21,092 research outputs found
Chandra imaging spectroscopy of 1E 1740.7 - 2942
We have observed the black hole candidate 1E 1740.7 - 2942, the brightest
persistent hard X-ray source within a few degrees of the Galactic centre, for
10 ksec with Chandra (ACIS-I) on August 2000. Attempting to compensate for
pile-up effects we found the spectra were well-fit by an absorbed power law,
with photon indices Gamma = 1.54 ^{+0.42}_{-0.37} (readout streak) and Gamma =
1.42^{+0.14}_{-0.14} (annulus), consistent with a black hole low/hard state. We
have analysed a public observation performed by Chandra which utilised short
frames in order to avoid severe pile-up effects: subtracting the core point
spread function from the whole image, we did not find evidence for any
elongated feature perpendicular to the radio jet axis, as reported in a recent
analysis of the same data. Moreover, comparing the radial profiles with those
of an unscattered X-ray point source, we found indication of an extended,
previously undetected, X-ray scattering halo. The measured halo fractional
intensity at 3 keV is between 30 and 40 percent within 40 arcsec but drops
below detectable levels at 5 keV. Finally, by placing a limit on the X-ray flux
from the radio emitting lobe which has been identified as the hot spot at the
end of the northern jet of 1E 1740.7 - 2942, we are able to constrain the
magnetic energy density in that region.Comment: Accepted for publication in MNRA
Photon spheres in Einstein and Einstein-Gauss-Bonnet theories and circular null geodesics in axially-symmetric spacetimes
In this article we extend a recent theorem proven by Hod (Phys. Lett. B, {\bf
727}, 345--348, 2013) to -dimensional Einstein and Einstein-Gauss-Bonnet
theories, which gives an upper bound for the photon sphere radii of spherically
symmetric black holes. As applications of these results we give a universal
upper bound for the real part of quasinormal modes in the WKB limit and a
universal lower bound for the position of the first relativistic image in the
strong lensing regime produced by these type of black holes. For the
axially-symmetric case, we also make some general comments (independent of the
underlying gravitational theory) on the relation between circular null
geodesics and the fastest way to circle a black hole.Comment: In this post-publisher version two typos were corrected: the
signature of the metric in eq.(1) and a factor in eq.(102). We thanks Gary
Gibbons and Chris Pope for bringing to our attention these typo
A Jacobian elliptic single-field inflation
In the scenario of single-field inflation, this field is done in terms of
Jacobian elliptic functions. This approach provides, when constrained to
particular cases, analytic solutions already known in the past, generalizing
them to a bigger family of analytical solutions. The emergent cosmology is
analysed using the Hamilton-Jacobi approach and then, the main results are
contrasted with the recent measurements obtained from the Planck 2015 data.Comment: 7 pages, 5 figure
Driving extreme variability: The evolving corona and evidence for jet launching in Markarian 335
Variations in the X-ray emission from the narrow line Seyfert 1 galaxy,
Markarian 335 (Mrk 335), are studied on both long and short timescales through
observations made between 2006 and 2013 with XMM-Newton, Suzaku and NuSTAR.
Changes in the geometry and energetics of the corona that give rise to this
variability are inferred through measurements of the relativistically blurred
reflection seen from the accretion disc. On long timescales, we find that
during the high flux epochs the corona has expanded, covering the inner regions
of the accretion disc out to a radius of 26(-7,+10)rg. The corona contracts to
within 12rg and 5rg in the intermediate and low flux epochs, respectively.
While the earlier high flux observation made in 2006 is consistent with a
corona extending over the inner part of the accretion disc, a later high flux
observation that year revealed that the X-ray source had become collimated into
a vertically-extended jet-like corona and suggested relativistic motion of
material upward. On short timescales, we find that an X-ray flare during a low
flux epoch in 2013 corresponded to a reconfiguration from a slightly extended
corona to one much more compact, within just 2~3rg of the black hole. There is
evidence that during the flare itself, the spectrum softened and the corona
became collimated and slightly extended vertically as if a jet-launching event
was aborted. Understanding the evolution of the X-ray emitting corona may
reveal the underlying mechanism by which the luminous X-ray sources in AGN are
powered.Comment: 21 pages, 9 figures. Accepted for publication in MNRA
On the correlation between radio and X-ray flux in Low/Hard state Black Holes
Radio emission from X-ray binary systems (XRBs) has developed in recent years
from being peculiar phenomenon to being recognised as an ubiquitous property of
several classes of XRBs. In this scenario the synchrotron emission is
interpreted as the radiative signature of jet-like outflows, some or all of
which may possess relativistic bulk motion. We have analysed a collection of
quasi-simultaneous radio/X-ray observations of Black Holes in the Low/Hard
X-ray state, finding evidence of a clear correlation between their fluxes over
many orders of magnitude in luminosity. Given that the correlation extends down
to GX 339-4 and V404 Cyg in quiescence, we can confidently assert that even at
accretion rates as low as ~ 10^{-5} dot{m}_{Edd} a powerful jet is being
formed. The normalisation of the correlation is very similar across a sample of
nine sources, implying that it is nearly independent of jet inclination angle.
Remarkably, V 404 Cyg is the second source (after GX 339-4) to show the
correlation S_{radio} proportional to S_{X}^{+0.7} from quiescent level up to
close to the High/Soft state transition. Moreover, assuming the same physics
and accretion:outflow coupling for all of these systems, the simplest
interpretation for the observed scenario is that outflows in Low/Hard state do
not have large bulk Lorentz factors.Comment: 4 pages, 3 figures, Proceedings of the 4th Microquasar Workshop, eds.
Ph Durouchoux, Y. Fuchs and J. Rodriguez, published by the Center for Space
Physics: Kolkat
Is HE 0436-4717 Anemic? A deep look at a bare Seyfert 1 galaxy
A multi-epoch, multi-instrument analysis of the Seyfert 1 galaxy HE 0436-4717
is conducted using optical to X-ray data from XMM-Newton and Swift (including
the BAT). Fitting of the UV-to-X-ray spectral energy distribution shows little
evidence of extinction and the X-ray spectral analysis does not confirm
previous reports of deep absorption edges from OVIII. HE 0436-4717 is a "bare"
Seyfert with negligible line-of-sight absorption making it ideal to study the
central X-ray emitting region. Three scenarios were considered to describe the
X-ray data: partial covering absorption, blurred reflection, and soft
Comptonization. All three interpretations describe the 0.5-10.0 keV spectra
well. Extrapolating the models to 100 keV results in poorer fits for the the
partial covering model. When also considering the rapid variability during one
of the XMM-Newton observations, the blurred reflection model appears to
describe all the observations in the most self-consistent manner. If adopted,
the blurred reflection model requires a very low iron abundance in HE
0436-4717. We consider the possibilities that this is an artifact of the
fitting process, but it appears possible that it is intrinsic to the object.Comment: 7 tables, 11 figures, 16 pages; accepted for publication in MNRAS 17
Feb. 201
Seyfert galaxies with Swift: giant flares, rapid drops, and other surprises
Swift has initiated a new era of understanding the extremes of active
galactic nuclei (AGN) variability, their drivers and underlying physics. This
is based on its rapid response, high sensitivity, good spatial resolution, and
its ability to collect simultaneously X--ray-to-optical SEDs. Here, we present
results from our recent monitoring campaigns with Swift of highly variable AGN,
including outbursts, deep low states, and unusual long-term trends in several
Seyfert galaxies including Mrk 335, WPVS007, and RXJ2314.9+2243. We also report
detection of a new X-ray and optical outburst of IC 3599 and our Swift
follow-ups. IC 3599 was previously known as one of the AGN with the
highest-amplitude outbursts. We briefly discuss implications of this second
outburst of IC 3599 for emission scenarios including accretion-disk
variability, repeat tidal disruption events, and the presence of a binary
supermassive black hole.Comment: to appear in "Swift: 10 years of discovery", Proceedings of Scienc
Probing the geometry and motion of AGN coronae through accretion disc emissivity profiles
To gain a better understanding of the inner disc region that comprises active
galactic nuclei it is necessary to understand the pattern in which the disc is
illuminated (the emissivity profile) by X-rays emitted from the continuum
source above the black hole (corona). The differences in the emissivity
profiles produced by various corona geometries are explored via general
relativistic ray tracing simulations. Through the analysis of various
parameters of the geometries simulated it is found that emissivity profiles
produced by point source and extended geometries such as cylindrical slabs and
spheroidal coronae placed on the accretion disc are distinguishable. Profiles
produced by point source and conical geometries are not significantly
different, requiring an analysis of reflection fraction to differentiate the
two geometries. Beamed point and beamed conical sources are also simulated in
an effort to model jet-like coronae, though the differences here are most
evident in the reflection fraction. For a point source we determine an
approximation for the measured reflection fraction with the source height and
velocity. Simulating spectra from the emissivity profiles produced by the
various geometries produce distinguishable differences. Overall spectral
differences between the geometries do not exceed 15 per cent in the most
extreme cases. It is found that emissivity profiles can be useful in
distinguishing point source and extended geometries given high quality spectral
data of extreme, bright sources over long exposure times. In combination with
reflection fraction, timing, and spectral analysis we may use emissivity
profiles to discern the geometry of the X-ray source.Comment: 15 pages, 12 figures. Accepted for publication in MNRA
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