is transient, suggesting it is produced
by a neutron star in an elliptical orbit around a stellar wind source. We
analyze the flaring source in several segments to look for spectral variation
and discuss the implications of the findings for the nature of the source. We
conclude that all four sources in our sample can be identified with the
emerging class of highly absorbed HMXBs, that one is a newly identified
transient X-ray pulsar and that at least one is a newly identified supergiant
fast X-ray transient (SFXT).Comment: 22 pages, 5 figures, submitted to Ap
A Chandra observation of the long-duration X-ray transient KS 1731-260 in quiescence: too cold a neutron star?
After more than a decade of actively accreting at about a tenth of the
Eddington critical mass accretion rate, the neutron-star X-ray transient KS
1731-260 returned to quiescence in early 2001. We present a Chandra/ACIS-S
observation taken several months after this transition. We detected the source
at an unabsorbed flux of ~2 x 10^{-13} erg/cm^2/s (0.5-10 keV). For a distance
of 7 kpc, this results in a 0.5-10 keV luminosity of ~1 x 10^{33} erg/s and a
bolometric luminosity approximately twice that. This quiescent luminosity is
very similar to that of the other quiescent neutron star systems. However, if
this luminosity is due to the cooling of the neutron star, this low luminosity
may indicate that the source spends at least several hundreds of years in
quiescence in between outbursts for the neutron star to cool. If true, then it
might be the first such X-ray transient to be identified and a class of
hundreds of similar systems may be present in the Galaxy. Alternatively,
enhanced neutrino cooling could occur in the core of the neutron star which
would cool the star more rapidly. However, in that case the neutron star in KS
1731-260 would be more massive than those in the prototypical neutron star
transients (e.g., Aql X-1 or 4U 1608-52).Comment: Accepted for publicaton in ApJ letters, 13 September 200
Search for L5 Earth Trojans with DECam
Most of the major planets in the Solar system support populations of co-orbiting bodies, known as Trojans, at their L4 and L5 Lagrange points. In contrast, Earth has only one known co-orbiting companion. This paper presents the results from a search for Earth Trojans (ETs) using the DECam instrument on the Blanco Telescope at CTIO. This search found no additional Trojans in spite of greater coverage compared to previous surveys of the L5 point. Therefore, the main result of this work is to place the most stringent constraints to date on the population of ETs. These constraints depend on assumptions regarding the underlying population properties, especially the slope of the magnitude distribution (which in turn depends on the size and albedo distributions of the objects). For standard assumptions, we calculate upper limits to a 90 perâcent confidence limit on the L5 population of N_(ET) < 1 for magnitude H < 15.5, N_(ET) = 60â85 for H < 19.7, and N_(ET) = 97 for H = 20.4. This latter magnitude limit corresponds to Trojans âŒ300 m in size for albedo 0.15. At H = 19.7, these upper limits are consistent with previous L4 ET constraints and significantly improve L5 constraints
Comparison between Windowed FFT and Hilbert-Huang Transform for Analyzing Time Series with Poissonian Fluctuations: A Case Study
Hilbert-Huang Transform (HHT) is a novel data analysis technique for
nonlinear and non-stationary data. We present a time-frequency analysis of both
simulated light curves and an X-ray burst from the X-ray burster 4U 1702-429
with both the HHT and the Windowed Fast Fourier Transform (WFFT) methods. Our
results show that the HHT method has failed in all cases for light curves with
Poissonian fluctuations which are typical for all photon counting instruments
used in astronomy, whereas the WFFT method can sensitively detect the periodic
signals in the presence of Poissonian fluctuations; the only drawback of the
WFFT method is that it cannot detect sharp frequency variations accurately.Comment: 10 pages, 12 figure
X-ray properties of an Unbiased Hard X-ray Detected Sample of AGN
The SWIFT gamma ray observatory's Burst Alert Telescope (BAT) has detected a
sample of active galactic nuclei (AGN) based solely on their hard X-ray flux
(14-195 keV). In this paper, we present for the first time {\it XMM-Newton}
X-ray spectra for 22 BAT AGNs with no previously analyzed X-ray spectra. If our
sources are a representative sample of the BAT AGN, as we claim, our results
present for the first time global X-ray properties of an unbiased towards
absorption (n), AGN
sample. We find 9/22 low absorption (n cm), simple power
law model sources, where 4 of these sources have a statistically significant
soft component. Among these sources, we find the presence of a warm absorber
statistically significant for only one Seyfert 1 source, contrasting with the
ASCA results of \citet{rey97} and \citet{geo98}, who find signatures of warm
absorption in half or more of their Seyfert 1 samples at similar redshifts.
Additionally, the remaining sources (14/22) have more complex spectra, well-fit
by an absorbed power law at keV. Five of the complex sources are
classified as Compton-thick candidates. Further, we find four more sources with
properties consistent with the hidden/buried AGN reported by Ueda {\it et al.}
(2007). Finally, we include a comparison of the {\it XMM-Newton} EPIC spectra
with available SWIFT X-ray Telescope (XRT) observations. From these
comparisons, we find 6/16 sources with varying column densities, 6/16 sources
with varying power law indices, and 13/16 sources with varying fluxes, over
periods of hours to months. Flux and power law index are correlated for objects
where both parameters vary.Comment: 39 pages, 16 figures, accepted to Ap
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