2,949 research outputs found
The Influence of Topography on the Formation of Temporary Bright Patches on Mars
Influence of topography on formation of bright patches on Mar
Spin frequency distributions of binary millisecond pulsars
Rotation-powered millisecond radio pulsars have been spun up to their present
spin period by a - yr long X-ray-bright phase of accretion of
matter and angular momentum in a low-to-intermediate mass binary system.
Recently, the discovery of transitional pulsars that alternate cyclically
between accretion and rotation-powered states on time scales of a few years or
shorter, has demonstrated this evolutionary scenario. Here, we present a
thorough statistical analysis of the spin distributions of the various classes
of millisecond pulsars to assess the evolution of their spin period between the
different stages. Accreting sources that showed oscillations exclusively during
thermonuclear type I X-ray bursts (nuclear-powered millisecond pulsars) are
found to be significantly faster than rotation-powered sources, while accreting
sources that possess a magnetosphere and show coherent pulsations (accreting
millisecond pulsars) are not. On the other hand, if accreting millisecond
pulsars and eclipsing rotation-powered millisecond pulsars form a common class
of transitional pulsars, these are shown to have a spin distribution
intermediate between the faster nuclear-powered millisecond pulsars and the
slower non-eclipsing rotation-powered millisecond pulsars. We interpret these
findings in terms of a spin-down due to the decreasing mass-accretion rate
during the latest stages of the accretion phase, and in terms of the different
orbital evolutionary channels mapped by the various classes of pulsars. We
summarize possible instrumental selection effects, showing that even if an
unbiased sample of pulsars is still lacking, their influence on the results of
the presented analysis is reduced by recent improvements in instrumentation and
searching techniques.Comment: Accepted for publication in A&A (6 pages, 4 figures
The puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence
We present an optical (gri) study during quiescence of the accreting
millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4m Gran
Telescopio Canarias (GTC) in August 2014. Despite the source being in
quiescence at the time of our observations, it showed a strong optical flaring
activity, more pronounced at higher frequencies (i.e. the g band). Once the
flares were subtracted, we tentatively recovered a sinusoidal modulation at the
system orbital period in all bands, even if a significant phase shift with
respect to an irradiated star, typical of accreting millisecond X-ray pulsars
is detected. We conclude that the observed flaring could be a manifestation of
the presence of an accretion disc in the system. The observed light curve
variability could be explained by the presence of a superhump, which might be
another proof of the formation of an accretion disc. In particular, the disc at
the time of our observations was probably preparing to the new outburst of the
source, that happened just a few months later, in 2015.Comment: 6 pages, 2 figures, 1 table. Accepted for publication in A&
Simultaneous Multi-band Radio & X-ray Observations of the Galactic Center Magnetar SGR 17452900
We report on multi-frequency, wideband radio observations of the Galactic
Center magnetar (SGR 17452900) with the Green Bank Telescope for 100
days immediately following its initial X-ray outburst in April 2013. We made
multiple simultaneous observations at 1.5, 2.0, and 8.9 GHz, allowing us to
examine the magnetar's flux evolution, radio spectrum, and interstellar medium
parameters (such as the dispersion measure (DM), the scattering timescale and
its index). During two epochs, we have simultaneous observations from the
Chandra X-ray Observatory, which permitted the absolute alignment of the radio
and X-ray profiles. As with the two other radio magnetars with published
alignments, the radio profile lies within the broad peak of the X-ray profile,
preceding the X-ray profile maximum by 0.2 rotations. We also find that
the radio spectral index is significantly negative between 2 and
9 GHz; during the final 30 days of our observations ,
which is typical of canonical pulsars. The radio flux has not decreased during
this outburst, whereas the long-term trends in the other radio magnetars show
concomitant fading of the radio and X-ray fluxes. Finally, our wideband
measurements of the DMs taken in adjacent frequency bands in tandem are
stochastically inconsistent with one another. Based on recent theoretical
predictions, we consider the possibility that the dispersion measure is
frequency-dependent. Despite having several properties in common with the other
radio magnetars, such as , an
increase in the radio flux during the X-ray flux decay has not been observed
thus far in other systems.Comment: 15 pages, 9 figures, 3 tables; accepted to Ap
The missing GeV {\gamma}-ray binary: Searching for HESS J0632+057 with Fermi-LAT
The very high energy (VHE; >100 GeV) source HESS J0632+057 has been recently
confirmed as a \gamma-ray binary, a subclass of the high mass X-ray binary
(HMXB) population, through the detection of an orbital period of 321 days. We
performed a deep search for the emission of HESS J0632+057 in the GeV energy
range using data from the Fermi Large Area Telescope (LAT). The analysis was
challenging due to the source being located in close proximity to the bright
\gamma-ray pulsar PSR J0633+0632 and lying in a crowded region of the Galactic
plane where there is prominent diffuse emission. We formulated a Bayesian block
algorithm adapted to work with weighted photon counts, in order to define the
off-pulse phases of PSR J0633+0632. A detailed spectral-spatial model of a 5
deg circular region centred on the known location of HESS J0632+057 was
generated to accurately model the LAT data. No significant emission from the
location of HESS J0632+057 was detected in the 0.1-100 GeV energy range
integrating over ~3.5 years of data; with a 95% flux upper limit of F_{0.1-100
GeV} < 3 x 10-8 ph cm-2 s-1. A search for emission over different phases of the
orbit also yielded no significant detection. A search for source emission on
shorter timescales (days--months) did not yield any significant detections. We
also report the results of a search for radio pulsations using the 100-m Green
Bank Telescope (GBT). No periodic signals or individual dispersed bursts of a
likely astronomical origin were detected. We estimated the flux density limit
of < 90/40 \mu Jy at 2/9 GHz. The LAT flux upper limits combined with the
detection of HESS J0632+057 in the 136-400 TeV energy band by the MAGIC
collaboration imply that the VHE spectrum must turn over at energies <136 GeV
placing constraints on any theoretical models invoked to explain the \gamma-ray
emission.Comment: 11 pages, 4 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS) Main Journa
SAX J1808.4-3658, an accreting millisecond pulsar shining in gamma rays?
We report the detection of a possible gamma-ray counterpart of the accreting
millisecond pulsar SAX J1808.4-3658. The analysis of ~6 years of data from the
Large Area Telescope on board the Fermi Gamma-ray Space Telescope (Fermi-LAT)
within a region of 15deg radius around the position of the pulsar reveals a
point gamma-ray source detected at a significance of ~6 sigma (Test Statistic
TS = 32), with position compatible with that of SAX J1808.4-3658 within 95%
Confidence Level. The energy flux in the energy range between 0.6 GeV and 10
GeV amounts to (2.1 +- 0.5) x 10-12 erg cm-2 s-1 and the spectrum is
well-represented by a power-law function with photon index 2.1 +- 0.1. We
searched for significant variation of the flux at the spin frequency of the
pulsar and for orbital modulation, taking into account the trials due to the
uncertainties in the position, the orbital motion of the pulsar and the
intrinsic evolution of the pulsar spin. No significant deviation from a
constant flux at any time scale was found, preventing a firm identification via
time variability. Nonetheless, the association of the LAT source as the
gamma-ray counterpart of SAX J1808.4-3658 would match the emission expected
from the millisecond pulsar, if it switches on as a rotation-powered source
during X-ray quiescence.Comment: 8 pages, 4 figures, accepted by MNRA
The Advanced X-ray Timing Array (AXTAR)
AXTAR is an X-ray observatory mission concept, currently under study in the
U.S., that combines very large collecting area, broadband spectral coverage,
high time resolution, highly flexible scheduling, and an ability to respond
promptly to time-critical targets of opportunity. It is optimized for
submillisecond timing of bright Galactic X-ray sources in order to study
phenomena at the natural time scales of neutron star surfaces and black hole
event horizons, thus probing the physics of ultradense matter, strongly curved
spacetimes, and intense magnetic fields. AXTAR's main instrument is a
collimated, thick Si pixel detector with 2-50 keV coverage and 8 square meters
collecting area. For timing observations of accreting neutron stars and black
holes, AXTAR provides at least an order of magnitude improvement in sensitivity
over both RXTE and Constellation-X. AXTAR also carries a sensitive sky monitor
that acts as a trigger for pointed observations of X-ray transients and also
provides continuous monitoring of the X-ray sky with 20 times the sensitivity
of the RXTE ASM. AXTAR builds on detector and electronics technology previously
developed for other applications and thus combines high technical readiness and
well understood cost.Comment: 4 pages with 1 figure, to appear in the proceedings of "A Decade of
Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds. R. Wijnands
et al. (AIP Conf. Proc.). Footnote and references adde
An XMM-Newton and INTEGRAL view on the hard state of EXO 1745-248 during its 2015 outburst
CONTEXT - Transient low-mass X-ray binaries (LMXBs) often show outbursts
lasting typically a few-weeks and characterized by a high X-ray luminosity
( erg/sec), while for most of the time they are
found in X-ray quiescence ( erg/sec). EXO 1745-248
is one of them. AIMS - The broad-band coverage, and the sensitivity of
instrument on board of {\xmm} and {\igr}, offers the opportunity to
characterize the hard X-ray spectrum during {\exo} outburst. METHODS - In this
paper we report on quasi-simultaneous {\xmm} and {\igr} observations of the
X-ray transient {\exo} located in the globular cluster Terzan 5, performed ten
days after the beginning of the outburst (on 2015 March 16th) shown by the
source between March and June 2015. The source was caught in a hard state,
emitting a 0.8-100 keV luminosity of ~{\lumcgs}. RESULTS - The
spectral continuum was dominated by thermal Comptonization of seed photons with
temperature keV, by a cloud with moderate optical depth
and electron temperature keV. A weaker soft
thermal component at temperature --0.7 keV and compatible
with a fraction of the neutron star radius was also detected. A rich emission
line spectrum was observed by the EPIC-pn on-board {\xmm}; features at energies
compatible with K- transitions of ionized sulfur, argon, calcium and
iron were detected, with a broadness compatible with either thermal Compton
broadening or Doppler broadening in the inner parts of an accretion disk
truncated at gravitational radii from the neutron star. Strikingly, at
least one narrow emission line ascribed to neutral or mildly ionized iron is
needed to model the prominent emission complex detected between 5.5 and 7.5
keV. (Abridged)Comment: 14 pages, 6 figure, 2 tables. Accepted for publication on A&A
(21/03/2017
A new low magnetic field magnetar: the 2011 outburst of Swift J1822.3-1606
We report on the long term X-ray monitoring with Swift, RXTE, Suzaku, Chandra
and XMM-Newton of the outburst of the newly discovered magnetar Swift
J1822.3-1606 (SGR 1822-1606), from the first observations soon after the
detection of the short X-ray bursts which led to its discovery, through the
first stages of its outburst decay (covering the time-span from July 2011,
until end of April 2012). We also report on archival ROSAT observations which
witnessed the source during its likely quiescent state, and on upper limits on
Swift J1822.3-1606's radio-pulsed and optical emission during outburst, with
the Green Bank Telescope (GBT) and the Gran Telescopio Canarias (GTC),
respectively. Our X-ray timing analysis finds the source rotating with a period
of P=8.43772016(2) s and a period derivative \dot{P}=8.3(2)x10^{-14} s s^{-1} ,
which entails an inferred dipolar surface magnetic field of B~2.7x10^{13} G at
the equator. This measurement makes Swift J1822.3-1606 the second lowest
magnetic field magnetar (after SGR 0418+5729; Rea et al. 2010). Following the
flux and spectral evolution from the beginning of the outburst, we find that
the flux decreased by about an order of magnitude, with a subtle softening of
the spectrum, both typical of the outburst decay of magnetars. By modeling the
secular thermal evolution of Swift J1822.3-1606, we find that the observed
timing properties of the source, as well as its quiescent X-ray luminosity, can
be reproduced if it was born with a poloidal and crustal toroidal fields of
B_{p}~1.5x10^{14} G and B_{tor}~7x10^{14} G, respectively, and if its current
age is ~550 kyr.Comment: 14 pages, 9 figures; new observations added; ApJ in pres
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