851 research outputs found
X-ray Spectral Analysis of the Steady States of GRS 1915+105
We report on the X-ray spectral behavior within the steady states of GRS
1915+105. Our work is based on the full data set on the source obtained using
the Proportional Counter Array on the Rossi X-ray Timing Explorer and 15 GHz
radio data obtained using the Ryle Telescope. The steady observations within
the X-ray data set naturally separated into two regions in the color-color
diagram and we refer to them as steady-soft and steady-hard. GRS 1915+105
displays significant curvature in the coronal component in both the soft and
hard data within the {\it RXTE}/PCA bandpass. A majority of the steady-soft
observations displays a roughly constant inner disk radius (R_in), while the
steady-hard observations display an evolving disk truncation which is
correlated to the mass accretion rate through the disk. The disk flux and
coronal flux are strongly correlated in steady-hard observations and very
weakly correlated in the steady-soft observations. Within the steady-hard
observations we observe two particular circumstances when there are
correlations between the coronal X-ray flux and the radio flux with log slopes
\eta~0.68 +/- 0.35 and \eta ~ 1.12 +/- 0.13. They are consistent with the upper
and lower tracks of Gallo et al. (2012), respectively. A comparison of model
parameters to the state definitions show that almost all steady-soft
observations match the criteria of either thermal or steep power law state,
while a large portion of the steady-hard observations match the hard state
criteria when the disk fraction constraint is neglected.Comment: 21 pages, 15 figures, 2 tables. Accepted for publication in Ap
Cygnus X-3 in outburst : quenched radio emission, radiation losses and variable local opacity
We present multiwavelength observations of Cygnus X-3 during an extended
outburst in 1994 February - March. Intensive radio monitoring at 13.3, 3.6 &
2.0 cm is complemented by observations at (sub)millimetre and infrared
wavelengths, which find Cyg X-3 to be unusually bright and variable, and
include the first reported detection of the source at 0.45 mm. We report the
first confirmation of quenched radio emission prior to radio flaring
independent of observations at Green Bank. The observations reveal evidence for
wavelength-dependent radiation losses and gradually decreasing opacity in the
environment of the radio jet. We find that the radiation losses are likely to
be predominantly inverse Compton losses experienced by the radio-emitting
electrons in the strong radiation field of a luminous companion to the compact
object. We interpret the decreasing opacity during the flare sequence as
resulting from a decreasing proportion of thermal electrons entrained in the
jet, reflecting a decreasing density in the region of jet formation. We
present, drawing in part on the work of other authors, a model based upon
mass-transfer rate instability predicting gamma-ray, X-ray, infrared and radio
trends during a radio flaring sequence.Comment: LaTeX, 11 pages, 6 figures. Submitted to MNRA
Discovery of Radio Outbursts in the Active Nucleus of M81
The low-luminosity active galactic nucleus of M81 has been monitored at
centimeter wavelengths since early 1993 as a by-product of radio programs to
study the radio emission from Supernova 1993J. The extensive data sets reveal
that the nucleus experienced several radio outbursts during the monitoring
period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of
1993 September and lasted for approximately three months; at longer
wavelengths, the maximum flux density decreases, and the onset of the burst is
delayed. These characteristics qualitatively resemble the standard model for
adiabatically expanding radio sources, although certain discrepancies between
the observations and the theoretical predictions suggest that the model is too
simplistic. In addition to the large-amplitude, prolonged variations, we also
detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on
short (less than 1 day) timescales. We discuss a possible association between
the radio activity and an optical flare observed during the period that the
nucleus was monitored at radio wavelengths.Comment: To appear in The Astronomical Journal. Latex, 18 pages including
embedded figures and table
A microlensing measurement of dark matter fractions in three lensing galaxies
Direct measurements of dark matter distributions in galaxies are currently
only possible through the use of gravitational lensing observations.
Combinations of lens modelling and stellar velocity dispersion measurements
provide the best constraints on dark matter distributions in individual
galaxies, however they can be quite complex. In this paper, we use observations
and simulations of gravitational microlensing to measure the smooth (dark)
matter mass fraction at the position of lensed images in three lens galaxies:
MG 0414+0534, SDSS J0924+0219 and Q2237+0305. The first two systems consist of
early-type lens galaxies, and both display a flux ratio anomaly in their close
image pair. Anomalies such as these suggest a high smooth matter percentage is
likely, and indeed we prefer ~50 per cent smooth matter in MG 0414+0534, and
~80 per cent in SDSS J0924+0219 at the projected locations of the lensed
images. Q2237+0305 differs somewhat in that its lensed images lie in the
central kiloparsec of the barred spiral lens galaxy, where we expect stars to
dominate the mass distribution. In this system, we find a smooth matter
percentage that is consistent with zero.Comment: 7 pages, 4 figures. Accepted for publication in Ap
The Enigmatic Radio Afterglow of GRB 991216
We present wide-band radio observations spanning from 1.4 GHz to 350 GHz of
the afterglow of GRB 991216, taken from 1 to 80 days after the burst. The
optical and X-ray afterglow of this burst were fairly typical and are explained
by a jet fireball. In contrast, the radio light curve is unusual in two
respects: (a) the radio light curve does not show the usual rise to maximum
flux on timescales of weeks and instead appears to be declining already on day
1 and (b) the power law indices show significant steepening from the radio
through the X-ray bands. We show that the standard fireball model, in which the
afterglow is from a forward shock, is unable to account for (b) and we conclude
that the bulk of the radio emission must arise from a different source. We
consider two models, neither of which can be ruled out with the existing data.
In the first (conventional) model, the early radio emission is attributed to
emission from the reverse shock as in the case of GRB 990123. We predict that
the prompt optical emission would have been as bright (or brighter) than 8th
magnitude. In the second (exotic) model, the radio emission originates from the
forward shock of an isotropically energetic fireball (10^54 erg) expanding into
a tenuous medium (10^-4 cm^-3). The resulting fireball would remain
relativistic for months and is potentially resolvable with VLBI techniques.
Finally, we note that the near-IR bump of the afterglow is similar to that seen
in GRB 971214 and no fireball model can explain this bump.Comment: ApJ, submitte
Continued Neutron Star Crust Cooling of the 11 Hz X-Ray Pulsar in Terzan 5: A Challenge to Heating and Cooling Models?
The transient neutron star low-mass X-ray binary and 11 Hz X-ray pulsar IGR
J17480-2446 in the globular cluster Terzan 5 exhibited an 11-week accretion
outburst in 2010. Chandra observations performed within five months after the
end of the outburst revealed evidence that the crust of the neutron star became
substantially heated during the accretion episode and was subsequently cooling
in quiescence. This provides the rare opportunity to probe the structure and
composition of the crust. Here, we report on new Chandra observations of Terzan
5 that extend the monitoring to ~2.2 yr into quiescence. We find that the
thermal flux and neutron star temperature have continued to decrease, but
remain significantly above the values that were measured before the 2010
accretion phase. This suggests that the crust has not thermally relaxed yet,
and may continue to cool. Such behavior is difficult to explain within our
current understanding of heating and cooling of transiently accreting neutron
stars. Alternatively, the quiescent emission may have settled at a higher
observed equilibrium level (for the same interior temperature), in which case
the neutron star crust may have fully cooled.Comment: Accepted to ApJ without revision. Updated references and fixed few
typos to match published version. 7 pages, 3 figures, 3 table
Cataclysmic Variables and Other Compact Binaries in the Globular Cluster NGC 362: Candidates from Chandra and HST
Highly sensitive and precise X-ray imaging from Chandra, combined with the
superb spatial resolution of HST optical images, dramatically enhances our
empirical understanding of compact binaries such as cataclysmic variables and
low mass X-ray binaries, their progeny, and other stellar X-ray source
populations deep into the cores of globular clusters. Our Chandra X-ray images
of the globular cluster NGC 362 reveal 100 X-ray sources, the bulk of which are
likely cluster members. Using HST color-magnitude and color-color diagrams, we
quantitatively consider the optical content of the NGC 362 Chandra X-ray error
circles, especially to assess and identify the compact binary population in
this condensed-core globular cluster. Despite residual significant crowding in
both X-rays and optical, we identify an excess population of H{\alpha}-emitting
objects that is statistically associated with the Chandra X-ray sources. The
X-ray and optical characteristics suggest that these are mainly cataclysmic
variables, but we also identify a candidate quiescent low mass X-ray binary. A
potentially interesting and largely unanticipated use of observations such as
these may be to help constrain the macroscopic dynamic state of globular
clusters.Comment: 6 pages, 6 figures, to appear in the proceedings of the conference
"Binary Star Evolution: Mass Loss, Accretion, and Mergers," Mykonos, Greece,
June 22-25, 201
Simultaneous X-ray and Radio Monitoring of the Unusual Binary LSI+61 303: Measurements of the Lightcurve and High-Energy Spectrum
The binary system, LSI+61 303, is unusual both because of the dramatic,
periodic, radio outbursts, and because of its possible association with the 100
MeV gamma-ray source, 2CG135+01. We have performed simultaneous radio and Rossi
X-ray Timing Explorer X-ray observations at eleven intervals over the 26.5 day
orbit, and in addition searched for variability on timescales ranging from
milliseconds to hours. We confirm the modulation of the X-ray emission on
orbital timescales originally reported by Taylor et al. (1996), and in addition
we find a significant offset between the peak of the X-ray and radio flux. We
argue that based on these results, the most likely X-ray emission mechanism is
inverse Compton scattering of stellar photons off of electrons accelerated at
the shock boundary between the relativistic wind of a young pulsar and the Be
star wind. In these observations we also detected 2 -- 150 keV flux from the
nearby low-redshift quasar QSO~0241+622. Comparing these measurements to
previous hard X-ray and gamma-ray observations of the region containing both
LSI+61 303 and QSO~0241+622, it is clear that emission from the QSO dominates.Comment: 23 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
Low frequency radio and X-ray properties of core-collapse supernovae
Radio and X-ray studies of young supernovae probe the interaction between the
supernova shock waves and the surrounding medium and give clues to the nature
and past of the progenitor star. Here we discuss the early emission from type
Ic SN 2002ap and argue that repeated Compton boosting of optical photons by hot
electrons presents the most natural explanation of the prompt X-ray emission.
We describe the radio spectrum of another type Ic SN 2003dh (GRB030329)
obtained with combined GMRT and VLA data. We report on the low frequency radio
monitoring of SN 1995N and our objectives of distinguishing between competing
models of X-ray emission from this SN and the nature of its progenitor by X-ray
spectroscopy. Radio studies on SN 2001gd, SN 2001ig and SN 2002hh are
mentioned.Comment: 5 pages, 4 figures. Uses svmult.cls. To appear in proceedings of IAU
Colloquium 192 "Supernovae (10 years of SN 1993J)", April 2003, Valencia,
Spain, eds. J. M. Marcaide and K. W. Weile
X-ray and optical observations of M55 and NGC 6366: evidence for primordial binaries
We present Chandra X-ray Observatory ACIS-S3 X-ray imaging observations and
VLT/FORS2 and Hubble Space Telescope optical observations of two low-density
Galactic globular clusters; NGC 6366 and M55. We detect 16 X-ray sources with
0.5-6.0 keV luminosities above Lx=4E30 erg/s within the half-mass radius of
M55, of which 8 or 9 are expected to be background sources, and 5 within the
half-mass radius of NGC 6366, of which 4 are expected to be background sources.
Optical counterparts are identified for several X-ray sources in both clusters
and from these we conclude that 3 of the X-ray sources in M55 and 2 or 3 of the
X-ray sources in NGC 6366 are probably related to the cluster. Combining these
results with those for other clusters, we find the best fit for a predicted
number of X-ray sources in a globular cluster Nc=1.2 Gamma+1.1 Mh, where Gamma
is the collision number and Mh is (half of) the cluster mass, both normalized
to the values for the globular cluster M4. Some sources tentatively classified
as magnetically active binaries are more luminous in X-rays than the upper
limit of Lx~0.001Lbol of such binaries in the solar neighbourhood. Comparison
with XMM and ROSAT observations lead us to conclude that the brightest X-ray
source in M55, a dwarf nova, becomes fainter in X-rays during the optical
outburst, in accordance with other dwarf novae. The brightest X-ray source in
NGC 6366 is a point source surrounded by a slightly offset extended source. The
absence of galaxies and Halpha emission in our optical observations argues
against a cluster of galaxies and against a planetary nebula, and we suggest
that the source may be an old nova.Comment: 13 pages, 9 figures, accepted for publication in A&
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