972 research outputs found
Models for gamma-ray production in low-mass microquasars
Unlike high-mass gamma-ray binaries, low-mass microquasars lack external
sources of radiation and matter that could produce high-energy emission through
interactions with relativistic particles. In this work we consider the
synchrotron emission of protons and leptons that populate the jet of a low-mass
microquasar. In our model photohadronic and inverse Compton (IC) interactions
with synchrotron photons produced by both protons and leptons result in a
high-energy tail of the spectrum. We also estimate the contribution from
secondary pairs injected through photopair production. The high-energy emission
is dominated by radiation of hadronic origin, so we can call these objects
proton microquasars.Comment: 4 pages, 2 figures, accepted for publication in the International
Journal of Modern Physics D, proceedings of HEPRO meeting, held in Dublin, in
September 200
Evidence for deceleration in the radio jets of GRS1915+105?
There is currently a clear discrepancy in the proper motions measured on
different angular scales in the approaching radio jets of the black hole X-ray
binary GRS1915+105. Lower velocities were measured with the Very Large Array
(VLA) prior to 1996 than were subsequently found from higher-resolution
observations made with the Very Long Baseline Array and the Multi-Element Radio
Linked Interferometer Network. We initiated an observing campaign to use all
three arrays to attempt to track the motion of the jet knots from the 2006
February outburst of the source, giving us unprecedented simultaneous coverage
of all angular scales, from milliarcsecond scales out to arcsecond scales. The
derived proper motion, which was dominated by the VLA measurements, was found
to be 17.0 mas per day, demonstrating that there has been no significant
permanent change in the properties of the jets since 1994. We find no
conclusive evidence for deceleration of the jet knots, unless this occurs
within 70 mas of the core. We discuss possible causes for the varying proper
motions recorded in the literature.Comment: Accepted for publication in MNRAS. 13 pages, 10 figure
Near-infrared jet emission in the microquasar XTE J1550-564
Context: Microquasars are accreting Galactic sources that are also observed
to launch relativistic jets. A key signature of the ejection is non-thermal
radio emission. The level of this jet component at high frequencies is still
poorly constrained. Aims: The X-ray binary and microquasar black hole candidate
XTE J1550-564 exhibited a faint X-ray outburst in April 2003 during which it
stayed in the X-ray low/hard state. We took optical and near-infrared (NIR)
observations with the ESO/NTT telescope during this outburst to disentangle the
various contributions to the spectral energy distribution (SED) and investigate
the presence of a jet component. Methods: Photometric and spectroscopic
observations allowed us to construct an SED and also to produce a high
time-resolution lightcurve. Results: The SED shows an abrupt change of slope
from the NIR domain to the optical. The NIR emission is attributed to
non-thermal synchrotron emission from the compact, self-absorbed jet that is
known to be present in the low/hard state. This is corroborated by the fast
variability, colours, lack of prominent spectral features and evidence for
intrinsic polarisation. The SED suggests the jet break from the optically thick
to the thin regime occurs in the NIR. Conclusions: The simultaneous optical-NIR
data allow an independent confirmation of jet emission in the NIR. The
transition to optically thin synchrotron occurs at NIR frequencies or below,
which leads to an estimated characteristic size greater than 2x10^8cm and
magnetic field less than 5T for the jet base, assuming a homogeneous one-zone
synchrotron model.Comment: Accepted for publication in Astronomy and Astrophysics (Section 7.
Stellar structure and evolution; 8 pages, 6 figures
High-energy gamma-ray observations of the accreting black hole V404 Cygni during its June 2015 outburst
We report on Fermi/Large Area Telescope observations of the accreting black
hole low-mass X-ray binary V404 Cygni during its outburst in June-July 2015.
Detailed analyses reveal a possible excess of -ray emission on 26 June
2015, with a very soft spectrum above MeV, at a position consistent with
the direction of V404 Cyg (within the confidence region and a chance
probability of ). This emission cannot be associated with any
previously-known Fermi source. Its temporal coincidence with the brightest
radio and hard X-ray flare in the lightcurve of V404 Cyg, at the end of the
main active phase of its outburst, strengthens the association with V404 Cyg.
If the -ray emission is associated with V404 Cyg, the simultaneous
detection of keV annihilation emission by INTEGRAL requires that the
high-energy rays originate away from the corona, possibly in a
Blandford-Znajek jet. The data give support to models involving a
magnetically-arrested disk where a bright -ray jet can re-form after
the occurrence of a major transient ejection seen in the radio.Comment: 5 pages, 3 figures, accepted for publication in MNRA
A transient relativistic radio jet from Cygnus X-1
We report the first observation of a transient relativistic jet from the
canonical black hole candidate, Cygnus X-1, obtained with the Multi-Element
Radio-Linked Interferometer Network (MERLIN). The jet was observed in only one
of six epochs of MERLIN imaging of the source during a phase of repeated X-ray
spectral transitions in 2004 Jan--Feb, and this epoch corresponded to the
softest 1.5-12 keV X-ray spectrum. With only a single epoch revealing the jet,
we cannot formally constrain its velocity. Nevertheless, several lines of
reasoning suggest that the jet was probably launched 0.5-4.0 days before this
brightening, corresponding to projected velocities of 0.2c < v_app < 1.6c, and
an intrinsic velocity of > 0.3c. We also report the occurrence of a major radio
flare from Cyg X-1, reaching a flux density of ~120 mJy at 15 GHz, and yet not
associated with any resolvable radio emission, despite a concerted effort with
MERLIN. We discuss the resolved jet in terms of the recently proposed 'unified
model' for the disc-jet coupling in black hole X-ray binaries, and tentatively
identify the 'jet line' for Cyg X-1. The source is consistent with the model in
the sense that a steady jet appears to persist initially when the X-ray
spectrum starts softening, and that once the spectral softening is complete the
core radio emission is suppressed and transient ejecta / shock observed.
However, there are some anomalies, and Cyg X-1 clearly does not behave like a
normal black hole transient in progressing to the canonical soft / thermal
state once the ejection event has happened.Comment: Accepted for publication in MNRA
Steady jets from radiatively efficient hard states in GRS 1915+105
Recent studies of different X-ray binaries (XRBs) have shown a clear
correlation between the radio and X-ray emission. We present evidence of a
close relationship found between the radio and X-ray emission at different
epochs for GRS1915+105, using observations from the Ryle Telescope and Rossi
X-ray Timing Explorer satellite. The strongest correlation was found during the
hard state (also known as the `plateau' state), where a steady AU-scale jet is
known to exist. Both the radio and X-ray emission were found to decay from the
start of most plateau states, with the radio emission decaying faster. An
empirical relationship of was then
fitted to data taken only during the plateau state, resulting in a power-law
index of , which is significantly higher than in other black
hole XRBs in a similar state. An advection-flow model was then fitted to this
relationship and compared to the universal XRB relationship as described by
Gallo et al. (2003). We conclude that either (I) the accretion disk in this
source is radiatively efficient, even during the continuous outflow of a
compact jet, which could also suggest a universal turn-over from radiatively
inefficient to efficient for all stellar-mass black holes at a critical mass
accretion rate ( g/s); or (II) the X-rays in
the plateau state are dominated by emission from the base of the jet and not
the accretion disk (e.g. via inverse Compton scattering from the outflow).Comment: 9 pages, 7 figures, accepted in A&
Absorption lines from magnetically-driven winds in X-ray binaries
High resolution X-ray spectra of black hole X-ray binaries (BHBs) show
blueshifted absorption lines from disk winds which seem to be equatorial. Winds
occur in the Softer (disk-dominated) states of the outburst and are less
prominent or absent in the Harder (power-law dominated) states. We use
self-similar magneto-hydrodynamic (MHD) accretion-ejection models to explain
the disk winds in BHBs. In our models, the density at the base of the outflow
from the accretion disk is not a free parameter, but is determined by solving
the full set of dynamical MHD equations. Thus the physical properties of the
outflow are controlled by the global structure of the disk. We studied
different MHD solutions characterized by different values of (a) the disk
aspect ratio () and (b) the ejection efficiency (). We use two
kinds of MHD solutions depending on the absence (cold solution) or presence
(warm solution) of heating at the disk surface. Such heating could be from e.g.
dissipation of energy due to MHD turbulence in the disk or from illumination.
We use each of these MHD solutions to predict the physical parameters of an
outflow; put limits on the ionization parameter (), column density and
timescales, motivated by observational results; and thus select regions within
the outflow which are consistent with the observed winds. The cold MHD
solutions cannot account for winds due to their low ejection efficiency. But
warm solutions can explain the observed physical quantities in the wind because
they can have sufficiently high values of (, implying larger
mass loading at the base of the outflow). Further from our thermodynamic
equilibrium curve analysis for the outflowing gas, we found that in the Hard
state a range of is thermodynamically unstable, and had to be excluded.
This constrain made it impossible to have any wind at all, in the Hard state.Comment: 16 Pages, 10 figures in the main body and 4 figures in the appendix.
Accepted for publication in A&
Studying the X-ray hysteresis in GX 339-4: the disc and iron line over one decade
We report on a comprehensive and consistent investigation into the X-ray
emission from GX 339-4. All public observations in the 11 year RXTE archive
were analysed. Three different types of model - single powerlaw, broken
powerlaw and a disc + powerlaw - were fitted to investigate the evolution of
the disc, along with a fixed gaussian component at 6.4 keV to investigate any
iron line in the spectrum. We show that the relative variation in flux and
X-ray colour between the two best sampled outbursts are very similar. The decay
of the disc temperature during the outburst is clearly seen in the soft state.
The expected decay is S_Disc \propto T^4; we measure T^4.75\pm0.23. This
implies that the inner disc radius is approximately constant in the soft state.
We also show a significant anti-correlation between the iron line significant
width and the X-ray flux in the soft state while in the hard state the EW is
independent of the flux. This results in hysteresis in the relation between
X-ray flux and both line flux and EW. To compare the X-ray binary outburst to
the behaviour seen in AGN, we construct a Disc Fraction Luminosity Diagram for
GX 339-4, the first for an X-ray binary. The shape qualitatively matches that
produced for AGN. Linking this with the radio emission from GX 339-4 the change
in radio spectrum between the disc and power-law dominated states is clearly
visible.Comment: Accepted for publication in MNRAS, 20 pages, 17 figures. For high-res
version see http://www.astro.soton.ac.uk/~r.j.dunn/publications.htm
The Distance to the Soft Gamma Repeater SGR 1627-41
We report millimeter observations of the line of sight to the recently
discovered Soft Gamma Repeater, SGR 1627-41, which has been tentatively
associated with the supernova remnant SNR G337.0-0.1 Among the eight molecular
clouds along the line of sight to SGR 1627-41, we show that SNR G337.0-0.1 is
probably interacting with one of the most massive giant molecular clouds (GMC)
in the Galaxy, at a distance of 11 kpc from the sun. Based on the high
extinction to the persistent X-ray counterpart of SGR 1627-41, we present
evidence for an association of this new SGR with the SNR G337.0-0.1; they both
appear to be located on the near side of the GMC. This is the second SGR
located near an extraordinarily massive GMC. We suggest that SGR 1627-41 is a
neutron star with a high transverse velocity (~ 1,000 \kms) escaping the young
(~ 5,000 years) supernova remnant G337.0-0.1Comment: 17 pages, including 2 figures. Accepted for publication in the
Astrophysical Journal Letter
A broadband leptonic model for gamma-ray emitting microquasars
Observational and theoretical studies point to microquasars (MQs) as possible
counterparts of a significant fraction of the unidentified gamma-ray sources
detected so far. At present, a proper scenario to explain the emission beyond
soft X-rays from these objects is not known, nor what the precise connection is
between the radio and the high-energy radiation. We develop a new model where
the MQ jet is dynamically dominated by cold protons and radiatively dominated
by relativistic leptons. The matter content and power of the jet are both
related with the accretion process. The magnetic field is assumed to be close
to equipartition, although it is attached to and dominated by the jet matter.
For the relativistic particles in the jet, their maximum energy depends on both
the acceleration efficiency and the energy losses. The model takes into account
the interaction of the relativistic jet particles with the magnetic field and
all the photon and matter fields. Such interaction produces significant amounts
of radiation from radio to very high energies through synchrotron, relativistic
Bremsstrahlung, and inverse Compton (IC) processes. Variability of the emission
produced by changes in the accretion process (e.g. via orbital eccentricity) is
also expected. The effects of the gamma-ray absorption by the external photon
fields on the gamma-ray spectrum have been taken into account, revealing clear
spectral features that might be observed. This model is consistent to the
accretion scenario, energy conservation laws, and current observational
knowledge, and can provide deeper physical information of the source when
tested against multiwavelength data.Comment: 15 pages, 12 figures, A&A, in press (text and plots improved after
minor corrections in calculations, text improved also by referee comments
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