140 research outputs found
BL Lac X-ray Spectra: simpler than we thought
We report results from {\it XMM-Newton} observations of thirteen X-ray bright
BL Lacertae objects, selected from the {\it Einstein} Slew Survey sample. The
spectra are generally well fit by power-law models, with four objects having
hard () spectra that indicates
synchrotron peaks at keV. None of our spectra show line features,
indicating that soft X-ray absorption ``notches'' must be rare amongst BL Lacs,
rather than common or ubiquitous as had previously been asserted. We find
significant curvature in most of the spectra. This curvature is almost
certainly intrinsic, as it appears nearly constant from 0.5 to 6 keV, an
observation which is inconsistent with the small columns seen in these sources.Comment: 4 pages, 1 figure; to be published in proceedings of the Cozumel
meeting on "Multiwavelength Surveys for AGN", Cozumel 200
X-ray Properties of the Weak Seyfert 1 Nucleus in NGC 4639
We obtained observations of NGC 4639 with ASCA in order to investigate its
mildly active Seyfert 1 nucleus at hard X-ray energies. Koratkar et al. (1995)
have previously shown that the nucleus is a pointlike source in the ROSAT soft
X-ray band. We detected in the 2-10 keV band a compact central source with a
luminosity of 8.3E+40 erg/s. Comparison of the ASCA data with archival data
taken with the Einstein and ROSAT satellites shows that the nucleus varies on
timescales of months to years. The variability could be intrinsic, or it could
be caused by variable absorption. More rapid variability, on a timescale of
\~10^4 s, may be present in the ASCA data. The spectrum from 0.5 to 10 keV is
well described by a model consisting of a lightly absorbed (N_H = 7.3E+20
cm^-2) power law with a photon index of 1.68. We find no evidence for
significant emission from a thermal plasma; if present, it can account for no
more than 25% of the flux in the 0.5-2.0 keV band. The limited photon
statistics of our data do not allow us to place significant limits on the
presence of iron K emission. (abridged)Comment: To appear in The Astrophysical Journal. LaTex, 18 pages including
embedded figures and table
Below the Lyman Edge: UV Polarimetry of Quasars
The Lyman edge at 912 \AA is an important diagnostic region for studying
quasi-stellar objects (QSOs). In particular, it reveals a great deal about the
physical conditions within the atmospheres of accretion disks, a ubiquitous
component of QSO theories. A robust prediction of accretion disk models is a
significant polarization due to electron scattering just longward (in
wavelength) of the Lyman edge because of the wavelength dependence of the
Hydrogen absorption opacity. Observations of the Lyman edge regions of QSOs
have shown scant evidence for the predicted features-few QSOs show the broad,
partial Lyman edges expected to be common according to most theories, and none
show the high polarizations expected longward of the Lyman edge. Still,
polarization spectra of a small number of QSOs have shown a rising polarization
(up to 20%) at wavelengths shortward of the Lyman edge. We have now doubled our
sample of intermediate-redshift QSOs observed with the HST/FOS
spectropolarimeter to determine the amount of polarization on both sides of the
Lyman limit. For this new sample of six objects, polarizations are low and
mostly consistent with zero below the Lyman edge.
Another important result of the new data is that it strengthens the
conclusion that quasars are generally not polarized significantly just longward
of the Lyman edge at $\sim 1000 \AA. There is no significant statistical
wavelength dependence to the polarization longward of the Lyman edge indicating
that simple plane-parallel atmospheres with scattering-dominated opacity are
not significant sources of UV flux in quasars.Comment: Accepted to Ap.J., 30 pages, 8 figure
Pandemic influenza A(H1N1) 2009 outbreak in a residential school at Panchgani, Maharashtra, India
Background & objectives: An outbreak of influenza was investigated between June 24 and July 30, 2009 in a residential school at Panchgani, Maharashtra, India. The objectives were to determine the aetiology, study the clinical features in the affected individuals and, important epidemiological and environmental factors. The nature of public health response and effectiveness of the control measures were also evaluated. Methods: Real time reverse transcriptase polymerase chain reaction was performed on throat swabs collected from 82 suspected cases to determine the influenza types (A or B) and sub-types [pandemic (H1N1) 2009, as well as seasonal influenza H1N1, H3N2]. Haemagglutination inhibition assay was performed on serum samples collected from entire school population (N = 415) to detect antibodies for pandemic (H1N1) 2009, seasonal H1N1, H3N2 and influenza B/Yamagata and B/Victoria lineages. Antibody titres â„ 10 for pandemic (H1N1) 2009 and â„ 20 for seasonal influenza A and B were considered as positive for these viruses. Results: Clinical attack rate for influenza-like illness was 71.1 per cent (295/415). The attack rate for pandemic (H1N1) 2009 cases was 42.4 per cent (176/415). Throat swabs were collected from 82 cases, of which pandemic (H1N1) 2009 virus was detected in 15 (18.3%), influenza type A in (6) 7.4 per cent and influenza type B only in one case. A serosurvey carried out showed haemagglutination inhibition antibodies to pandemic (H1N1) 2009 in 52 per cent (216) subjects in the school and 9 per cent (22) in the community. Interpretation & conclusion: Our findings confirmed an outbreak of pandemic (H1N1) 2009 due to local transmission among students in a residential school at Panchgani, Maharashtra, India
Extragalactic MeV gamma-ray emission from cocoons of young radio galaxies
Strong -ray emission from cocoons of young radio galaxies is newly
predicted. Considering the process of adiabatic injection of the shock
dissipation energy and mass of the relativistic jet in active nuclei (AGNs)
into the cocoon, while assuming thermalizing electron plasma interactions, we
find that the thermal electron temperature of the cocoon is typically predicted
in MeV, which is determined only by the bulk Lorentz factor of the
relativistic jet. Together with the time-dependent dynamics of the cocoon
expansion, we find that young cocoons can yield thermal bremsstrahlung
emissions at energies MeV.Comment: 5pages, 1figure, MNRAS accepte
Ultraviolet and Multiwavelength Variability of the Blazar 3C 279: Evidence for Thermal Emission
The gamma-ray blazar 3C 279 was monitored on a nearly daily basis with IUE,
ROSAT and EGRET for three weeks between December 1992 and January 1993. During
this period, the blazar was at a historical minimum at all wavelengths. Here we
present the UV data obtained during the above multiwavelength campaign. A
maximum UV variation of ~50% is detected, while during the same period the
X-ray flux varied by no more than 13%. At the lowest UV flux level the average
spectrum in the 1230-2700 A interval is unusually flat for this object
(~1). The flattening could represent the lowest energy tail of the
inverse Compton component responsible for the X-ray emission, or could be due
to the presence of a thermal component at ~20000 K possibly associated with an
accretion disk. The presence of an accretion disk in this blazar object, likely
observable only in very low states and otherwise hidden by the beamed, variable
synchrotron component, would be consistent with the scenario in which the seed
photons for the inverse Compton mechanism producing the gamma-rays are external
to the relativistic jet. We further discuss the long term correlation of the UV
flux with the X-ray and gamma-ray fluxes obtained at various epochs. All UV
archival data are included in the analysis. Both the X- and gamma-ray fluxes
are generally well correlated with the UV flux, approximately with square root
and quadratic dependences, respectively.Comment: 22 pages, Latex, 7 PostScript figures, to appear in The Astrophysical
Journa
Towards a New Standard Model for Black Hole Accretion
We briefly review recent developments in black hole accretion disk theory,
emphasizing the vital role played by magnetohydrodynamic (MHD) stresses in
transporting angular momentum. The apparent universality of accretion-related
outflow phenomena is a strong indicator that large-scale MHD torques facilitate
vertical transport of angular momentum. This leads to an enhanced overall rate
of angular momentum transport and allows accretion of matter to proceed at an
interesting rate. Furthermore, we argue that when vertical transport is
important, the radial structure of the accretion disk is modified at small
radii and this affects the disk emission spectrum. We present a simple model
demonstrating how energetic, magnetically-driven outflows modify the emergent
disk emission spectrum with respect to that predicted by standard accretion
disk theory. A comparison of the predicted spectra against observations of
quasar spectral energy distributions suggests that mass accretion rates
inferred using the standard disk model may severely underestimate their true
values.Comment: To appear in the Fifth Stromlo Symposium Proceedings special issue of
ApS
Rms-flux relation in the optical fast variability data of BL Lacertae object S5 0716+714
The possibility that BL Lac S5 0716+714 exhibits a linear root mean square
(rms)-flux relation in its IntraDay Variability (IDV) is analysed. The results
may be used as an argument in the existing debate regarding the source of
optical IDV in Active Galactic Nuclei. 63 time series in different optical
bands were used. A linear rms-flux relation at a confidence level higher than
65% was recovered for less than 8% of the cases. We were able to check if the
magnitude is log-normally distributed for eight timeseries and found, with a
confidence > 95%, that this is not the case.Comment: Accepted by Astrophysics and Space Scienc
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