2,539 research outputs found
Radio Luminosities and Classificatory Criteria of BL Lacertae Objects
Using the sample of radio selected BL Lacertae objects (RBLs) and X-ray
selected BL Lacertae objects (XBLs) presented by Sambruna et al. (1996), we
calculated the luminosities of radio, optical and X-ray of each source and made
the statistical analysis among the luminosities at different wave-bands,
broad-band spectral indices from radio to X-ray () and peak
frequencies (). Our results are as follows: (i) there is a positive
correlation between radio luminosity and and a
negative correlation between and . High-energy peak BL Lacs
(HBLs) and low-energy peak BL Lacs (LBLs) can be distinguished very well, the
dividing lines are probably those of (erg/sec) and
(or )0.75 for - plot and
those of (erg/sec) and for the
- plot; (ii) there is a weak positive correlation between
optical luminosity and and a negatively weak
correlation between and ; (iii) there is no correlation
between X-ray luminosity and or between and
. From our analysis, we find that synchrotron radiation is the main
X-ray radiation mechanism for HBLs while inverse Compton scattering for LBLs.Comment: 9 pages, 3 figures. Submitted to A&
The discovery of 12min X-ray pulsations from 1WGA J1958.2+3232
During a systematic search for periodic signals in a sample of ROSAT PSPC
(0.1-2.4 keV) light curves, we discovered 12min large amplitude X-ray
pulsations in 1WGA J1958.2+3232, an X-ray source which lies close to the
galactic plane. The energy spectrum is well fit by a power law with a photon
index of 0.8, corresponding to an X-ray flux of about 10E-12 ergs cmE-2 sE-1.
The source is probably a long period, low luminosity X-ray pulsar, similar to X
Per, or an intermediate polar.Comment: 5 pages (figures included). Accepted for publication on MNRA
BeppoSAX Observations of 1-Jy BL Lacertae Objects - II
We present new BeppoSAX LECS and MECS observations, covering the energy range
0.1 - 10 keV (observer's frame), of four BL Lacertae objects selected from the
1 Jy sample. All sources display a flat (alpha_x ~ 0.7) X-ray spectrum, which
we interpret as inverse Compton emission. One object shows evidence for a
low-energy steepening (Delta alpha_x ~ 0.9) which is likely due to the
synchrotron component merging into the inverse Compton one around ~ 2 keV. A
variable synchrotron tail would explain why the ROSAT spectra of our sources
are typically steeper than the BeppoSAX ones (Delta alpha_x} ~ 0.7). The
broad-band spectral energy distributions fully confirm this picture and model
fits using a synchrotron inverse Compton model allow us to derive the physical
parameters (intrinsic power, magnetic field, etc.) of our sources. By combining
the results of this paper with those previously obtained on other sources we
present a detailed study of the BeppoSAX properties of a well-defined
sub-sample of 14 X-ray bright (f_x (0.1 - 10 keV) > 3 x 10^{-12} erg/cm^2/s)
1-Jy BL Lacs. We find a very tight proportionality between nearly simultaneous
radio and X-ray powers for the 1-Jy sources in which the X-ray band is
dominated by inverse Compton emission, which points to a strong link between
X-ray and radio emission components in these objects.Comment: 13 pages, 6 figures. Accepted for publication in MNRAS. Postscript
file also available at http://www.stsci.edu/~padovani/xrayspectra_papers.htm
The 0.1-200 keV spectrum of the blazar PKS 2005-489 during an active state
The bright BL Lac object PKS 2005-489 was observed by BeppoSAX on November
1-2, 1998, following an active X-ray state detected by RossiXTE. The source,
detected between 0.1 and 200 keV, was in a very high state with a continuum
well fitted by a steepening spectrum due to synchrotron emission only. Our
X-ray spectrum is the flattest ever observed for this source. The different
X-ray spectral slopes and fluxes, as measured by various satellites, are
consistent with relatively little changes of the peak frequency of the
synchrotron emission, always located below 10^{17} Hz. We discuss these results
in the framework of synchrotron self-Compton models. We found that for the
BeppoSAX observation, the synchrotron peak frequency is between 10^{15} and
2.5x10^{16} Hz, depending on the model assumptions.Comment: 7 pages, 4 figures, accepted for publication in A&
Low power BL Lacertae objects and the blazar sequence: Clues on the particle acceleration process
The spectral properties of blazars seem to follow a phenomenological sequence
according to the source luminosity. By inferring the source physical parameters
through (necessarily) modeling the blazar spectra, we have previously proposed
that the sequence arises because the particles responsible for most of the
emission suffer increasing radiative losses as the luminosity increases. Here
we extend those results by considering the widest possible range of blazar
spectral properties. We find a new important ingredient for shaping the spectra
of the lowest power objects, namely the role of a finite timescale for the
injection of relativistic particles. Only high energy particles radiatively
cool in such timescale leading to a break in the particle distribution:
particles with this break energy are those emitting most of the power, and this
gives raise to a link between blazar spectra and total energy density inside
the source, which controls the cooling timescale. The emerging picture requires
two phases for the particle acceleration: a first pre-heating phase in which
particles reach a characteristic energy as the result of balancing heating and
radiative cooling, and a more rapid acceleration phase which further accelerate
these particles to form a power law distribution. While in agreement with
standard shock theory, this scenario also agrees with the idea that the
luminosity of blazars is produced through internal shocks, which naturally lead
to shocks lasting for a finite time.Comment: 10 pages, 5 figures, accepted for publication in A&
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
New Extreme Synchrotron BL Lac Objects
We report on the BeppoSAX observations of four "extreme" BL Lacs, selected to
have high synchrotron peak frequencies. All have been detected also in the PDS
band. For 1ES 0120+340, PKS 0548-322 and H 2356-309 the spectrum is well fitted
by a convex broken power-law, thus locating the synchrotron peak around 1 - 4
keV. 1ES 1426+428 presents a flat energy spectral index (alpha_x= 0.92) up to
100 keV, thus constraining the synchrotron peak to lie near or above that
value. For their extreme properties, all sources could be strong TeV emitters.Comment: To appear in the proceedings of the conference "X-Ray Astronomy '99",
Bologna, Italy, September 1999; 4 pages, 3 figures, uses bo99.sty. Corrected
authors' lis
VHE Gamma Rays from PKS 2155-304
The close X-ray selected BL Lac PKS 2155-304 has been observed using the
University of Durham Mark 6 very high energy (VHE) gamma ray telescope during
1996 September/October/November and 1997 October/November. VHE gamma rays with
energy > 300 GeV were detected from this object with a time-averaged integral
flux of (4.2 +/- 0.7 (stat) +/- 2.0 (sys)) x 10^(-11) per cm2 per s. There is
evidence for VHE gamma ray emission during our observations in 1996 September
and 1997 October/November, with the strongest emission being detected in 1997
November, when the object was producing the largest flux ever recorded in
high-energy X-rays and was detected in > 100 MeV gamma-rays. The VHE and X-ray
fluxes show evidence of a correlation.Comment: 14 pages, 6 figures, accepted for publication in Ap.
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