380 research outputs found
Rapid X-ray Variability of the BL Lacertae Object PKS 2155-304
(Abridged) We present a detailed power density spectrum and cross-correlation
analysis of the X-ray light curves of the BL Lac object PKS 2155-304, observed
with BeppoSAX in 1997 and 1996, aimed at exploring the rapid variability
properties and the inter-band cross correlations in the X-rays. We also perform
the same analysis on the (archival) X-ray light curve obtained with ASCA in
1994.Comment: 47 pages, 11 figures, AAS Latex macros V4.0, accepted for publication
in the Astrophysical Journa
Spectral Evolution of PKS 2155-304 observed with BeppoSAX during an Active Gamma-ray Phase
We present the results of BeppoSAX observations of PKS 2155-304 during an
intense gamma-ray flare. The source was in a high X-ray state. A temporal
analysis of the data reveals a tendency of the amplitude of variations to
increase with energy, and the presence of a soft lag with a timescale of the
order 10^3 s. A curved continuum spectrum, with no evidence of spectral
features, extends up to ~50 keV, while there is indication of a flatter
component emerging at higher energies, consistent with the interpretation of
the broad band spectral energy distribution (SED) as due to synchrotron
self-Compton (SSC) emission from a single region. Notably, the fitting of the
SED with such a model is consistent with an interpretation of the detected soft
lag as due to radiative cooling, supporting the idea that radiation losses play
an important role in variability. The observed shifts of the SED peaks between
the lowest and highest flux levels can be accounted for by an increase of the
break energy in the relativistic particle spectrum. The model predicts emission
at TeV energies in good agreement with the recently reported detection.Comment: 36 pages (8 figures), Latex with AAS macros, etc), accepted for
publication on Astrophysical Journa
Searches for very high energy gamma rays from blazars with CANGAROO-III telescope in 2005-2009
We have searched for very high energy (VHE) gamma rays from four blazars
using the CANGAROO-III imaging atmospheric Cherenkov telescope. We report the
results of the observations of H 2356-309, PKS 2155-304, PKS 0537-441, and 3C
279, performed from 2005 to 2009, applying a new analysis to suppress the
effects of the position dependence of Cherenkov images in the field of view. No
significant VHE gamma ray emission was detected from any of the four blazars.
The GeV gamma-ray spectra of these objects were obtained by analyzing Fermi/LAT
archival data. Non-simultaneous wide range (radio to VHE gamma-ray bands)
spectral energy distributions (SEDs) including CANGAROO-III upper limits, GeV
gamma-ray spectra, and archival data are discussed using a one-zone synchrotron
self-Compton (SSC) model in combination with a external Compton (EC) radiation.
The HBLs (H 2356-309 and PKS 2155-304) can be explained by a simple SSC model,
and PKS 0537-441 and 3C 279 are well modeled by a combination of SSC and EC
model. We find a consistency with the blazar sequence in terms of strength of
magnetic field and component size.Comment: 11 pages, 8 figures, Accepted for publication in Astroparticle
Physic
Nebular spectra and abundance tomography of the Type Ia supernova SN 2011fe: a normal SN Ia with a stable Fe core
A series of optical and one near-infrared nebular spectra covering the first year of the Type Ia supernova SN 2011fe are presented and modelled. The density profile that proved best for the early optical/ultraviolet spectra, â?-11feâ, was extended to lower velocities to include the regions that emit at nebular epochs. Model ?-11fe is intermediate between the fast deflagration model W7 and a low-energy delayed-detonation. Good fits to the nebular spectra are obtained if the innermost ejecta are dominated by neutron-rich, stable Fe-group species, which contribute to cooling but not to heating. The correct thermal balance can thus be reached for the strongest [Fe ii] and [Fe iii] lines to be reproduced with the observed ratio. The 56Ni mass thus obtained is ?0.47 ± 0.05?M?. The bulk of 56Ni has an outermost velocity of ?8500 km s?1. The mass of stable iron is ?0.23 ± 0.03?M?. Stable Ni has low abundance, ?10?2?M?. This is sufficient to reproduce an observed emission line near 7400 Ă
. A sub-Chandrasekhar explosion model with mass 1.02?M? and no central stable Fe does not reproduce the observed line ratios. A mock model where neutron-rich Fe-group species are located above 56Ni following recent suggestions is also shown to yield spectra that are less compatible with the observations. The densities and abundances in the inner layers obtained from the nebular analysis, combined with those of the outer layers previously obtained, are used to compute a synthetic bolometric light curve, which compares favourably with the light curve of SN 2011fe
WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase. Detection of the little and big blue bumps
The blazar 3C 454.3 underwent an unprecedented optical outburst in spring
2005. This was first followed by a mm and then by a cm radio outburst, which
peaked in February 2006. We report on follow-up observations by the WEBT to
study the multiwavelength emission in the post-outburst phase. XMM-Newton
observations on July and December 2006 added information on the X-ray and UV
fluxes. The source was in a faint state. The radio flux at the higher
frequencies showed a fast decreasing trend, which represents the tail of the
big radio outburst. It was followed by a quiescent state, common at all radio
frequencies. In contrast, moderate activity characterized the NIR and optical
light curves, with a progressive increase of the variability amplitude with
increasing wavelength. We ascribe this redder-when-brighter behaviour to the
presence of a "little blue bump" due to line emission from the broad line
region, which is clearly visible in the source SED during faint states.
Moreover, the data from the XMM-Newton OM reveal a rise of the SED in the UV,
suggesting the existence of a "big blue bump" due to thermal emission from the
accretion disc. The X-ray spectra are well fitted with a power-law model with
photoelectric absorption, possibly larger than the Galactic one. However, the
comparison with previous X-ray observations would imply that the amount of
absorbing matter is variable. Alternatively, the intrinsic X-ray spectrum
presents a curvature, which may depend on the X-ray brightness. In this case,
two scenarios are possible.Comment: 9 pages, 7 figures, accepted for publication in A&
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