95,411 research outputs found
The quiescent counterpart of the peculiar X-ray burster SAX J2224.9+5421
SAX J2224.9+5421 is an extraordinary neutron star low-mass X-ray binary.
Albeit discovered when it exhibited a ~10-s long thermonuclear X-ray burst, it
had faded to a 0.5-10 keV luminosity of Lx<8E32 (D/7.1 kpc)^2 erg/s only ~8 hr
later. It is generally assumed that neutron stars are quiescent (i.e., not
accreting) at such an intensity, raising questions about the trigger conditions
of the X-ray burst and the origin of the faint persistent emission. We report
on a ~51 ks XMM-Newton observation aimed to find clues explaining the unusual
behavior of SAX J2224.9+5421. We identify a likely counterpart that is detected
at Lx~5E31 (D/7.1 kpc)^2 erg/s (0.5-10 keV) and has a soft X-ray spectrum that
can be described by a neutron star atmosphere model with a temperature of ~50
eV. This would suggest that SAX J2224.9+5421 is a transient source that was in
quiescence during our XMM-Newton observation and experienced a very faint
(ceasing) accretion outburst at the time of the X-ray burst detection. We
consider one other potential counterpart that is detected at Lx~5E32 (D/7.1
kpc)^2 erg/s and displays an X-ray spectrum that is best described by power law
with a photon index of ~1.7. Similarly hard X-ray spectra are seen for a few
quiescent neutron stars and may be indicative of a relatively strong magnetic
field or the occurrence of low-level accretion.Comment: 7 pages, 2 figures, 2 tables. Accepted to Ap
A direct measurement of the heat release in the outer crust of the transiently accreting neutron star XTE J1709-267
The heating and cooling of transiently accreting neutron stars provides a
powerful probe of the structure and composition of their crust. Observations of
superbursts and crust cooling of accretion-heated neutron stars require more
heat release than is accounted for in current models. Obtaining firm
constraints on the depth and magnitude of this extra heat is challenging and
therefore its origin remains uncertain. We report on Swift and XMM-Newton
observations of the transient neutron star low-mass X-ray binary XTE J1709-267,
which were made in 2012 September-October when it transitioned to quiescence
after a ~10-week long accretion outburst. The source is detected with
XMM-Newton at a 0.5-10 keV luminosity of Lx~2E34 (D/8.5 kpc)^2 erg/s. The X-ray
spectrum consists of a thermal component that fits to a neutron star atmosphere
model and a non-thermal emission tail, which each contribute ~50% to the total
emission. The neutron star temperature decreases from ~158 to ~152 eV during
the ~8-hour long observation. This can be interpreted as cooling of a crustal
layer located at a column density of y~5E12 g/cm^2 (~50 m inside the neutron
star), which is just below the ignition depth of superbursts. The required heat
generation in the layers on top would be ~0.06-0.13 MeV per accreted nucleon.
The magnitude and depth rule out electron captures and nuclear fusion reactions
as the heat source, but it may be accounted for by chemical separation of light
and heavy nuclei. Low-level accretion offers an alternative explanation for the
observed variability.Comment: 6 pages, 4 figures, 1 table, accepted to ApJ Letters. Minor changes
according to referee report, revised version includes a discussion on the
alternative interpretation of residual accretio
The X-ray flaring properties of Sgr A* during six years of monitoring with Swift
Starting in 2006, Swift has been targeting a region of ~21'X21' around
Sagittarius A* (Sgr A*) with the onboard X-ray telescope. The short,
quasi-daily observations offer an unique view of the long-term X-ray behavior
of the supermassive black hole. We report on the data obtained between 2006
February and 2011 October, which encompasses 715 observations with a total
accumulated exposure time of ~0.8 Ms. A total of six X-ray flares were detected
with Swift, which all had an average 2-10 keV luminosity of Lx (1-4)E35 erg/s
(assuming a distance of 8 kpc). This more than doubles the number of such
bright X-ray flares observed from Sgr A*. One of the Swift-detected flares may
have been softer than the other five, which would indicate that flares of
similar intensity can have different spectral properties. The Swift campaign
allows us to constrain the occurrence rate of bright (Lx > 1E35 erg/s) X-ray
flares to be ~0.1-0.2 per day, which is in line with previous estimates. This
analysis of the occurrence rate and properties of the X-ray flares seen with
Swift offers an important calibration point to asses whether the flaring
behavior of Sgr A* changes as a result of its interaction with the gas cloud
that is projected to make a close passage in 2013.Comment: 8 pages, 5 figures, 3 tables. Shortened, accepted to Ap
Compressibility and probabilistic proofs
We consider several examples of probabilistic existence proofs using
compressibility arguments, including some results that involve Lov\'asz local
lemma.Comment: Invited talk for CiE 2017 (full version
Telecommunications and data acquisition support for the Pioneer Venus Project: Pioneers 12 and 13, prelaunch through March 1984
The support provided by the Telecommunications and Data Acquisition organization of the Jet Propulsion Laboratory (JPL) to the Pioneer Venus missions is described. The missions were the responsibility of the Ames Research Center (ARC). The Pioneer 13 mission and its spacecraft design presented one of the greatest challenges to the Deep Space Network (DSN) in the implementation and operation of new capabilities. The four probes that were to enter the atmosphere of Venus were turned on shortly before arrival at Venus, and the DSN had to acquire each of these probes in order to recover the telemetry being transmitted. Furthermore, a science experiment involving these probes descending through the atmosphere required a completed new data type to be generated at the ground stations. This new data type is known as the differential very long baseline interferometry. Discussions between ARC and JPL of the implementation requirements involved trade-offs in spacecraft design and led to a very successful return of science data. Specific implementation and operational techniques are discussed, not only for the prime mission, but also for the extended support to the Pioneer 12 spacecraft (in orbit around Venus) with its science instruments including that for radar observations of the planet
The Swift X-ray monitoring campaign of the center of the Milky Way
In 2006 February, shortly after its launch, Swift began monitoring the center
of the Milky Way with the onboard X-Ray Telescope using short 1-ks exposures
performed every 1-4 days. Between 2006 and 2014, over 1200 observations have
been obtained, amounting to ~1.2 Ms of exposure time. This has yielded a wealth
of information about the long-term X-ray behavior of the supermassive black
hole Sgr A*, and numerous transient X-ray binaries that are located within the
25'x25' region covered by the campaign. In this review we highlight the
discoveries made during these first nine years, which includes 1) the detection
of seven bright X-ray flares from Sgr A*, 2) the discovery of the magnetar SGR
J1745-29, 3) the first systematic analysis of the outburst light curves and
energetics of the peculiar class of very-faint X-ray binaries, 4) the discovery
of three new transient X-ray sources, 5) exposing low-level accretion in
otherwise bright X-ray binaries, and 6) the identification of a candidate X-ray
binary/millisecond radio pulsar transitional object. We also reflect on future
science to be done by continuing this Swift's legacy campaign of the Galactic
center, which includes high-cadence monitoring of how the interaction between
the gaseous object `G2' and Sgr A* plays out in the future.Comment: 13 pages, 6 figures, 4 tables. Invited review to appear in Elsevier's
Journal of High Energy Astrophysics dedicated issue "Swift: 10 years of
discovery
The Galactic center X-ray transients AX J1745.6-2901 and GRS 1741-2853
AX J1745.6-2901 and GRS 1741-2853 are two transient neutron star low-mass
X-ray binaries that are located within ~10' from the Galactic center.
Multi-year monitoring observations with the Swift/XRT has exposed several
accretion outbursts from these objects. We report on their updated X-ray light
curves and renewed activity that occurred in 2010-2013.Comment: 2 pages, 1 figure, 1 table. To appear in conference proceedings of
IAU symposium 303 "The Galactic Center: Feeding and Feedback in a Normal
Galactic Nucleus
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