83 research outputs found
Eclipse Timings of the LMXB XTE J1710-281: Orbital Period Glitches
We present an X-ray eclipse timing analysis of the transient low mass X-ray
binary XTE J1710-281. We report observations of 57 complete X-ray eclipses,
made with the proportional counter array detectors aboard the RXTE satellite.
Using the eclipse timing technique, we have derived a constant orbital period
of 0.1367109674 (3) d, during the period from MJD 52132 upto MJD 54410; and
1{\sigma} limits of -1.6 10^ -12 d d-1 and 0.2 10^ -12 d d-1
, on the period derivative. This puts constraints on the minimum timescale of
secular orbital period evolution of 2.34 10^8 yr for a period decay
and 18.7 10^8 yr for a period increase. We also report detection of
two instances of discontinuity in the mid-eclipse time, one before and one
after the above MJD range. These results are interpreted as three distinct
epochs of orbital period in XTE J1710-281. We have put lower limits of 1.4 ms
and 0.9 ms on orbital period change at successive epochs.Comment: Accepted for publication in MNRAS Main Journal, 5 pages, 3 figure
Measuring Gravitational Lensing Flexions in Abell 1689 Using an Analytic Image Model
Measuring dark matter substructure within galaxy cluster haloes is a
fundamental probe of the Lambda-CDM model of structure formation. Gravitational
lensing is a technique for measuring the total mass distribution which is
independent of the nature of the gravitating matter, making it a vital tool for
studying these dark-matter dominated objects. We present a new method for
measuring weak gravitational lensing flexions, the gradients of the lensing
shear field, to measure mass distributions on small angular scales. While
previously published methods for measuring flexions focus on measuring derived
properties of the lensed images, such as shapelet coefficients or surface
brightness moments, our method instead fits a
mass-sheet-transformation-invariant Analytic Image Model (AIM) to the each
galaxy image. This simple parametric model traces the distortion of lensed
image isophotes and constrains the flexion fields. We test the AIM method using
simulated data images with realistic noise and a variety of unlensed image
properties, and show that it successfully reproduces the input flexion fields.
We also apply the AIM method for flexion measurement to Hubble Space Telescope
observations of Abell 1689, and detect mass structure in the cluster using
flexions measured with the AIM method.Comment: 44 pages, 4 figures, 3 tables. Accepted to ApJ. V2 (published
version) has minor changes from V1; ApJ 736 (2011
The long-term evolution of the spin, pulse shape, and orbit of the accretion-powered millisecond pulsar SAX J1808.4-3658
We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4-3658,
an X-ray transient with a recurrence time of ~2 yr, using data from the Rossi
X-ray Timing Explorer covering 4 transient outbursts (1998-2005). We verify
that the 401 Hz pulsation traces the spin frequency fundamental and not a
harmonic. Substantial pulse shape variability, both stochastic and systematic,
was observed during each outburst. Analysis of the systematic pulse shape
changes suggests that, as an outburst dims, the X-ray "hot spot" on the pulsar
surface drifts longitudinally and a second hot spot may appear. The overall
pulse shape variability limits the ability to measure spin frequency evolution
within a given X-ray outburst (and calls previous nudot measurements of this
source into question), with typical upper limits of |nudot| < 2.5x10^{-14} Hz/s
(2 sigma). However, combining data from all the outbursts shows with high (6
sigma) significance that the pulsar is undergoing long-term spin down at a rate
nudot = (-5.6+/-2.0)x10^{-16} Hz/s, with most of the spin evolution occurring
during X-ray quiescence. We discuss the possible contributions of magnetic
propeller torques, magnetic dipole radiation, and gravitational radiation to
the measured spin down, setting an upper limit of B < 1.5x10^8 G for the
pulsar's surface dipole magnetic field and and Q/I < 5x10^{-9} for the
fractional mass quadrupole moment. We also measured an orbital period
derivative of Pdot = (3.5+/-0.2)x10^{-12} s/s. This surprising large Pdot is
reminiscent of the large and quasi-cyclic orbital period variation observed in
the so-called "black widow" millisecond radio pulsars, supporting speculation
that SAX J1808.4-3658 may turn on as a radio pulsar during quiescence. In an
appendix we derive an improved (0.15 arcsec) source position from optical data.Comment: 22 pages, 10 figures; accepted for publication in Ap
An XTE Archival Search for Coherent X-ray Pulsations in LMXB 4U 1820-30
As part of a large-scale search for coherent pulsations from LMXBs in the
RXTE archive, we have completed a detailed series of searches for coherent
pulsations of 4U 1820-30 -- an ultracompact LMXB with a binary period of 11.4
min, located in the globular cluster NGC6624. The short binary period implies
any coherent signal would be highly accelerated, so we used phase modulation
searches, orbital-parameter-fitting coherent searches, and standard
acceleration searches to give significant sensitivity to millisecond
pulsations. We searched, in four energy bands and at a range of luminosities, a
total of 34 archival RXTE observations, 32 of which had on-source integration
times longer than 10 ks, and some of which were made consecutively which
allowed us to combine them. We found no pulsations. Using our phase modulation
search technique, which we ran on all 34 observations, we have been able to
place the first stringent (95% confidence) pulsed fraction limits of <~0.8% for
all realistic spin frequencies (i.e. <~2kHz) and likely companion masses
(0.02Msun <= M_c <= 0.3Msun). Using our orbital-parameter-fitting coherent
search, which we ran on only 11 selected observations, we have placed a pulsed
fraction limit of <~0.3% for spin frequencies <~1.25kHz and companion masses M_
<= 0.106Msun. By contrast, all five LMXBs known to emit coherent pulsations
have intrinsic pulsed fractions in the range 3% to 7% when pulsations are
observed. Hence, our searches rule out pulsations with significantly lower
pulsed fractions than those already observed.Comment: 10 pages, 5 figures, published in ApJ, the text of this astro-ph
version is now identical to the text of the published versio
GALEX FUV Observations of Comet C/2004 Q2 (Machholz): The Ionization Lifetime of Carbon
We present a measurement of the lifetime of ground state atomic carbon,
C(^3P), against ionization processes in interplanetary space and compare it to
the lifetime expected from the dominant physical processes likely to occur in
this medium. Our measurement is based on analysis of a far ultraviolet (FUV)
image of comet C/2004 Q2 (Machholz) recorded by the Galaxy Evolution Explorer
(GALEX) on 2005 March 1. The bright CI 1561 A and 1657 A multiplets dominate
the GALEX FUV band. We used the image to create high S/N radial profiles that
extended beyond one million km from the comet nucleus. Our measurements yielded
a total carbon lifetime of 7.1 -- 9.6 x 10^5 s (scaled to 1 AU). Which compares
favorably to calculations assuming solar photoionization, solar wind proton
change exchange and solar wind electron impact ionization are the dominant
processes occurring in this medium and that comet Machholz was embedded in the
slow solar wind. The shape of the CI profiles inside 3x10^5 km suggests that
either the CO lifetime is shorter than previously thought and/or a
shorter-lived carbon-bearing parent molecule, such as CH_4 is providing the
majority of the carbon in this region of the coma of comet Machholz.Comment: 26 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Does GRS 1915+105 exhibit "canonical" black-hole states?
We have analysed RXTE data of the superluminal source GRS 1915+105 in order
to investigate if, despite its extreme variability, it also exhibits the
canonical source states that characterise other black-hole candidates. The
phenomenology of GRS 1915+105 has been described in terms of three states
(named A, B and C) based on their hardness ratios and position in the
colour-colour diagram. We have investigated the connection between these states
and the canonical behaviour and found that the shape of the power spectral
continuum and the values of the best-fit model parameters to the noise
components in all three states indicate that the source shows properties
similar to the canonical very high state.Comment: 5 pages, 3 figures, accepted for publication in A&
A powerful bursting radio source towards the Galactic Centre
Transient astronomical sources are typically powered by compact objects and
usually signify highly explosive or dynamic events. While radio astronomy has
an impressive record of obtaining high time resolution observations, usually it
is achieved in quite narrow fields-of-view. Consequently, the dynamic radio sky
is poorly sampled, in contrast to the situation in the X- and gamma-ray bands
in which wide-field instruments routinely detect transient sources. Here we
report a new transient source, GCRT J1745-3009, detected in 2002 during a
moderately wide-field radio transient monitoring program of the Galactic center
(GC) region at 0.33 GHz. The characteristics of its bursts are unlike those
known for any other class of radio transient. If located in or near the GC, its
brightness temperature (~10^16 K) and the implied energy density within GCRT
J1745-3009 vastly exceeds that observed in most other classes of radio
astronomical sources, and is consistent with coherent emission processes rarely
observed. We conclude that GCRT J1745-3009 is the first member of a new class
of radio transient sources, the first of possibly many new classes to be
identified through current and upcoming radio surveys.Comment: 16 pages including 3 figures. Appears in Nature, 3 March 200
BlackCAT: A catalogue of stellar-mass black holes in X-ray transients
During the last ~50 years, the population of black hole candidates in X-ray
binaries has increased considerably with 59 Galactic objects detected in
transient low-mass X-ray binaries, plus a few in persistent systems (including
~5 extragalactic binaries). We collect near-infrared, optical and X-ray
information spread over hundreds of references in order to study the population
of black holes in X-ray transients as a whole. We present the most updated
catalogue of black hole transients, which contains X-ray, optical and
near-infrared observations together with their astrometric and dynamical
properties. It provides new useful information in both statistical and
observational parameters providing a thorough and complete overview of the
black hole population in the Milky Way. Analysing the distances and spatial
distribution of the observed systems, we estimate a total population of ~1300
Galactic black hole transients. This means that we have already discovered less
than ~5% of the total Galactic distribution. The complete version of this
catalogue will be continuously updated online and in the Virtual Observatory,
including finding charts and data in other wavelengths.Comment: http://www.astro.puc.cl/BlackCAT - Accepted for publication in
Astronomy & Astrophysics. 20 pages, 8 figures, 5 Table
Infrared Dark Clouds in the Small Magellanic Cloud?
We have applied the unsharp-masking technique to the 24 m image of the
Small Magellanic Cloud (SMC), obtained with the Spitzer Space Telescope, to
search for high-extinction regions. This technique has been used to locate very
dense and cold interstellar clouds in the Galaxy, particularly infrared dark
clouds (IRDCs). Fifty five candidate regions of high-extinction, namely
high-contrast regions (HCRs), have been identified from the generated
decremental contrast image of the SMC. Most HCRs are located in the southern
bar region and mainly distributed in the outskirts of CO clouds, but most
likely contain a significant amount of H2. HCRs have a peak-contrast at 24
m of 2 - 2.5 % and a size of 8 - 14 pc. This corresponds to the size of
typical and large Galactic IRDCs, but Galactic IRDCs are 2 - 3 times darker at
24 m than our HCRs. To constrain the physical properties of the HCRs, we
have performed NH3, N2H+, HNC, HCO+, and HCN observations toward one of the
HCRs, HCR LIRS36-EAST, using the Australia Telescope Compact Array and the
Mopra single-dish radio telescope. We did not detect any molecular line
emission, however, our upper limits to the column densities of molecular
species suggest that HCRs are most likely moderately dense with n ~ 10^{3}
cm-3. This volume density is in agreement with predictions for the cool atomic
phase in low metallicity environments. We suggest that HCRs may be tracing
clouds at the transition from atomic to molecule-dominated medium, and could be
a powerful way to study early stages of gas condensation in low metallicity
galaxies. Alternatively, if made up of dense molecular clumps < 0.5 pc in size,
HCRs could be counterparts of Galactic IRDCs, and/or regions with highly
unusual abundance of very small dust grains.Comment: accepted for publication in the Astronomical Journa
NICER and Fermi GBM Observations of the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124
Swift J0243.6+6124 is a newly discovered Galactic Be/X-ray binary, revealed in late 2017 September in a giant outburst with a peak luminosity of 2 × 10[superscript 39](d/7 kpc)[superscript 2] erg s[superscript -1] (0.1-10 keV), with no formerly reported activity. At this luminosity, Swift J0243.6+6124 is the first known galactic ultraluminous X-ray pulsar. We describe Neutron star Interior Composition Explorer (NICER) and Fermi Gamma-ray Burst Monitor (GBM) timing and spectral analyses for this source. A new orbital ephemeris is obtained for the binary system using spin frequencies measured with GBM and 15-50 keV fluxes measured with the Neil Gehrels Swift Observatory Burst Alert Telescope to model the system's intrinsic spin-up. Power spectra measured with NICER show considerable evolution with luminosity, including a quasi-periodic oscillation near 50 mHz that is omnipresent at low luminosity and has an evolving central frequency. Pulse profiles measured over the combined 0.2-100 keV range show complex evolution that is both luminosity and energy dependent. Near the critical luminosity of L ∼ 10[superscript 38] erg s[superscript -1], the pulse profiles transition from single peaked to double peaked, the pulsed fraction reaches a minimum in all energy bands, and the hardness ratios in both NICER and GBM show a turnover to softening as the intensity increases. This behavior repeats as the outburst rises and fades, indicating two distinct accretion regimes. These two regimes are suggestive of the accretion structure on the neutron star surface transitioning from a Coulomb collisional stopping mechanism at lower luminosities to a radiation-dominated stopping mechanism at higher luminosities. This is the highest observed (to date) value of the critical luminosity, suggesting a magnetic field of B ∼ 10[superscript 13] G.United States. National Aeronautics and Space Administratio
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