1,485 research outputs found
The identification of MAXI J1659-152 as a black hole candidate
We report on the analysis of all 65 pointed Rossi X-ray Timing Explorer
observations of the recently discovered soft X-ray transient MAXI J1659-152
(initially referred to as GRB 100925A). The source was studied in terms of its
evolution through the hardness-intensity diagram (HID) as well as its X-ray
variability properties. MAXI J1659-152 traced out an anti-clockwise loop in the
HID, which is commonly seen in transient low-mass X-ray binaries. The
variability properties of the source, in particular the detection of type-B and
type-C low-frequency quasi-periodic oscillations, and the way they evolve along
the HID track, indicate that MAXI J1659-152 is a black hole candidate. The
spectral and variability properties of MAXI J1659-152 imply that the source was
observed in the hard and soft intermediate states during the RXTE observations,
with several transitions between these two states.Comment: 6 pages, 4 figures, accepted for publication in ApJ
The variable radio counterpart and possible large-scale jet of the new Z-source XTE J1701-462
We report radio observations, made with the Australia Telescope Compact
Array, of the X-ray transient XTE J1701-462. This system has been classified as
a new `Z' source, displaying characteristic patterns of behaviour probably
associated with accretion onto a low magnetic field neutron star at close to
the Eddington limit. The radio counterpart is highly variable, and was detected
in six of sixteen observations over the period 2006 January -- April. The
coupling of radio emission to X-ray state, despite limited sampling, appears to
be similar to that of other `Z' sources, in that there is no radio emission on
the flaring branch. The mean radio and X-ray luminosities are consistent with
the other Z sources for a distance of 5--15 kpc. The radio spectrum is
unusually flat, or even inverted, in contrast to the related sources, Sco X-1
and Cir X-1, which usually display an optically thin radio spectrum. Deep
wide-field observations indicate an extended structure three arcminutes to the
south which is aligned with the X-ray binary. This seems to represent a
significant overdensity of radio sources for the field and so, although a
background source remains a strong possibility, we consider it plausible that
this is a large-scale jet associated with XTE J1701-462.Comment: Accepted for publication as a Letter in MNRA
On the nature of the first transient Z-source XTE J1701-462: its accretion disk structure, neutron star magnetic field strength, and hard tail
Using the data from the Rossi X-Ray Timing Explorer satellite, we investigate
the spectral evolution along a "Z" track and a "v" track on the
hardness-intensity diagrams of the first transient Z source XTE J1701-462. The
spectral analyses suggest that the inner disk radius depends on the mass
accretion rate, in agreement with the model prediction, R_in \propto
((dM/dt)_disk)^{2/7}, for a radiation pressure dominated accretion disk
interacting with the magnetosphere of a neutron star (NS). The changes in the
disk mass accretion rate (dM/dt)_disk are responsible for the evolution of the
"Z" or "v" track. The radiation pressure thickens the disk considerably, and
also produces significant outflows. The NS surface magnetic field strength,
derived from the interaction between the magnetosphere and the radiation
pressure dominated accretion disk, is ~(1--3)X10^9 G, which is possibly between
normal atoll and Z sources. A significant hard tail is detected in the
horizontal branches and we discuss several possible origins of the hard tail
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
Full-Stokes polarimetry with circularly polarized feeds - Sources with stable linear and circular polarization in the GHz regime
We present a pipeline that allows recovering reliable information for all
four Stokes parameters with high accuracy. Its novelty relies on the treatment
of the instrumental effects already prior to the computation of the Stokes
parameters contrary to conventional methods, such as the M\"uller matrix one.
The instrumental linear polarization is corrected across the whole telescope
beam and significant Stokes and can be recovered even when the recorded
signals are severely corrupted. The accuracy we reach in terms of polarization
degree is of the order of 0.1-0.2 %. The polarization angles are determined
with an accuracy of almost 1. The presented methodology was applied
to recover the linear and circular polarization of around 150 Active Galactic
Nuclei. The sources were monitored from July 2010 to April 2016 with the
Effelsberg 100-m telescope at 4.85 GHz and 8.35 GHz with a cadence of around
1.2 months. The polarized emission of the Moon was used to calibrate the
polarization angle. Our analysis showed a small system-induced rotation of
about 1 at both observing frequencies. Finally, we identify five
sources with significant and stable linear polarization; three sources remain
constantly linearly unpolarized over the period we examined; a total of 11
sources have stable circular polarization degree and four of
them with non-zero . We also identify eight sources that maintain
a stable polarization angle over the examined period. All this is provided to
the community for polarization observations reference. We finally show that our
analysis method is conceptually different from the traditionally used ones and
performs better than the M\"uller matrix method. Although it was developed for
a system equipped with circularly polarized feeds it can easily be modified for
systems with linearly polarized feeds as well.Comment: 19 pages, 17 figures, accepted for publication in Astronomy &
Astrophysics on May 30, 201
INTEGRAL/RossiXTE high-energy observation of a state transition of GX 339-4
On 2004 August 15, we observed a fast (shorter than 10 hours) state
transition in the bright black-hole transient GX 339-4 simultaneously with
RossiXTE and INTEGRAL. This transition was evident both in timing and spectral
properties. Combining the data from PCA, HEXTE and IBIS, we obtained good
quality broad-band (3-200 keV) energy spectra before and after the transition.
These spectra indicate that the hard component steepened. Also, the high-energy
cutoff that was present at ~70 keV before the transition was not detected after
the transition. This is the first time that an accurate determination of the
broad-band spectrum across such a transition has been measured on a short time
scale. It shows that, although some spectral parameters do not change abruptly
through the transition, the high-energy cutoff increases/disappears rather
fast. These results constitute a benchmark on which to test theoretical models
for the production of the hard component in these systems.Comment: Accepted for publication in MNRAS (9 pages, 6 figures
Discovery of two simultaneous non-harmonically related Quasi-Periodic Oscillations in the 2005 outburst of the black-hole binary GRO J1655-40
We studied the low-frequency quasi-periodic oscillations (LFQPOs) in the
black hole GRO J1655-40 during the 2005 outburst, using data from the Rossi
X-ray Timing Explorer. All LFQPOs could be identified as either type B or type
C using previously proposed classification schemes. In the soft state of the
outburst the type-C LFQPOs reached frequencies that are among the highest ever
seen for LFQPOs in black holes. At the peak of the outburst, in the
ultra-luminous state, the power spectrum showed two simultaneous,
non-harmonically related peaks which we identified as a type-B and a type-C
QPO. The simultaneous presence of a type-C and type-B QPO shows that at least
two of the three known LFQPO types are intrinsically different and likely the
result of distinct physical mechanisms. We also studied the properties of a
broad peaked noise component in the power spectra of the ultra-luminous state.
This noise component becomes more coherent with count rate and there are strong
suggestions that it evolves into a type-B QPO at the highest observed count
rates.Comment: 14 pages, 6 figures, 2 tables, accepted for publication in MNRA
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