413 research outputs found
X-ray vs. Optical Variations in the Seyfert 1 Nucleus NGC 3516: A Puzzling Disconnectedness
We present optical broadband (B and R) observations of the Seyfert 1 nucleus
NGC 3516, obtained at Wise Observatory from March 1997 to March 2002,
contemporaneously with X-ray 2-10 keV measurements with RXTE. With these data
we increase the temporal baseline of this dataset to 5 years, more than triple
to the coverage we have previously presented for this object. Analysis of the
new data does not confirm the 100-day lag of X-ray behind optical variations,
tentatively reported in our previous work. Indeed, excluding the first year's
data, which drive the previous result, there is no significant correlation at
any lag between the X-ray and optical bands. We also find no correlation at any
lag between optical flux and various X-ray hardness ratios. We conclude that
the close relation observed between the bands during the first year of our
program was either a fluke, or perhaps the result of the exceptionally bright
state of NGC 3516 in 1997, to which it has yet to return. Reviewing the results
of published joint X-ray and UV/optical Seyfert monitoring programs, we
speculate that there are at least two components or mechanisms contributing to
the X-ray continuum emission up to 10 keV: a soft component that is correlated
with UV/optical variations on timescales >1 day, and whose presence can be
detected when the source is observed at low enough energies (about 1 keV), is
unabsorbed, or is in a sufficiently bright phase; and a hard component whose
variations are uncorrelated with the UV/optical.Comment: 9 pages, AJ, in pres
An extreme, blueshifted iron line profile in the Narrow Line Seyfert 1 PG 1402+261; an edge-on accretion disk or highly ionized absorption?
We report on a short XMM-Newton observation of the radio-quiet Narrow Line
Seyfert 1 PG 1402+261. The EPIC X-ray spectrum of PG 1402+261 shows a strong
excess of counts between 6-9 keV in the rest frame. This feature can be modeled
by an unusually strong (equivalent width 2 keV) and very broad (FWHM velocity
of 110000 km/s) iron K-shell emission line. The line centroid energy at 7.3 keV
appears blue-shifted with respect to the iron Kalpha emission band between
6.4-6.97 keV, while the blue-wing of the line extends to 9 keV in the quasar
rest frame. The line profile can be fitted by reflection from the inner
accretion disk, but an inclination angle of >60 deg is required to model the
extreme blue-wing of the line. Furthermore the extreme strength of the line
requires a geometry whereby the hard X-ray emission from PG 1402+261 above 2
keV is dominated by the pure-reflection component from the disk, while little
or none of the direct hard power-law is observed. Alternatively the spectrum
above 2 keV may instead be explained by an ionized absorber, if the column
density is sufficiently high (N_H > 3 x 10^23 cm^-2) and if the matter is
ionized enough to produce a deep (tau~1) iron K-shell absorption edge at 9 keV.
This absorber could originate in a large column density, high velocity outflow,
perhaps similar to those which appear to be observed in several other high
accretion rate AGN. Further observations, especially at higher spectral
resolution, are required to distinguish between the accretion disk reflection
or outflow scenarios.Comment: Accepted for publication in ApJ (18 pages, 5 figures, 1 table
Can there be neutrino oscillation in Gamma-Ray Bursts fireball ?
The central engine which powers the Gamma-Ray Burst (GRB) fireball, produces
neutrinos in the energy range of about 5-20 MeV. Fractions of these neutrinos
may propagate through the fireball which is far away from the central engine.
We have studied the propagation of these neutrinos through the fireball which
is contaminated by baryons and have shown that, resonant conversion of
neutrinos are possible for the oscillations of nu_e to nu_{mu,tau}, nu_e to
nu_s and anti-nu_(mu,tou) to anti-nu_s if the neutrino mass square difference
and mixing angle are in the atmospheric and/or LSND range. On the other hand it
is probably difficult for neutrinos to have resonant oscillation if the
neutrino parameters are in the solar neutrino range. From the resonance
condition we have estimated the fireball temperature and the baryon load in it.Comment: 4 pages, two column text, To be published in Phys. Rev.
Time dependent numerical model for the emission of radiation from relativistic plasma
We describe a numerical model constructed for the study of the emission of
radiation from relativistic plasma under conditions characteristic, e.g., to
gamma-ray bursts (GRB's) and active galactic nuclei (AGN's). The model solves
self consistently the kinetic equations for e^\pm and photons, describing
cyclo-synchrotron emission, direct Compton and inverse Compton scattering, pair
production and annihilation, including the evolution of high energy
electromagnetic cascades. The code allows calculations over a wide range of
particle energies, spanning more than 15 orders of magnitude in energy and time
scales. Our unique algorithm, which enables to follow the particle
distributions over a wide energy range, allows to accurately derive spectra at
high energies, >100 \TeV. We present the kinetic equations that are being
solved, detailed description of the equations describing the various physical
processes, the solution method, and several examples of numerical results.
Excellent agreement with analytical results of the synchrotron-SSC model is
found for parameter space regions in which this approximation is valid, and
several examples are presented of calculations for parameter space regions
where analytic results are not available.Comment: Minor changes; References added, discussion on observational status
added. Accepted for publication in Ap.
Flash-Heating of Circumstellar Clouds by Gamma Ray Bursts
The blast-wave model for gamma-ray bursts (GRBs) has been called into
question by observations of spectra from GRBs that are harder than can be
produced through optically thin synchrotron emission. If GRBs originate from
the collapse of massive stars, then circumstellar clouds near burst sources
will be illuminated by intense gamma radiation, and the electrons in these
clouds will be rapidly scattered to energies as large as several hundred keV.
Low-energy photons that subsequently pass through the hot plasma will be
scattered to higher energies, hardening the intrisic spectrum. This effect
resolves the "line-of-death" objection to the synchrotron shock model.
Illuminated clouds near GRBs will form relativistic plasmas containing large
numbers of electron-positron pairs that can be detected within ~ 1-2 days of
the explosion before expanding and dissipating. Localized regions of pair
annihilation radiation in the Galaxy would reveal past GRB explosions.Comment: 9 pages, 1 figure, submitted to ApJ Letter
Gamma Rays from Compton Scattering in the Jets of Microquasars: Application to LS 5039
Recent HESS observations show that microquasars in high-mass systems are
sources of VHE gamma-rays. A leptonic jet model for microquasar gamma-ray
emission is developed. Using the head-on approximation for the Compton cross
section and taking into account angular effects from the star's orbital motion,
we derive expressions to calculate the spectrum of gamma rays when nonthermal
jet electrons Compton-scatter photons of the stellar radiation field.
Calculations are presented for power-law distributions of nonthermal electrons
that are assumed to be isotropically distributed in the comoving jet frame, and
applied to -ray observations of LS 5039. We conclude that (1) the TeV
emission measured with HESS cannot result only from Compton-scattered stellar
radiation (CSSR), but could be synchrotron self-Compton (SSC) emission or a
combination of CSSR and SSC; (2) fitting both the HESS data and the EGRET data
associated with LS 5039 requires a very improbable leptonic model with a very
hard electron spectrum. Because the gamma rays would be variable in a leptonic
jet model, the data sets are unlikely to be representative of a simultaneously
measured gamma-ray spectrum. We therefore attribute EGRET gamma rays primarily
to CSSR emission, and HESS gamma rays to SSC emission. Detection of periodic
modulation of the TeV emission from LS 5039 would favor a leptonic SSC or
cascade hadron origin of the emission in the inner jet, whereas stochastic
variability alone would support a more extended leptonic model. The puzzle of
the EGRET gamma rays from LS 5039 will be quickly solved with GLAST. (Abridged)Comment: 17 pages, 11 figures, ApJ, in press, June 1, 2006, corrected eq.
Peak energy clustering and efficiency in compact objects
We study the properties of plasmas containing a low energy thermal photon
component at comoving temperature \theta \equiv kT'/m_e c^2 \sim 10^{-5} -
10^{-2} interacting with an energetic electron component, characteristic of,
e.g., the dissipation phase of relativistic outflows in gamma-ray bursts
(GRB's), X-ray flashes, and blazars. We show that, for scattering optical
depths larger than a few, balance between Compton and inverse-Compton
scattering leads to the accumulation of electrons at values of . For optical depths larger than ~ 100, this leads to a peak in the
comoving photon spectrum at 1-10 keV, very weakly dependent on the values of
the free parameters. In particular, these results are applicable to the
internal shock model of GRB, as well as to slow dissipation models, e.g. as
might be expected from reconnection, if the dissipation occurs at a
sub-photospheric radii. For GRB bulk Lorentz factors ~ 100, this results in
observed spectral peaks clustering in the 0.1-1 MeV range, with conversion
efficiencies of electron into photon energy in the BATSE range of ~ 30%.Comment: Extended explanations about the electron energy balance; Refine
figures; Accepted for publication in Ap.
Detecting Compton Reflection and a Broad Iron Line in MCG-5-23-16 with the Rossi X-ray Timing Explorer
We report the detection with the Rossi X-ray Timing Explorer of a Compton
reflection signature in the Seyfert galaxy MCG-5-23-16. RXTE also resolves the
Fe K-alpha fluorescence line with FWHM ~48,000 km s^{-1}. This measurement
provides the first independent confirmation of ASCA detections in Seyfert
galaxies of broad Fe K-alpha lines that are thought to be the signature of
emission from the inner regions of an accretion disk orbiting a black hole.
Under the assumption that reflection arises from an isotropic source located
above a neutral accretion disk, and using a theoretical model that accounts for
the dependence of the reflected spectrum on inclination angle, we derive a 90%
confidence range for the disk inclination of i = 50 to 81 degrees. The large
inclination is consistent with that expected from the unified model for
MCG-5-23-16 based on its Seyfert 1.9 classification. If we assume that the
high-energy cutoff in the incident spectrum lies at energies larger than a few
hundred keV, then the equivalent width of the Fe K-alpha line is much larger
than predicted for the amount of reflection. This implies either an enhanced
iron abundance, a covering factor of reflecting material > 0.5, or a cutoff in
the incident spectrum at energies between ~60 and ~200 keV.Comment: Accepted for publication in ApJ, LaTeX. 14 pages including 3 figures,
with 1 table as a separate postscript file. Typo corrected in abstrac
X-ray Rich GRB, Photospheres and Variability
We investigate the relationship between the quasi-thermal baryon-related
photosphere in relativistic outflows, and the internal shocks arising outside
them, which out to a limiting radius may be able to create enough pairs to
extend the optically thick region. Variable gamma-ray light curves are likely
to arise outside this limiting pair-forming shock radius, while X-ray excess
bursts may arise from shocks occurring below it; a possible relation to X-ray
flashes is discussed. This model leads to a simple physical interpretation of
the observational gamma-ray variability-luminosity relation.Comment: accepted in Ap.J. 6/20/02; subm 5/8/02; aaspp4, 12 pages, 1 fi
Discovery of the Narrow-Line Seyfert 1 galaxy Mkn 335 in an historical low X-ray flux state
We report the discovery of the Narrow-Line Seyfert 1 galaxy Mkn 335 in an
extremely low X-ray state. A comparison of Swift observations obtained in May
and June/July 2007 with all previous X-ray observations between 1971 to 2006
show the AGN to have diminished in flux by a factor of more than 30, the lowest
X-ray flux Mkn 335 has ever been observed in. The Swift observations show an
extremely hard X-ray spectrum at energies above 2 keV. Possible interpretations
include partial covering absorption or X-ray reflection from the disk. In this
letter we consider the partial covering interpretation. The Swift observations
can be well fit by a strong partial covering absorber with varying absorption
column density N_H= 1-4 x 10^{23} cm-2 and a covering fraction f_c=0.9 - 1.
When corrected for intrinsic absorption, the X-ray flux of Mkn 335 varies by
only factors of 4-6. In the UV Mkn 335 shows variability in the order of 0.2
mag. We discuss the similarity of Mkn 335 with the highly variable NLS1
WPVS007, and speculate about a possible link between NLS1 galaxies and
broad-absorption line quasars.Comment: ApJ Letter accepted; 8 pages, 2 figures; The new version has three
more sentences in the introduction and three references added to the
discussio
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