126 research outputs found
A Continuous Injection Plasma Model for the X-Ray/Radio Knots in Kpc-Scale Jets of AGN
We consider the evolution of a spherically expanding plasma cloud, where
there is continuous injection of non-thermal electrons. We compute the time
dependent electron distribution and resultant photon spectra taking into
account synchrotron, adiabatic and inverse Compton cooling. This model is
different from previous works where, instead of a continuous injection of
particles, a short injection period was assumed. We apply this model to the
radio/optical knots in the large scale jets of AGN, detected in X-rays by {\it
Chandra} and find that the overall broadband spectral features can be
reproduced. It is shown that for some sources, constraints on the X-ray
spectral index (by a longer {\it Chandra} observation) will be able to
differentiate between the different models. This in turn will put a strong
constraint on the acceleration mechanism active in these sources.Comment: Accepted for publications in the Astrophysical Journal Letter
Continuum radiation from active galactic nuclei: A statistical study
The physics of the continuum spectrum of active galactic nuclei (AGNs) was examined using a large data set and rigorous statistical methods. A data base was constructed for 469 objects which include radio selected quasars, optically selected quasars, X-ray selected AGNs, BL Lac objects, and optically unidentified compact radio sources. Each object has measurements of its radio, optical, X-ray core continuum luminosity, though many of them are upper limits. Since many radio sources have extended components, the core component were carefully selected out from the total radio luminosity. With survival analysis statistical methods, which can treat upper limits correctly, these data can yield better statistical results than those previously obtained. A variety of statistical tests are performed, such as the comparison of the luminosity functions in different subsamples, and linear regressions of luminosities in different bands. Interpretation of the results leads to the following tentative conclusions: the main emission mechanism of optically selected quasars and X-ray selected AGNs is thermal, while that of BL Lac objects is synchrotron; radio selected quasars may have two different emission mechanisms in the X-ray band; BL Lac objects appear to be special cases of the radio selected quasars; some compact radio sources show the possibility of synchrotron self-Compton (SSC) in the optical band; and the spectral index between the optical and the X-ray bands depends on the optical luminosity
Two component model for X-ray emission of radio selected QSO's
Using a large database of radio, optical, and x ray luminosities of AGNs with survival analysis, it was found that the x ray emission of the radio selected quasars has two components. One is related to the optical luminosity and the other is related to the radio luminosity
Towards a robust estimate of the merger rate evolution using near-IR photometry
We use a combination of deep, high angular resolution imaging data from the
CDFS (HST/ACS GOODS survey) and ground based near-IR images to derive the
evolution of the galaxy major merger rate in the redshift range . We select galaxies on the sole basis of their J-band rest-frame,
absolute magnitude, which is a good tracer of the stellar mass. We find steep
evolution with redshift, with the merger rate for
optically selected pairs, and for pairs selected
in the near-IR. Our result is unlikely to be affected by luminosity evolution
which is relatively modest when using rest-frame J band selection. The
apparently more rapid evolution that we find in the visible is likely caused by
biases relating to incompleteness and spatial resolution affecting the ground
based near IR photometry, underestimating pair counts at higher redshifts in
the near-IR. The major merger rate was 5.6 times higher at
than at the current epoch. Overall 41%(0.5\gyr/) of all
galaxies with have undergone a major merger in the last \sim8
\gyr, where is the merger timescale. Interestingly, we find no effect
on the derived major merger rate due to the presence of the large scale
structure at in the CDFS.Comment: Accepted for Publication in ApJ. 9 Figure
Accretion disc-corona and jet emission from the radio-loud narrow-line Seyfert 1 galaxy RX J1633.3+4719
We perform X-ray/ultraviolet (UV) spectral and X-ray variability studies of
the radio-loud narrow-line Seyfert 1 (NLS1) galaxy RX J1633.3+4719 using
XMM-Newton and Suzaku observations from 2011 and 2012. The 0.3-10 keV spectra
consist of an ultrasoft component described by an accretion disc blackbody
(kT_in = 39.6^{+11.2}_{-5.5} eV) and a power law due to the thermal
Comptonization ({\Gamma} = 1.96^{+0.24}_{-0.31}) of the disc emission. The disc
temperature inferred from the soft excess is at least a factor of 2 lower than
that found for the canonical soft excess emission from radio-quiet NLS1s. The
UV spectrum is described by a power law with photon index 3.05^{+0.56}_{-0.33}.
The observed UV emission is too strong to arise from the accretion disc or the
host galaxy, but can be attributed to a jet. The X-ray emission from RX
J1633.3+4719 is variable with fractional variability amplitude =13.5 per cent. In contrast to radio-quiet active galactic nuclei
(AGN), X-ray emission from the source becomes harder with increasing flux. The
fractional rms variability increases with energy and the rms spectrum is well
described by a constant disc component and a variable power-law continuum with
the normalization and photon index being anticorrelated. Such spectral
variability cannot be caused by variations in the absorption and must be
intrinsic to the hot corona. Our finding of possible evidence for emission from
the inner accretion disc, jet and hot corona from RX J1633.3+4719 in the
optical to X-ray bands makes this object an ideal target to probe the disc-jet
connection in AGN.Comment: 12 pages, 11 figures, 3 tables, Published in MNRA
CCD surface photometry of galaxies
We present here the results based on analysis of broad band optical images of radio loud and radio quiet galaxies selected from a set of x-ray galaxies. Data reduction techniques, and surface photometry using the IRAF data reduction package are described. The radial surface brightness profile of each galaxy is obtained. The disc, bulge, and nucleus of the galaxy are modeled based on the estimated brightness profile. The model of the galaxy is constructed, which is subtracted from the observed image to enhance the small scale features in the galaxy. Color maps of the galaxy are obtained and compared with those of normal galaxies. The optical properties of the galaxy are compared with its x-ray and radio properties
The non-linear behavior of the black hole system GRS 1915+105
Using non-linear time series analysis, along with surrogate data analysis, it
is shown that the various types of long term variability exhibited by the black
hole system GRS 1915+105, can be explained in terms of a deterministic
non-linear system with some inherent stochastic noise. Evidence is provided for
a non-linear limit cycle origin of one of the low frequency QPO detected in the
source, while some other types of variability could be due to an underlying low
dimensional chaotic system. These results imply that the partial differential
equations which govern the magneto-hydrodynamic flow of the inner accretion
disk, can be approximated by a small number () of non-linear but
{\it ordinary} differential equations. While this analysis does not reveal the
exact nature of these approximate equations, they may be obtained in the
future, after results of magneto-hydrodynamic simulation of realistic accretion
disks become available.Comment: Accepted for publication in ApJ. Numerical code that is used in the
analysis is available at http://www.iucaa.ernet.in/~rmisra/NL
Tracking pulsar dispersion measures using the giant metrewave radio telescope
In this paper, we describe a novel experiment for the accurate estimation of pulsar dispersion measures (DMs) using the Giant Metrewave Radio Telescope. This experiment was carried out for a sample of 12 pulsars, over a period of more than one year (2001 January to 2002 May) with observations about once every fortnight. At each epoch, the pulsar DMs were obtained from simultaneous dual-frequency observations, without requiring any absolute timing information. The DM estimates were obtained from both the single-pulse data streams and from the average profiles. The accuracy of the DM estimates at each epoch is ~1 part in 104 or better, making the data set useful for many different kinds of studies. The time-series of DMs shows significant variations on time-scales of weeks to months for most of the pulsars. An analysis of the mean DM values from these data shows significant deviations from catalogue values (as well as from other estimates in the literature) for some of the pulsars, with PSR B1642-03 showing the most notable differences. From our analysis results it appears that the constancy of pulsar DMs (at the level of 1 in 103 or better) cannot be taken for granted. For PSR B2217+47, we see evidence of a large-scale DM gradient over a 1-yr period, which is modelled as being due to a blob of enhanced electron density sampled by the line of sight. For some pulsars, including pulsars with fairly simple profiles such as PSR B1642-03, we find evidence for small changes in DM values for different frequency pairs of measurement, a result that needs to be investigated in detail. Another interesting result is that we find significant differences in DM values obtained from average profiles and single-pulse data
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