7,030 research outputs found
A Multiwavelength Investigation of the Relationship Between 2CG135+1 and LSI+61o 303
We present the results of a multiwavelength monitoring campaign targeting the
gamma-ray source 2CG 135+1 in an attempt to confirm the association of this
object with the radio/Be/X-ray binary system LSI +61o 303. The campaign
included simultaneous radio, optical, infrared, and hard x-ray/gamma-ray
observations carried out with a variety of instruments, covering (not
continously) almost three binary cycles of LSI +61o 303 during the period
April-July 1994. Three separate OSSE observations of the gamma-ray source were
carried out, covering different phases of the radio lightcurve. Hard
X-ray/gamma-ray emission was detected from the direction of 2CG 135+1 during
the first of these OSSE observations. The signal to noise ratio of the OSSE
observations was insufficient to establish a spectral or intensity correlation
of the high-energy emission with simultaneous radio, optical and infrared
emission of LSI +61o 303. We briefly discuss the theoretical implications of
our observations.Comment: 17 pages, 9 figures, 6 tables to be published in Astrophysical
Journal, 10 April 199
One-sided jet at milliarcsecond scales in LSI+61303
We present Very Long Baseline Interferometry (VLBI) observations of the high
mass X-ray binary LSI+61303, carried out with the European VLBI Network (EVN).
Over the 11 hour observing run, performed 10 days after a radio outburst, the
radio source showed a constant flux density, which allowed sensitive imaging of
the emission distribution. The structure in the map shows a clear extension to
the southeast. Comparing our data with previous VLBI observations we interpret
the extension as a collimated radio jet as found in several other X-ray
binaries. Assuming that the structure is the result of an expansion that
started at the onset of the outburst, we derive an apparent expansion velocity
of 0.003 c, which, in the context of Doppler boosting, corresponds to an
intrinsic velocity of at least 0.4 c for an ejection close to the line of
sight. From the apparent velocity in all available epochs we are able to
establish variations in the ejection angle which imply a precessing accretion
disk. Finally we point out that LSI+61303, like SS433 and Cygnus X-1, shows
evidence for an emission region almost orthogonal to the relativistic jet.Comment: 7 pages, 4 figures, LaTeX, uses aa.cls. Accepted for publication in
A&
Chandra Observations of the Gamma-ray Binary LSI+61303: Extended X-ray Structure?
We present a 50 ks observation of the gamma-ray binary LSI+61303 carried out
with the ACIS-I array aboard the Chandra X-ray Observatory. This is the highest
resolution X-ray observation of the source conducted so far. Possible evidence
of an extended structure at a distance between 5 and 12 arcsec towards the
North of LSI+61303 have been found at a significance level of 3.2 sigma. The
asymmetry of the extended emission excludes an interpretation in the context of
a dust-scattered halo, suggesting an intrinsic nature. On the other hand, while
the obtained source flux, of F_{0.3-10 keV}=7.1^{+1.8}_{-1.4} x 10^{-12}
ergs/cm^2/s, and hydrogen column density, N_{H}=0.70+/-0.06 x 10^{22} cm^{-2},
are compatible with previous results, the photon index Gamma=1.25+/-0.09 is the
hardest ever found. In light of these new results, we briefly discuss the
physics behind the X-ray emission, the location of the emitter, and the
possible origin of the extended emission ~0.1 pc away from LSI+61303.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letter
New universality class for the three-dimensional XY model with correlated impurities: Application to He in aerogels
Encouraged by experiments on He in aerogels, we confine planar spins in
the pores of simulated aerogels (diffusion limited cluster-cluster aggregation)
in order to study the effect of quenched disorder on the critical behavior of
the three-dimensional XY model. Monte Carlo simulations and finite-size scaling
are used to determine critical couplings and exponents. In agreement with
experiments, clear evidence of change in the thermal critical exponents
and is found at nonzero volume fractions of impurities. These changes
are explained in terms of {\it hidden} long-range correlations within disorder
distributions.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Long-Term Flux Monitoring of LSI +61 303 at 2.25 and 8.3 GHz
LSI +61 303 is an exotic binary system consisting of a ~10 Msun B star and a
compact object which is probably a neutron star. The system is associated with
the interesting radio source GT0236+610 that exhibits bright radio outbursts
with a period of 26.5 days. We report the results of continuous daily radio
interferometric observations of GT0236+610 at 2.25 and 8.3 GHz from 1994
January to 1996 February. The observations cover 25 complete (and 3 partial)
cycles with multiple observations each day. We detect substantial
cycle-to-cycle variability of the radio emission characterized by a rapid onset
of the radio flares followed by a more gradual decrease of the emission. We
detect a systematic change of the radio spectral index alpha which typically
becomes larger than zero at the onset of the radio outbursts. This behavior is
suggestive of expansion of material initially optically thick to radio
frequencies, indicating either that synchrotron or inverse Compton cooling are
important or that the free-free optical depth to the source is rapidly
changing. After two years of observations, we see only weak evidence for the
proposed 4-year periodic modulation in the peak flux of the outbursts. We
observe a secular trend in the outburst phases according the the best published
ephemeris. This trend indicates either orbital period evolution, or a drift in
outburst orbital phase in response to some other change in the system.Comment: 23 pages, LaTex, 7 figures, to appear in ApJ, v491, Dec 10th issue,
for associated info and preprints see
http://www.srl.caltech.edu/personnel/paulr/lsi.htm
Formation and Collapse of Quiescent Cloud Cores Induced by Dynamic Compressions
(Abridged) We present numerical hydrodynamical simulations of the formation,
evolution and gravitational collapse of isothermal molecular cloud cores. A
compressive wave is set up in a constant sub-Jeans density distribution of
radius r = 1 pc. As the wave travels through the simulation grid, a
shock-bounded spherical shell is formed. The inner shock of this shell reaches
and bounces off the center, leaving behind a central core with an initially
almost uniform density distribution, surrounded by an envelope consisting of
the material in the shock-bounded shell, with a power-law density profile that
at late times approaches a logarithmic slope of -2 even in non-collapsing
cases. The resulting density structure resembles a quiescent core of radius <
0.1 pc, with a Bonnor-Ebert-like (BE-like) profile, although it has significant
dynamical differences: it is initially non-self-gravitating and confined by the
ram pressure of the infalling material, and consequently, growing continuously
in mass and size. With the appropriate parameters, the core mass eventually
reaches an effective Jeans mass, at which time the core begins to collapse.
Thus, there is necessarily a time delay between the appearance of the core and
the onset of its collapse, but this is not due to the dissipation of its
internal turbulence as it is often believed. These results suggest that
pre-stellar cores may approximate Bonnor-Ebert structures which are however of
variable mass and may or may not experience gravitational collapse, in
qualitative agreement with the large observed frequency of cores with BE-like
profiles.Comment: Accepted for publication in ApJ. Associated mpeg files can be found
in http://www.astrosmo.unam.mx/~g.gomez/publica.htm
On the origin of the X-ray emission from a narrow-line radioquasar at z>1
We present new XMM-Newton X-ray observations of the z=1.246 narrow-line
radioquasar RX J1011.2+5545 serendipitously discovered by ROSAT. The flat X-ray
spectrum previously measured by ROSAT and ASCA is shown to be the result of a
steep Gamma~1.8 power law spectrum seen through a moderate intrinsic absorbing
column NH~4E21 cm^-2. The position of the X-ray source is entirely coincident
with the nucleus of the radio source that we have resolved in new sensitive VLA
observations at 3.6 and 6 cm, implying that scattering in the radio lobes is
not responsible for the bulk of X-ray emission. In the EPIC pn image, a faint
patch of X-ray emission is apparent 14'' to the NE of the main X-ray source.
The former is positionally coincident with an apparently extended optical
object with R~21.9, but there is no associated radio emission, thus ruling out
the possibility that this represents a hotspot in a jet emanating from the
primary X-ray source. No reflection features are detected in the X-ray spectrum
of the narrow-line radioquasar, although an Fe line with equivalent width of up
to 600 eV cannot be ruled out.Comment: 7 pages, 6 figures, MNRAS in the pres
H-alpha Emission Variability in the gamma-ray Binary LS I +61 303
LS I +61 303 is an exceptionally rare example of a high mass X-ray binary
(HMXB) that also exhibits MeV-TeV emission, making it one of only a handful of
"gamma-ray binaries". Here we present H-alpha spectra that show strong
variability during the 26.5 day orbital period and over decadal time scales. We
detect evidence of a spiral density wave in the Be circumstellar disk over part
of the orbit. The H-alpha line profile also exhibits a dramatic emission burst
shortly before apastron, observed as a redshifted shoulder in the line profile,
as the compact source moves almost directly away from the observer. We
investigate several possible origins for this red shoulder, including an
accretion disk, mass transfer stream, and a compact pulsar wind nebula that
forms via a shock between the Be star's wind and the relativistic pulsar wind.Comment: Accepted to Ap
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