258,543 research outputs found
Is the Universe More Transparent to Gamma Rays Than Previously Thought?
The MAGIC collaboration has recently reported the detection of the strong
gamma-ray blazar 3C279 during a 1-2 day flare. They have used their spectral
observations to draw conclusions regarding upper limits on the opacity of the
Universe to high energy gamma-rays and, by implication, upper limits on the
extragalactic mid-infrared background radiation. In this paper we examine the
effect of gamma-ray absorption by the extragalactic infrared radiation on
intrinsic spectra for this blazar and compare our results with the
observational data on 3C279. We find agreement with our previous results,
contrary to the recent assertion of the MAGIC group that the Universe is more
transparent to \gray s than our calculations indicate. Our analysis indicates
that in the energy range between ~80 and ~500 GeV, 3C279 has a best-fit
intrinsic spectrum with a spectral index ~1.78 using our fast evolution model
and ~2.19 using our baseline model. However, we also find that spectral indices
in the range of 0.0 to 3.0 are almost as equally acceptable as the best fit
spectral indices. Assuming the same intrinsic spectral index for this flare as
for the 1991 flare from 3C279 observed by EGRET, viz., 2.02, which lies between
our best fit indices, we estimate that the MAGIC flare was ~3 times brighter
than the EGRET flare observed 15 years earlier.Comment: version accepted for publication in ApJ Letter
X-ray Spectroscopy of QSOs with Broad Ultraviolet Absorption Lines
For the population of QSOs with broad ultraviolet absorption lines, we are
just beginning to accumulate X-ray observations with enough counts for spectral
analysis at CCD resolution. From a sample of eight QSOs [including four Broad
Absorption Line (BAL) QSOs and three mini-BAL QSOs] with ASCA or Chandra
spectra with more than 200 counts, general patterns are emerging. Their
power-law X-ray continua are typical of normal QSOs with Gamma~2.0, and the
signatures of a significant column density [N_H~(0.1-4)x10^{23} cm^{-2}] of
intrinsic, absorbing gas are clear. Correcting the X-ray spectra for intrinsic
absorption recovers a normal ultraviolet-to-X-ray flux ratio, indicating that
the spectral energy distributions of this population are not inherently
anomalous. In addition, a large fraction of our sample shows significant
evidence for complexity in the absorption. The subset of BAL QSOs with broad
MgII absorption apparently suffers from Compton-thick absorption completely
obscuring the direct continuum in the 2-10 keV X-ray band, complicating any
measurement of their intrinsic X-ray spectral shapes.Comment: 9 pages, 6 figures, uses AASTeX. Accepted to the Astrophysical
Journa
The X-ray variability of the Seyfert~1 galaxy MCG-6-30-15 from long ASCA and RXTE observations
We present an analysis of the long RXTE observation of the Seyfert~1 galaxy
MCG-6-30-15, taken in July 1997. Our results show that the behaviour is
complicated. We find clear evidence from colour ratios and direct spectral
fitting that changes to the intrinsic photon index are taking place. Spectral
hardening is evident during periods of diminished intensity; in particular, a
general trend for harder spectra is seen in the period following the hardest
RXTE flare. Flux-correlated studies further show that the 3-10 keV photon index
steepens while that in the 10-20 keV band, flattens with flux. The largest
changes come from the spectral index below 10keV; however, changes in the
intrinsic power law slope, and reflection both contribute in varying degrees to
the overall spectral variability. We find that the iron line flux is consistent
with being constant over large time intervals on the order of days (although
the ASCA and RXTE spectra show that changes on shorter time
intervals of order < 10ks), and equivalent width which anticorrelates with the
continuum flux, and reflection fraction. Flux-correlated studies point at
possible ionization signatures, while detailed spectral analysis of short time
intervals surrounding flare events hint tentatively at observed spectral
responses to the flare. We present a simple model for partial ionization where
the bulk of the variability comes from within 6r_g. Temporal analysis further
provides evidence for possible time (< 1000s) and phase (phi~0.6 rad) lags.
Finally, we report an apparent break in the power density spectrum (~ 4-5 x
10^{-6}Hz) and a possible 33 hr period. Estimates for the mass of the black
hole in MCG-6-30-15 are discussed in the context of spectral and temporal
findings.Comment: 19 pages, 38 figures total (19 figure captions), accepted for
publication in MNRAS July 200
Spectral Variations in Early-Type Galaxies as a Function of Mass
We report on the strengths of three spectral indicators - Mg_2, Hbeta, and
Hn/Fe - in the integrated light of a sample of 100 field and cluster E/S0
galaxies. The measured indices are sensitive to age and/or and metallicity
variations within the galaxy sample. Using linear regression analysis for data
with non-uniform errors, we determine the intrinsic scatter present among the
spectral indices of our galaxy sample as a function of internal velocity
dispersion. Our analysis indicates that there is significantly more intrinsic
scatter in the two Balmer line indices than in the Mg_2 index, indicating that
the Balmer indices provide more dynamic range in determining the age of a
stellar population than does the Mg_2 index. Furthermore, the scatter is much
larger for the low velocity dispersion galaxies, indicating that star formation
has occurred more recently in the lower mass galaxies.Comment: 4 pages, 1 figure, 1 table, to appear in the Astrophysical Journal
Letter
The X-ray spectral properties of X-ray selected AGN : ROSAT spectra of EMSS AGN
Using a sample of 63 AGNs extracted from the Extended Medium
Sensitivity Survey (EMSS), we study the X-ray spectral properties of X-ray
selected AGN in the 0.12.4 keV ROSAT band. These objects are all the EMSS
AGN detected with more than 300 net counts in ROSAT PSPC images available from
the public archive (as of May 31, 1995). A Maximum-Likelihood analysis is used
to find the mean power-law spectral index and the intrinsic
dispersion $\sigma_p$. We find =1.42 with =0.44. This
value is significantly steeper (0.4) than the mean
/IPC spectral index obtained applying the ML analysis on the whole
sample of EMSS AGN. This result shows that the soft excess already noted in
optically selected AGN is present also in X-ray selected AGN. The relatively
high value obtained for the intrinsic dispersion confirms that in the soft band
AGN are characterized by a variety of spectral indices and the increase with
respect to results obtained from the analysis of Einstein data (0.16) suggests a further broadening of the spectral index
distribution as one moves to softer energies. A comparison between the mean
spectral index of Radio-quiet and Radio-loud subsamples shows that the mean
index of the RL sample is flatter than that of RQ, both in the IPC (0.3) and in the PSPC (0.4) data. This suggests
that the additional X-ray component in RL AGN dominates the X-ray emission of
RL AGN over almost two decades of energy (0.110 keV).Comment: 8 pages LaTex file; mn.sty macro (enclosed), 5 LaTex Tables, 12
Postscript figures; accepted for publication in MNRA
A short-graph Fourier transform via personalized PageRank vectors
The short-time Fourier transform (STFT) is widely used to analyze the spectra
of temporal signals that vary through time. Signals defined over graphs, due to
their intrinsic complexity, exhibit large variations in their patterns. In this
work we propose a new formulation for an STFT for signals defined over graphs.
This formulation draws on recent ideas from spectral graph theory, using
personalized PageRank vectors as its fundamental building block. Furthermore,
this work establishes and explores the connection between local spectral graph
theory and localized spectral analysis of graph signals. We accompany the
presentation with synthetic and real-world examples, showing the suitability of
the proposed approach
Chandra Observations of 3C Radio Sources with z<0.3: Nuclei, Diffuse Emission, Jets and Hotspots
We report on our Chandra Cycle 9 program to observe half of the 60
(unobserved by Chandra) 3C radio sources at z<0.3 for 8 ksec each. Here we give
the basic data: the X-ray intensity of the nuclei and any features associated
with radio structures such as hot spots and knots in jets. We have measured
fluxes in soft, medium and hard bands and are thus able to isolate sources with
significant intrinsic column density. For the stronger nuclei, we have applied
the standard spectral analysis which provides the best fit values of X-ray
spectral index and column density. We find evidence for intrinsic absorption
exceeding a column density of 10^{22} cm^{-2} for one third of our sources.Comment: 12 pages, 37 figures (the complete version of the paper with all
figures is available on line, see appendix for details), ApJ accepte
The XMM-Newton wide-field survey in the COSMOS field. IV: X-ray spectral properties of Active Galactic Nuclei
We present a detailed spectral analysis of point-like X-ray sources in the
XMM-COSMOS field. Our sample of 135 sources only includes those that have more
than 100 net counts in the 0.3-10 keV energy band and have been identified
through optical spectroscopy. The majority of the sources are well described by
a simple power-law model with either no absorption (76%) or a significant
intrinsic, absorbing column (20%).As expected, the distribution of intrinsic
absorbing column densities is markedly different between AGN with or without
broad optical emission lines. We find within our sample four Type-2 QSOs
candidates (L_X > 10^44 erg/s, N_H > 10^22 cm^-2), with a spectral energy
distribution well reproduced by a composite Seyfert-2 spectrum, that
demonstrates the strength of the wide field XMM/COSMOS survey to detect these
rare and underrepresented sources.Comment: 16 pages, ApJS COSMOS Special Issue, 2007 in press. The
full-resolution version is available at
http://www.mpe.mpg.de/XMMCosmos/PAPERS/mainieri_cosmos.ps.g
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