767 research outputs found
Footprints in the wind of Vela X-1 traced with MAXI
The stellar wind around the compact object in luminous wind-accreting high
mass X-ray binaries is expected to be strongly ionized with the X-rays coming
from the compact object. The stellar wind of hot stars is mostly driven by
light absorption in lines of heavier elements, and X-ray photo-ionization
significantly reduces the radiative force within the so-called Stroemgren
region leading to wind stagnation around the compact object. In close binaries
like Vela X-1 this effect might alter the wind structure throughout the system.
Using the spectral data from Monitor of All-sky X-ray Image (MAXI), we study
the observed dependence of the photoelectric absorption as function of orbital
phase in Vela X-1, and find that it is inconsistent with expectations for a
spherically-symmetric smooth wind. Taking into account previous investigations
we develop a simple model for wind structure with a stream-like photoionization
wake region of slower and denser wind trailing the neutron star responsible for
the observed absorption curve.Comment: 5 pages, 3 figures, accepted in A&
Black hole candidate XTE J1752-223: Swift observations of canonical states during outburst
We present Swift broadband observations of the recently discovered black hole
candidate, X-ray transient, XTE J1752-223, obtained over the period of outburst
from October 2009 to June 2010. From Swift-UVOT data we confirm the presence of
an optical counterpart which displays variability correlated, in the soft
state, to the X-ray emission observed by Swift-XRT. The optical counterpart
also displays hysteretical behaviour between the states not normally observed
in the optical bands, suggesting a possible contribution from a synchrotron
emitting jet to the optical emission in the rising hard state. We offer a
purely phenomenological treatment of the spectra as an indication of the
canonical spectral state of the source during different periods of the
outburst. We find that the high energy hardness-intensity diagrams over two
separate bands follows the canonical behavior, confirming the spectral states.
Our XRT timing analysis shows that in the hard state there is significant
variability below 10Hz which is more pronounced at low energies, while during
the soft state the level of variability is consistent with being minimal. These
properties of XTE J1752-223 support its candidacy as a black hole in the
Galactic centre region.Comment: 8 pages, 8 figures; MNRAS in pres
Combined Spectral and Timing Analysis of the Black Hole Candidate MAXI J1659-152 Discovered by MAXI and Swift
We report on X-ray spectral and timing results of the new black hole
candidate (BHC) MAXI J1659-152 with the orbital period of 2.41 hours (shortest
among BHCs) in the 2010 outburst from 65 Rossi X-ray Timing Explorer (RXTE)
observations and 8 simultaneous Swift and RXTE observations. According to the
definitions of the spectral states in Remillard & McClintock (2006), most of
the observations have been classified into the intermediate state. All the
X-ray broadband spectra can be modeled by a multi-color disk plus a power-law
with an exponential cutoff or a multi-color disk plus a Comptonization
component. During the initial phase of the outburst, a high energy cutoff was
visible at 30-40 keV. The innermost radius of the disk gradually decreased by a
factor of more than 3 from the onset of the outburst and reached a constant
value of 35 d_10 cos i^-1/2 km, where d_10 is the distance in units of 10 kpc
and is the inclination. The type-C quasi-periodic oscillation (QPO)
frequency varied from 1.6 Hz to 7.3 Hz in association with a change of the
innermost radius, while the innermost radius remained constant during the
type-B QPO detections at 1.6-4.1 Hz. Hence, we suggest that the origin of the
type-B QPOs is different from that of type-C QPOs, the latter of which would
originate from the disk truncation radius. Assuming the constant innermost
radius in the latter phase of the outburst as the innermost stable circular
orbit, the black hole mass in MAXI J1659-152 is estimated to be 3.6-8.0 M_solar
for a distance of 5.3-8.6 kpc and an inclination angle of 60-75 degrees.Comment: 27 pages, 14 figures, accepted for publication in PAS
The Peculiar X-ray transient Swift J0840.7-3516: an unusual low-mass X-ray binary or a tidal disruption event?
We report on the X-ray properties of the new transient Swift J0840.7-3516, discovered with Swift/BAT in 2020 February, using extensive data from Swift, MAXI, NICER, and NuSTAR. The source flux increased for similar to 10(3) s after the discovery, decayed rapidly over similar to 5 orders of magnitude in five days, and then remained almost constant over nine months. Large-amplitude short-term variations on timescales of 1-10(4) s were observed throughout the decay. In the initial flux rise, the source showed a hard power-law-shaped spectrum with a photon index of similar to 1.0 extending up to similar to 30 keV, above which an exponential cutoff was present. The photon index increased in the following rapid decay and became similar to 2 at the end of the decay. A spectral absorption feature at 3-4 keV was detected in the decay. It is not straightforward to explain all the observed properties by any known class of X-ray sources. We discuss the possible nature of the source, including a Galactic low-mass X-ray binary with multiple extreme properties and a tidal disruption event by a supermassive black hole or a Galactic neutron star
Radiation dose during relativistic electron precipitation events at the International Space Station
AbstractWe provide a quantitative estimate of the radiation dose during relativistic electron precipitation (REP) events at the International Space Station (ISS). To this goal, we take advantage of the data collected by the CALorimetric Electron Telescope, the Monitor of Allâsky Xâray Image, and the Space Environment Data Acquisition equipmentâAttached Payload. The three ISS detectors offer complementary REP observations, including energy spectra and flux directional information, during a period of approximately 2.5 years, from November 2015 to March 2018. We have identified 762 REP events during this period from which we obtain the distribution of radiation dose, relevant to extravehicular activities outside the ISS
On-orbit Operations and Offline Data Processing of CALET onboard the ISS
The CALorimetric Electron Telescope (CALET), launched for installation on the
International Space Station (ISS) in August, 2015, has been accumulating
scientific data since October, 2015. CALET is intended to perform long-duration
observations of high-energy cosmic rays onboard the ISS. CALET directly
measures the cosmic-ray electron spectrum in the energy range of 1 GeV to 20
TeV with a 2% energy resolution above 30 GeV. In addition, the instrument can
measure the spectrum of gamma rays well into the TeV range, and the spectra of
protons and nuclei up to a PeV.
In order to operate the CALET onboard ISS, JAXA Ground Support Equipment
(JAXA-GSE) and the Waseda CALET Operations Center (WCOC) have been established.
Scientific operations using CALET are planned at WCOC, taking into account
orbital variations of geomagnetic rigidity cutoff. Scheduled command sequences
are used to control the CALET observation modes on orbit. Calibration data
acquisition by, for example, recording pedestal and penetrating particle
events, a low-energy electron trigger mode operating at high geomagnetic
latitude, a low-energy gamma-ray trigger mode operating at low geomagnetic
latitude, and an ultra heavy trigger mode, are scheduled around the ISS orbit
while maintaining maximum exposure to high-energy electrons and other
high-energy shower events by always having the high-energy trigger mode active.
The WCOC also prepares and distributes CALET flight data to collaborators in
Italy and the United States.
As of August 31, 2017, the total observation time is 689 days with a live
time fraction of the total time of approximately 84%. Nearly 450 million events
are collected with a high-energy (E>10 GeV) trigger. By combining all operation
modes with the excellent-quality on-orbit data collected thus far, it is
expected that a five-year observation period will provide a wealth of new and
interesting results.Comment: 11 pages, 7 figures, published online 27 February 201
XTE J1752-223 in outburst: a persistent radio jet, dramatic flaring, multiple ejections and linear polarisation
The black hole candidate, XTE J1752-223, was discovered in 2009 October when
it entered an outburst. We obtained radio data from the Australia Telescope
Compact Array for the duration of the ~9 month event. The lightcurves show that
the radio emission from the compact jet persisted for the duration of an
extended hard state and through the transition to the intermediate state. The
flux then rose rapidly by a factor of 10 and the radio source entered a series
of at least 7 maxima, the first of which was likely to be emission associated
with the compact jet. The subsequent 6 flares were accompanied by variable
behaviour in terms of radio spectrum, degree of linear polarisation, morphology
and associated X-ray behaviour. They were, however, remarkably similar in terms
of the estimated minimum power required to launch such an ejection event. We
compare the timing of radio peaks with the location of the ejecta, imaged by
contemporaneous VLBI experiments. We then discuss the mechanism behind the
events, in terms of whether discrete ejections is the most likely description
of the behaviour. One ejection, at least, appears to be travelling with
apparent superluminal motion. The range of properties, however, suggests that
mutiple mechanisms may be relevant and that at least some of the emission is
coming from shocked interactions amongst the ejecta and between the ejecta and
the interstellar medium. We also compare the radio flux density with the X-ray
source during the hard state and conclude that XTE J1752-223 is a
radio-weak/X-ray-bright outlier on the universal correlation for black hole
transient sources.Comment: 14 pages; Accepted for publication in MNRA
- âŠ