112 research outputs found
Comptonization, the X-ray-radio correlation and the long-term periodicity in the chi-states of GRS 1915+105
We analyzed 139 chi-state observations of GRS 1915+105 with RXTE from 1997 to
2000 and found i) that the observations fall into two groups with different
Comptonization behavior, ii) that the slope of the hard X-ray component
correlates with the radio flux, thus revealing the interaction of jet and
corona, and iii) a 590 days long term periodicity in the hard X-ray and radio
components.Comment: 4 pages, 3 figures, Proceedings of the 4th Microquasar Workshop,
2002, eds. Durouchoux, Fuchs, & Rodriguez, published by the Center for Space
Physics: Kolkat
Constraining the GRB Collimation with a Survey for Orphan Afterglows
Gamma-ray bursts are believed to be produced in highly-relativistic
collimated outflows. Support for this comes among others from the association
of the times of detected breaks in the decay of afterglow light curves with the
collimation angle of the jets. An alternative approach to estimate a limit on
the collimation angle uses GRB afterglows without detected prompt-emission
counterparts. Here we report on the analysis of a dedicated survey for the
search of these orphan afterglows using the Wide Field Imager at the 2.2m
MPI/ESO telescope at La Silla, Chile. We monitored ~12 square degrees. in up to
25 nights typically spaced by one to two nights with a limiting magnitude of
R=23. Four previously unknown optical transients were discovered and three of
these associated with a flare star, a cataclysmic variable and a dwarf nova.
The fourth source shows indications for an extragalactic origin but the sparse
sampling of the light curve prevents a reliable classification. We discuss the
results in the context of the collimation of GRBs.Comment: 11 pages, A&A 449, 79-8
Six Years of Bursts with the SPI-ACS
The anticoincidence system of the INTEGRAL spectrometer has been an essential component of the
interplanetary network since launch. It has observed about 600 events which have been confirmed as
either soft gamma repeaters or cosmic gamma ray bursts by other instruments in the IPN. It has also
observed over 130 events which are unconfirmed, but which are almost certainly weak bursts below the
thresholds of the other IPN experiments. We review the highlights of these observations, which include
gamma-ray bursts, soft gamma repeaters, and one or two extragalactic giant magnetar flares
New high-z BL Lacs using the photometric method with Swift and SARA
BL Lacertae (BL Lac) objects are the prominent members of the third {\it
Fermi} Large Area Telescope catalog of -ray sources. Half of the BL Lac
population ( 300) lack redshift measurements, which is due to the absence
of lines in their optical spectrum, thereby making it difficult to utilize
spectroscopic methods. Our photometric drop-out technique can be used to
establish the redshift for a fraction of these sources. This work employed 6
filters mounted on the -UVOT and 4 optical filters on two telescopes,
the 0.65 m SARA-CTIO in Chile and 1.0 m SARA-ORM in the Canary Islands, Spain.
A sample of 15 sources was extracted from the archival data for which 6
filter UVOT observations were conducted. By complementing the {\it Swift}
observations with the SARA ones, we were able to discover two high redshift
sources: 3FGL J1155.4-3417 and 3FGL J1156.7-2250 at
and , respectively, resulting from the dropouts in the
powerlaw template fits to these data. The discoveries add to the important (26
total) sample of high-redshift BL Lacs. While the sample of high-z BL Lacs is
still rather small, these objects do not seem to fit well within known schemes
of the blazar population and represent the best probes of the extragalactic
background light.Comment: 10 pages, 6 figures (accepted by ApJ
Clustering of galaxies around GRB sight-lines
There is evidence of an overdensity of strong intervening MgII absorption
line systems distributed along the lines of sight towards GRB afterglows
relative to quasar sight-lines. If this excess is real, one should also expect
an overdensity of field galaxies around GRB sight-lines, as strong MgII tends
to trace these sources. In this work, we test this expectation by calculating
the two point angular correlation function of galaxies within
120 ( at ) of GRB afterglows. We compare the Gamma-ray burst Optical and
Near-infrared Detector (GROND) GRB afterglow sample -- one of the largest and
most homogeneous samples of GRB fields -- with galaxies and AGN found in the
COSMOS-30 photometric catalog. We find no significant signal of anomalous
clustering of galaxies at an estimated median redshift of around GRB
sight-lines, down to . This result is contrary to the
expectations from the MgII excess derived from GRB afterglow spectroscopy,
although many confirmed galaxy counterparts to MgII absorbers may be too faint
to detect in our sample -- especially those at . We note that the addition
of higher sensitivity Spitzer IRAC or HST WFC3 data for even a subset of our
sample would increase this survey's depth by several orders of magnitude,
simultaneously increasing statistics and enabling the investigation of a much
larger redshift space.}Comment: 10 pages, 6 figures. A&A accepte
On the HU Aquarii planetary system hypothesis
In this work, we investigate the eclipse timing of the polar binary HU
Aquarii that has been observed for almost two decades. Recently, Qian et al.
attributed large (O-C) deviations between the eclipse ephemeris and
observations to a compact system of two massive jovian companions. We improve
the Keplerian, kinematic model of the Light Travel Time (LTT) effect and
re-analyse the whole currently available data set. We add almost 60 new, yet
unpublished, mostly precision light curves obtained using the time
high-resolution photo-polarimeter OPTIMA, as well as photometric observations
performed at the MONET/N, PIRATE and TCS telescopes. We determine new
mid--egress times with a mean uncertainty at the level of 1 second or better.
We claim that because the observations that currently exist in the literature
are non-homogeneous with respect to spectral windows (ultraviolet, X-ray,
visual, polarimetric mode) and the reported mid--egress measurements errors,
they may introduce systematics that affect orbital fits. Indeed, we find that
the published data, when taken literally, cannot be explained by any unique
solution. Many qualitatively different and best-fit 2-planet configurations,
including self-consistent, Newtonian N-body solutions may be able to explain
the data. However, using high resolution, precision OPTIMA light curves, we
find that the (O-C) deviations are best explained by the presence of a single
circumbinary companion orbiting at a distance of ~4.5 AU with a small
eccentricity and having ~7 Jupiter-masses. This object could be the next
circumbinary planet detected from the ground, similar to the announced
companions around close binaries HW Vir, NN Ser, UZ For, DP Leo or SZ Her, and
planets of this type around Kepler-16, Kepler-34 and Kepler-35.Comment: 20 pages, 18 figures, accepted to Monthly Notices of the Royal
Astronomical Society (MNRAS
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