519 research outputs found
A fit to the simultaneous broadband spectrum of Cygnus X-1 using the transition disk model
We have used the transition disk model to fit the simultaneous broad band
( keV) spectrum of Cygnus X-1 from OSSE and Ginga observations. In this
model, the spectrum is produced by saturated Comptonization within the inner
region of the accretion disk, where the temperature varies rapidly with radius.
In an earlier attempt, we demonstrated the viability of this model by fitting
the data from EXOSAT, XMPC balloon and OSSE observations, though these were not
made simultaneously. Since the source is known to be variable, however, the
results of this fit were not conclusive. In addition, since only once set of
observations was used, the good agreement with the data could have been a
chance occurrence. Here, we improve considerably upon our earlier analysis by
considering four sets of simultaneous observations of Cygnus X-1, using an
empirical model to obtain the disk temperature profile. The vertical structure
is then obtained using this profile and we show that the analysis is self-
consistent. We demonstrate conclusively that the transition disk spectrum is a
better fit to the observations than that predicted by the soft photon
Comptonization model. Since the temperature profile is obtained by fitting the
data, the unknown viscosity mechanism need not be specified. The disk structure
can then be used to infer the viscosity parameter , which appears to
vary with radius and luminosity. This behavior can be understood if
depends intrinsically on the local parameters such as density, height and
temperature. However, due to uncertainties in the radiative transfer,
quantitative statements regarding the variation of cannot yet be made.Comment: 8 figures. uses aasms4.sty, accepted by ApJ (Mar 98
Broad band X-ray spectrum of Cygnus X-1
We present the hard X-ray (20-100 keV) observations of Cygnus X-1 obtained
using a large area balloon-borne Xenon filled Multi-anode Proportional Counter
(XMPC) telescope. The observations were carried out during the gamma_2 state of
the source and we obtain a power law photon index of 1.62 +- 0.07. To constrain
the spectral shape of the source, we have analyzed the archival EXOSAT ME argon
and GSPC data in the low energies (2-20 keV band) as well as the archival OSSE
data in the high energies (50-500 keV). The data in different energy bands are
not obtained in simultaneous observations, but they pertain to the gamma_2
state of the source. We have attempted a combined fit to the wide band data
using appropriate mutual detector calibrations. A combined fit to the EXOSAT
and XMPC data (2-100 keV) shows that the observed spectrum requires a low
energy absorption corresponding to the Galactic interstellar absorption, a low
energy excess modeled as blackbody, a narrow emission line due to iron K_alpha
and a continuum. The continuum can be either modeled as a power law with a
reflection bump or a Comptonisation model with an additional bump which can be
modeled as the partial covering with a heavy absorber. To resolve between these
two models, we have attempted combined fit to the 2-500 keV data obtained from
EXOSAT, XMPC and OSSE. We find that two component Comptonisation model
adequately represents the data. We explore the possible emission region that is
responsible for the observed spectrum.Comment: 25 pages including 8 figures. To appear in Astronomy and Astrophysic
Broadband study of blazar 1ES 1959+650 during flaring state in 2016
Aim : The nearby TeV blazar 1ES 1959+650 (z=0.047) was reported to be in
flaring state during June - July 2016 by Fermi-LAT, FACT, MAGIC and VERITAS
collaborations. We studied the spectral energy distributions (SEDs) in
different states of the flare during MJD 57530 - 57589 using simultaneous
multiwaveband data to understand the possible broadband emission scenario
during the flare. Methods : The UV/optical and X-ray data from UVOT and XRT
respectively on board Swift and high energy -ray data from Fermi-LAT
are used to generate multiwaveband lightcurves as well as to obtain high flux
states and quiescent state SEDs. The correlation and lag between different
energy bands is quantified using discrete correlation function. The synchrotron
self Compton (SSC) model was used to reproduce the observed SEDs during flaring
and quiescent states of the source. Results : A decent correlation is seen
between X-ray and high energy -ray fluxes. The spectral hardening with
increase in the flux is seen in X-ray band. The powerlaw index vs flux plot in
-ray band indicates the different emission regions for 0.1 - 3 GeV and
3-300 GeV energy photons. Two zone SSC model satisfactorily fits the observed
broadband SEDs. The inner zone is mainly responsible for producing synchrotron
peak and high energy -ray part of the SED in all states. The second
zone is mainly required to produce less variable optical/UV and low energy
-ray emission. Conclusions : Conventional single zone SSC model does
not satisfactorily explain broadband emission during observation period
considered. There is an indication of two emission zones in the jet which are
responsible for producing broadband emission from optical to high energy
-rays.Comment: 11 pages, 12 figures, Accepted in A&
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