517 research outputs found
The very flat radio - millimetre spectrum of Cygnus X-1
We present almost-simultaneous detections of Cygnus X-1 in the radio and mm
regimes, obtained during the low/hard X-ray state. The source displays a flat
spectrum between 2 and 220 GHz, with a spectral index flatter than 0.15
(3sigma). There is no evidence for either a low- or high-frequency cut-off, but
in the mid-infrared (~30 microns) thermal emission from the OB-type companion
star becomes dominant. The integrated luminosity of this flat-spectrum emission
in quiescence is > 2 x 10^{31} erg/s (2 x 10^{24} W). Assuming the emission
originates in a jet for which non-radiative (e.g. adiabatic expansion) losses
dominate, this is a very conservative lower limit on the power required to
maintain the jet. A comparison with Cyg X-3 and GRS 1915+105, the other X-ray
binaries for which a flat spectrum at shorter than cm wavelengths has been
observed, shows that the jet in Cyg X-1 is significantly less luminous and less
variable, and is probably our best example to date of a continuous, steady,
outflow from an X-ray binary. The emissive mechanism reponsible for such a flat
spectral component remains uncertain. Specifically, we note that the radio-mm
spectra observed from these X-ray binaries are much flatter than those of the
`flat-spectrum' AGN, and that existing models of synchrotron emission from
partially self-absorbed radio cores, which predict a high-frequency cut-off in
the mm regime, are not directly applicable.Comment: Accepted for publication in MNRA
A highly polarised radio jet during the 1998 outburst of the black hole transient XTE J1748-288
XTE J1748-288 is a black hole X-ray transient which went into outburst in
1998 June. The X-ray lightcurves showed canonical morphologies, with minor
variations on the ``Fast Rise Exponential Decay'' profile. The radio source,
however, reached an unusually high flux density of over 600 mJy. This high
radio flux was accompanied by an exceptional (>20%) fractional linear
polarisation, the variability of which was anti-correlated with the flux
density. We use this variability to discuss possible depolarisation mechanisms
and to predict the underlying behaviour of the (unresolved) core/jet
components.Comment: Accepted for publication in MNRA
On the peak radio and X-ray emission from neutron star and black hole candidate X-ray transients
We have compiled and analysed reports from the literature of
(quasi-)simultaneous observations of X-ray transients at radio and X-ray
wavelengths and compared them with each other and with more unusual
radio-bright sources such as Cygnus X-3, GRS 1915+105 and Circinus X-1. There
exists a significant (>97% likelihood) positive (rank) correlation between the
peak X-ray flux P_X and radio flux density P_R for the black hole candidate
(BHC) systems, and a marginally significant positive (rank) correlation for the
neutron star (NS) systems. This is further evidence for a coupling between
accretion and outflows in X-ray binary systems, in this case implying a
relation between peak disc-accretion-rate and the number of synchroton-emitting
electrons ejected. However, we also show that the distribution of `radio
loudness', P_R/P_X, is significantly different for the two samples, in the
sense that the BHCs generally have a higher ratio of P_R/P_X. The origin of
this discrepancy is uncertain, but probably reflects differences in the
energetics and/or radiative efficiency of flows around the neutron stars and
black holes; we briefly discuss some of these possibilities. We conclude that
these data point to the formation of a mildly relativistic jet whose luminosity
is a function of the accretion rate, in the majority, if not all, of X-ray
transient outbursts, but whose relation to the observed X-ray emission is
dependent on the nature of the accreting compact object. (Abridged).Comment: Accepted for publication in MNRA
Simultaneous radio and X-ray observations of Galactic Centre low mass X-ray binaries
We have performed simultaneous X-ray and radio observations of thirteen
Galactic centre low-mass X-ray binaries in 1998 April using the Wide-Field
Cameras onboard BeppoSAX and the Australia Telescope Compact Array, the latter
simultaneously at 4.8 and 8.64 GHz. We detect two Z sources, GX 17+2 and GX
5-1, and the unusual `hybrid' source GX 13+1. Upper limits, which are
significantly deeper than previous non-detections, are placed on the radio
emission from two more Z sources and seven atoll sources. Hardness-Intensity
diagrams constructed from the Wide-Field Camera data reveal GX 17+2 and GX 5-1
to have been on the lower part of the horizontal branch and/or the upper part
of the normal branch, at the time of the observations, and the two non-detected
Z sources, GX 340+0 and GX 349+2, to have been on the lower part of the normal
branch. This is consistent with the previous empirically-determined relation
between radio and X-ray emission from Z sources, in which radio emission is
strongest on the Horizontal branch and weakest on the Flaring branch. For the
first time we have information on the X-ray state of atoll sources, which are
clearly radio-quiet relative to the Z sources, during periods of observed radio
upper limits. We place limits on the linear polarisation from the three
detected sources, and use accurate radio astrometry of GX 17+2 to confirm that
it is probably not associated with the optical star NP Ser. Additionally we
place strong upper limits on the radio emission from the X-ray binary 2S
0921-630, disagreeing with suggestions that it is a Z-source viewed edge-on.Comment: Accepted for publication in MNRA
V723 Cas (Nova Cassiopeiae 1995): MERLIN observations from 1996 to 2001
MERLIN observations of the unusually slow nova V723 Cas are presented. Nine
epochs of 6-cm data between 1996 and 2001 are mapped, showing the initial
expansion and brightening of the radio remnant, the development of structure
and the final decline. A radio light curve is presented and fitted by the
standard Hubble flow model for radio emission from novae in order to determine
the values of various physical parameters for the shell. The model is
consistent with the overall development of the radio emission. Assuming a
distance of 2.39 (+/-0.38) kpc and a shell temperature of 17000 K, the model
yields values for expansion velocity of 414 +/- 0.1 km s^-1 and shell mass of
1.13 +/- 0.04 * 10^-4 Msolar. These values are consistent with those derived
from other observations although the ejected masses are rather higher than
theoretical predictions. The structure of the shell is resolved by MERLIN and
shows that the assumption of spherical symmetry in the standard model is
unlikely to be correct.Comment: 6 pages, 6 figure
Programmability in AIPS++
AIPS++ is an Astronomical Information Processing System being designed and implemented by an international consortium of NRAO and six other radio astronomy institutions in Australia, India, the Netherlands, the United Kingdom, Canada, and the USA. AIPS++ is intended to replace the functionality of AIPS, to be more easily programmable, and will be implemented in C++ using object-oriented techniques. Programmability in AIPS++ is planned at three levels. The first level will be that of a command-line interpreter with characteristics similar to IDL and PV-Wave, but with an intensive set of operations appropriate to telescope data handling, image formation, and image processing. The third level will be in C++ with extensive use of class libraries for both basic operations and advanced applications. The third level will allow input and output of data between external FORTRAN programs and AIPS++ telescope and image databases. In addition to summarizing the above programmability characteristics, this talk will given an overview of the classes currently being designed for telescope data calibration and editing, image formation, and the 'toolkit' of mathematical 'objects' that will perform most of the processing in AIPS++
Adiabatic Mass Loss and the Outcome of the Common Envelope Phase of Binary Evolution
We have developed a new method for calculating common envelope (CE) events
based on explicit consideration of the donor star's structural response to
adiabatic mass loss. In contrast to existing CE prescriptions, which specify a
priori the donor's remnant mass, we determine this quantity self-consistently
and find it depends on binary and CE parameters. This aspect of our model is
particularly important to realistic modeling for upper main sequence star
donors without strongly degenerate cores (and hence without a clear
core/envelope boundary). We illustrate the central features of our method by
considering CE events involving 10 solar mass donors on or before their red
giant branch. For such donors, the remnant core mass can be as much as 30%
larger than the star's He-core mass. Applied across a population of such
binaries, our methodology results in a significantly broader remnant mass and
final orbital separation distribution and a 20% increase in CE survival rates
as compared to previous prescriptions for the CE phase.Comment: 4 pages, 3 figures; corrected typo in equation (1); updated reference
dat
ASCA Observations of the Jet Source XTE J1748-288
XTE J1748-288 is a new X-ray transient with a one-sided radio jet. It was
observed with ASCA on 1998/09/06 and 1998/09/26, 100 days after the onset of
the radio-X-ray outburst. The spectra were fitted with an attenuated power-law
model, and the 2-6-keV flux was 4.6 * 10^{-11} erg s^{-1} cm^{-2} and 2.2 *
10^{-12} on 09/06 and 09/26, respectively. The light curve showed that the
steady exponential decay with an e-folding time of 14 days lasted over 100 days
and 4 orders of magnitude from the peak of the outburst. The celestial region
including the source had been observed with ASCA on 1993/10/01 and 1994/09/22,
years before the discovery. In those period, the flux was < 10^{-13} erg s^{-1}
cm^{-2}, below ASCA's detection limit. The jet blob colliding to the
environmental matter was supposedly not the X-ray source, although the emission
mechanism has not been determined. A possible detection of a K line from highly
ionized iron is discussed.Comment: 11 pages, 4 figures, submitted to ApJL. Fig2 is replaced with correct
on
International Ultraviolet Explorer observations of amorphous hot galaxies
In order to better understand the nature of star formation processes in amorphous galaxies, short wavelength International Ultraviolet Explorer (IUE) spectra of galaxies NGC 1705 and NGC 1800 were obtained. The IUE data for NGC 1705 were of excellent quality while the low signal-to-noise NGC 1800 observation was useful only as a rough guide to the ultraviolet energy distribution. It was found that NGC 1705 contains a normal mix of OB stars, which is consistent with the nearly constant recent star formation rate inferred from new optical data. The NGC 1800 is likely to have similar properties, and blue galaxies with amorphous structures thus do not show evidence for anomalies in stellar mass distributions. The UV spectra of amorphous galaxies and a variety of other hot extragalactic stellar systems have similar characteristics, which suggests OB stellar populations often are homogeneous in their properties
A decelerating jet observed by the EVN and VLBA in the X-ray transient XTE J1752-223
The recently discovered Galactic X-ray transient XTE J1752-223 entered its
first known outburst in 2010, emitting from the X-ray to the radio regimes. Its
general X-ray properties were consistent with those of a black hole candidate
in various spectral states, when ejection of jet components is expected. To
verify this, we carried out very long baseline interferometry (VLBI)
observations. The measurements were carried out with the European VLBI Network
(EVN) and the Very Long Baseline Array (VLBA) at four epochs in 2010 February.
The images at the first three epochs show a moving jet component that is
significantly decelerated by the last epoch, when a new jet component appears
that is likely to be associated with the receding jet side. The overall picture
is consistent with an initially mildly relativistic jet, interacting with the
interstellar medium or with swept-up material along the jet. The brightening of
the receding ejecta at the final epoch can be well explained by initial Doppler
deboosting of the emission in the decelerating jet.Comment: Accepted for publication in MNRAS Letters. 5 pages, 2 figure
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