89 research outputs found
Radio Emission from GRO J1655-40 during the 1994 Jet Ejection Episodes
We report multifrequency radio observations of GRO J1655-40 obtained with the
Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope
and the Hartebeesthoek Radio Astronomy Observatory at the time of the major
hard X-ray and radio outbursts in 1994 August-September. The radio emission
reached levels of the order of a few Jy and was found to be linearly polarized
by up to 10%, indicating a synchrotron origin. The light curves are in good
agreement with those measured with the VLA, but our closer time sampling has
revealed two new short-lived events and significant deviations from a simple
exponential decay. The polarization data show that the magnetic field is well
ordered and aligned at right angles to the radio jets for most of the
monitoring period. The time evolution of the polarization cannot be explained
solely in terms of a simple synchrotron bubble model, and we invoke a hybrid
`core-lobe' model with a core which contributes both synchrotron and free-free
emission and `lobes' which are classical synchrotron emitters.Comment: 36 pages, 5 tables, 9 figures; accepted for publication in Ap
The X-ray source population of the globular cluster M15: Chandra high-resolution imaging
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.The globular cluster M15 was observed on three occasions with the High Resolution Camera on-board Chandra in 2001 in order to investigate the X-ray source population in the cluster centre. After subtraction of the two bright central sources, four faint sources were identified within 50 arcsec of the core. One of these sources is probably the planetary nebula K648, making this the first positive detection of X-rays from a planetary nebula inside a globular cluster. Another two are identified with UV variables (one previously known), which we suggest are cataclysmic variables (CVs). The nature of the fourth source is more difficult to ascertain, and we discuss whether it is possibly a quiescent soft X-ray transient or also a CV.DCH is grateful to the Academy of Finland and to PPARC for financial support. MBD gratefully acknowledges the support of a Swedish Royal Academy of Sciences (KVA) Research Fellowship. The authors thank Craig Heinke, Bruce Balick and Joel Kastner for valuable comments. The authors also wish to thank Jonathan C. McDowell for useful suggestions, Miriam Krauss at the Chandra HelpDesk, and the anonymous referee for useful comments. DCH is grateful to Panu Muhli for useful comments. This research has made use of NASA's Astrophysics Data System, SAOImage DS9, developed by Smithsonian Astrophysical Observatory, and of the SIMBAD database operated at CDS, Strasbourg, France. Part of this work was based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555
The nature of the hard state of Cygnus X-3
The X-ray binary Cygnus X-3 (Cyg X-3) is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at âŒ20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf-Rayet companion, it has remained unclear whether this state represents an intrinsic hard state, with truncation of the inner disc, or whether it is just a result of increased local absorption. We study the X-ray light curves from RXTE/ASM and CGRO/BATSE in terms of distributions and correlations of flux and hardness and find several signs of a bimodal behaviour of the accretion flow that are not likely to be the result of increased absorption in a surrounding medium. Using INTEGRAL observations, we model the broad-band spectrum of Cyg X-3 in its apparent hard state. We find that it can be well described by a model of a hard state with a truncated disc, despite the low cut-off energy, provided the accreted power is supplied to the electrons in the inner flow in the form of acceleration rather than thermal heating, resulting in a hybrid electron distribution and a spectrum with a significant contribution from non-thermal Comptonization, usually observed only in soft states. The high luminosity of this non-thermal hard state implies that either the transition takes place at significantly higher L/LE than in the usual advection models, or the mass of the compact object is âł20 Mâ, possibly making it the most-massive black hole observed in an X-ray binary in our Galaxy so far. We find that an absorption model as well as a model of almost pure Compton reflection also fit the data well, but both have difficulties explaining other results, in particular the radio/X-ray correlatio
Spectral variability in Cygnus X-3
We model the broad-band X-ray spectrum of Cyg X-3 in all states displayed by
this source as observed by the Rossi X-ray Timing Explorer. From our models, we
derive for the first time unabsorbed spectral shapes and luminosities for the
full range of spectral states. We interpret the unabsorbed spectra in terms of
Comptonization by a hybrid electron distribution and strong Compton reflection.
We study the spectral evolution and compare with other black hole as well as
neutron star sources. We show that a neutron star accretor is not consistent
with the spectral evolution as a function of Ledd and especially not with the
transition to a hard state. Our results point to the compact object in Cyg X-3
being a massive, ~30 Msun black hole.Comment: 14 pages, 9 figures, accepted for publication in MNRA
Characterizing a new class of variability in GRS 1915+105 with simultaneous INTEGRAL/RXTE observations
We report on the analysis of 100 ks INTEGRAL observations of the Galactic
microquasar GRS 1915+105. We focus on INTEGRAL Revolution number 48 when the
source was found to exhibit a new type of variability as preliminarily reported
in Hannikainen et al. (2003). The variability pattern, which we name , is
characterized by a pulsing behaviour, consisting of a main pulse and a shorter,
softer, and smaller amplitude precursor pulse, on a timescale of 5 minutes in
the JEM-X 3-35 keV lightcurve. We also present simultaneous RXTE data. From a
study of the individual RXTE/PCA pulse profiles we find that the rising phase
is shorter and harder than the declining phase, which is opposite to what has
been observed in other otherwise similar variability classes in this source.
The position in the colour-colour diagram throughout the revolution corresponds
to State A (Belloni et al. 2000) but not to any previously known variability
class. We separated the INTEGRAL data into two subsets covering the maxima and
minima of the pulses and fitted the resulting two broadband spectra with a
hybrid thermal--non-thermal Comptonization model. The fits show the source to
be in a soft state characterized by a strong disc component below ~6 keV and
Comptonization by both thermal and non-thermal electrons at higher energies.Comment: Accepted for publication in A&A. 11 pages, 10 figures, 4 in colour.
Original figures can be found at
http://www.astro.helsinki.fi/~diana/grs1915_rev48. Author affiliations
correcte
Sources of Relativistic Jets in the Galaxy
Black holes of stellar mass and neutron stars in binary systems are first
detected as hard X-ray sources using high-energy space telescopes. Relativistic
jets in some of these compact sources are found by means of multiwavelength
observations with ground-based telescopes. The X-ray emission probes the inner
accretion disk and immediate surroundings of the compact object, whereas the
synchrotron emission from the jets is observed in the radio and infrared bands,
and in the future could be detected at even shorter wavelengths. Black-hole
X-ray binaries with relativistic jets mimic, on a much smaller scale, many of
the phenomena seen in quasars and are thus called microquasars. Because of
their proximity, their study opens the way for a better understanding of the
relativistic jets seen elsewhere in the Universe. From the observation of
two-sided moving jets it is inferred that the ejecta in microquasars move with
relativistic speeds similar to those believed to be present in quasars. The
simultaneous multiwavelength approach to microquasars reveals in short
timescales the close connection between instabilities in the accretion disk
seen in the X-rays, and the ejection of relativistic clouds of plasma observed
as synchrotron emission at longer wavelengths. Besides contributing to a deeper
comprehension of accretion disks and jets, microquasars may serve in the future
to determine the distances of jet sources using constraints from special
relativity, and the spin of black holes using general relativity.Comment: 39 pages, Tex, 8 figures, to appear in vol. 37 (1999) of Annual
Reviews of Astronomy and Astrophysic
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