735 research outputs found
Radio observations of NGC 6388: an upper limit on the mass of its central black hole
We present the results of deep radio observations with the Australia
Telescope Compact Array (ATCA) of the globular cluster NGC 6388. We show that
there is no radio source detected (with a r.m.s. noise level of 27 uJy) at the
cluster centre of gravity or at the locations of the any of the Chandra X-ray
sources in the cluster. Based on the fundamental plane of accreting black holes
which is a relationship between X-ray luminosity, radio luminosity and black
hole mass, we place an upper limit of 1500 M_sun on the mass of the putative
intermediate-mass black hole located at the centre of NGC 6388. We discuss the
uncertainties of this upper limit and the previously suggested black hole mass
of 5700 M_sun based on surface density profile analysis.Comment: 6 pages, 2 figures, accepted for publication in MNRA
X-Ray Emission from the Jets of XTE J1550-564
We report on X-ray observations of the the large-scale jets recently
discovered in the radio and detected in X-rays from the black hole candidate
X-ray transient and microquasar XTE J1550-564. On 11 March 2002, X-ray emission
was detected 23 arcsec to the West of the black hole candidate and was extended
along the jet axis with a full width at half maximum of 1.2 arcsec and a full
width at 10% of maximum intensity of 5 arcsec. The morphology of the X-ray
emission matched well to that of the radio emission at the same epoch. The jet
moved by 0.52 +/- 0.13 arcsec between 11 March and 19 June 2002. The apparent
speed during that interval was 5.2 +/- 1.3 mas/day. This is significantly less
than the average apparent speed of 18.1 +/- 0.4 mas/day from 1998 to 2002,
assuming that the jet was ejected in September 1998, and indicates that the jet
has decelerated. The X-ray spectrum is adequately described by a powerlaw with
a photon index near 1.8 subject to interstellar absorption. The unabsorbed
X-ray flux was 3.4 x 10^-13 erg cm^-2 s^-1 in the 0.3-8 keV band in March 2002,
and decreased to 2.9 x 10^-13 erg cm^-2 s^-1 in June. We also detect X-rays
from the eastern jet in March 2002 and show that it has decelerated and dimmed
since the previous detections in 2000.Comment: accepted for publication in ApJ, 11 pages, several figures in colo
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
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
Optimisation des traitements utilises en lutte anti-vectorielle : associations synergiques repulsif / insecticide. Société française de Mycologie médicale
Mosquito borne-disease vector isolated neurons: a new biological model for optimizing insecticide/repellent efficacy while reducing adverse effects on ecosystem
Chandra Observations of the Interacting NGC 4410 Galaxy Group
We present high resolution X-ray imaging data from the ACIS-S instrument on
the Chandra telescope of the nearby interacting galaxy group NGC 4410. Four
galaxies in the inner portion of this group are clearly detected by Chandra,
including the peculiar low luminosity radio galaxy NGC 4410A. In addition to a
nuclear point source, NGC 4410A contains diffuse X-ray emission, including an
X-ray ridge extending out to about 12" (6 kpc) to the northwest of the nucleus.
This ridge is coincident with an arc of optical emission-line gas, which has
previously been shown to have optical line ratios consistent with shock
ionization. This structure may be due to an expanding superbubble of hot gas
caused by supernovae and stellar winds or by the active nucleus. The Chandra
observations also show four or five possible compact ultra-luminous X-ray (ULX)
sources (L(x) >= 10^39 erg/s) associated with NGC 4410A. At least one of these
candidate ULXs appears to have a radio counterpart, suggesting that it may be
due to an X-ray binary with a stellar-mass black hole, rather than an
intermediate mass black hole. In addition, a faint diffuse intragroup X-ray
component has been detected between the galaxies (L(x) ~ 10^41 erg/s). This
supports the hypothesis that the NGC 4410 group is in the process of evolving
via mergers from a spiral-dominated group (which typically have no
X-ray-emitting intragroup gas) to an elliptical-dominated group (which often
have a substantial intragroup medium).Comment: 27 pages, 14 figures; Accepted by Astronomical Journal; color images
at http://www.etsu.edu/physics/bsmith/research/n4410.htm
Positive allosteric modulation of muscarinic receptors is responsible for the synergistic effect occurring between the repellent DEET and carbamate in insect central nervous system
Going with the flow: can the base of jets subsume the role of compact accretion disk coronae?
The hard state of X-ray binaries (XRBs) is characterized by a power law
spectrum in the X-ray band, and a flat/inverted radio/IR spectrum associated
with occasionally imaged compact jets. It has generally been thought that the
hard X-rays result from Compton upscattering of thermal accretion disk photons
by a hot, coronal plasma whose properties are inferred via spectral fitting.
Interestingly, these properties-especially those from certain magnetized corona
models-are very similar to the derived plasma conditions at the jet footpoints.
Here we explore the question of whether the `corona' and `jet base' are in fact
related, starting by testing the strongest premise that they are synonymous. In
such models, the radio through the soft X-rays are dominated by synchrotron
emission, while the hard X-rays are dominated by inverse Compton at the jet
base - with both disk and synchrotron photons acting as seed photons. The
conditions at the jet base fix the conditions along the rest of the jet, thus
creating a direct link between the X-ray and radio emission. We also add to
this model a simple iron line and convolve the spectrum with neutral
reflection. After forward-folding the predicted spectra through the detector
response functions, we compare the results to simultaneous radio/X-ray data
obtained from the hard states of the Galactic XRBs GX339-4 and Cygnus X-1.
Results from simple Compton corona model fits are also presented for
comparison. We demonstrate that the jet model fits are statistically as good as
the single-component corona model X-ray fits, yet are also able to address the
simultaneous radio data.Comment: Accepted to the Astrophysical Journal. 14 pages, emulateapj.st
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