23 research outputs found
Multifrequency Observations of the Virgo Blazars 3C 273 and 3C 279 in CGRO Cycle 8
We report first observational results of multifrequency campaigns on the
prominent Virgo blazars 3C 273 and 3C 279 which were carried out in January and
February 1999. Both blazars are detected from radio to gamma-ray energies. We
present the measured X- to gamma-ray spectra of both sources, and for 3C 279 we
compare the 1999 broad-band (radio to gamma-ray) spectrum to measured previous
ones.Comment: 5 pages including 3 figures, latex2e, to appear in: 'Proc. of the 5th
Compton Symposium', AIP, in pres
Long-term variability of radio-bright BL Lacertae objects
Radio-bright BL Lacertae objects (BLOs) are typically variable and exhibit
prominent flaring. We use a sample of 24 BLOs to get a clear idea of their
flaring behavior and to find possible commonalities in their variability
patterns. Our goal was to compare the results given by computational time
scales and the observed variability parameters determined directly from the
flux curves. Also, we wanted to find out if the BLO flares adhere to the
generalized shock model. We use long-term monitoring data from 4.8, 8, 14.5,
22, 37, 90 and 230 GHz. The structure function, discrete correlation function
and Lomb-Scargle periodogram time scales, calculated in a previous study, are
analyzed in more detail. We determine flare durations, rise and decay times,
absolute and relative peak fluxes from the monitoring data. We find that BLOs
demonstrate a wide range of variability behavior. BLOs include sources with
fast and strong variability, such as OJ 287, PKS 1749+096 and BL Lac, but also
sources with more rolling fluctuations like PKS 0735+178. The most extreme
flares can last for up to 13 years or have peak fluxes of approximately 12 Jy
in the observer's frame. When the Doppler boosting effect is taken into
account, the peak flux of a flare does not depend on the duration of the flare.
A rough analysis of the time lags and peak flux evolution indicates that BLO
flares in the mm - cm wavelengths are high-peaking, i.e., are in the adiabatic
stage. Thus, the results concur with the generalized shock model.Comment: Published in the Astronomical Journa
BeppoSAX observations of PKS 0528+134
We report on the BeppoSAX observations of the gamma-ray blazar PKS 0528+134
performed in Feb and Mar 1997, during a multiwavelength campaign involving
EGRET and ground based telescopes. The source was in a faint and hard state,
with energy spectral index alpha=0.48+-0.04 between 0.1 and 10 keV, and [2-10]
keV flux of 2.7E-12 erg/cm2/s. No significant variability was observed. The
source was detected in the 20-120 keV band by the PDS, with a flux lying
slightly above the extrapolation from lower X-ray energies. Comparing this low
state with previous higher states of the source, there is an indication that
the X-ray spectrum hardens and the gamma-ray spectrum steepens when the source
is fainter.Comment: 4 pages, LateX, 5 figures (included). Uses espcrc2.sty and epsf.sty
(included). To appear in The Active X-ray Sky: Results from BeppoSAX and
Rossi-XTE, Rome, Italy, 21-24 October, 1997. Eds.: L. Scarsi, H. Bradt, P.
Giommi and F. Fior
Multiwavelength observations of Mkn 501 during the 1997 high state
During the observation period 1997, the nearby Blazar Mkn 501 showed
extremely strong emission and high variability. We examine multiwavelength
aspects of this event using radio, optical, soft and hard X-ray and TeV data.
We concentrate on the medium-timescale variability of the broadband spectra,
averaged over weekly intervals.
We confirm the previously found correlation between soft and hard X-ray
emission and the emission at TeV energies, while the source shows only minor
variability at radio and optical wavelengths. The non-linear correlation
between hard X-ray and TeV fluxes is consistent with a simple analytic estimate
based on an SSC model in which Klein-Nishina effects are important for the
highest-energy electrons in the jet, and flux variations are caused by
variations of the electron density and/or the spectral index of the electron
injection spectrum.
The time-averaged spectra are fitted with a Synchrotron Self-Compton (SSC)
dominated leptonic jet model, using the full Klein-Nishina cross section and
following the self-consistent evolution of relativistic particles along the
jet, accounting for gamma-gamma absorption and pair production within the
source as well as due to the intergalactic infrared background radiation. The
contribution from external inverse-Compton scattering is tightly constrained by
the low maximum EGRET flux and found to be negligible at TeV energies. We find
that high levels of the X-ray and TeV fluxes can be explained by a hardening of
the energy spectra of electrons injected at the base of the jet, in remarkable
contrast to the trend found for gamma-ray flares of the flat-spectrum radio
quasar PKS 0528+134.Comment: accepted for publication in ApJ, 31 pages, 11 figure
Day-Scale Variability of 3C 279 and Searches for Correlations in Gamma-Ray, X-Ray, and Optical Bands
Light curves of 3C 279 are presented in optical (R-band), X-rays (RXTE/PCA),
and gamma rays (CGRO/EGRET) for 1999 Jan-Feb and 2000 Jan-Mar. During both of
those epochs the gamma-ray levels were high, and all three observed bands
demonstrated substantial variation, on time scales as short as one day.
Correlation analyses provided no consistent pattern, although a rather
significant optical/gamma-ray correlation was seen in 1999, with a gamma-ray
lag of ~2.5 days, and there are other suggestions of correlations in the light
curves. For comparison, correlation analysis is also presented for the
gamma-ray and X-ray light curves during the large gamma ray flare in 1996 Feb
and the two gamma-bright weeks leading up to it; the correlation at that time
was strong, with a gamma-ray/X-ray offset of no more than 1 day.Comment: 20 pages, including 7 figures; accepted by The Astrophysical Journa
Multi-Epoch Multiwavelength Spectra and Models for Blazar 3C~279
Of the blazars detected by EGRET in GeV gamma rays, 3C 279 is not only the
best-observed by EGRET, but also one of the best-monitored at lower
frequencies. We have assembled eleven spectra, from GHz radio through GeV gamma
rays, from the time intervals of EGRET observations. Although some of the data
have appeared in previous publications, most are new, including data taken
during the high states in early 1999 and early 2000. All of the spectra show
substantial gamma-ray contribution to the total luminosity of the object; in a
high state, the gamma-ray luminosity dominates over that at all other
frequencies by a factor of more than 10. There is no clear pattern of time
correlation; different bands do not always rise and fall together, even in the
optical, X-ray, and gamma-ray bands.
The spectra are modeled using a leptonic jet, with combined synchrotron
self-Compton + external Compton gamma-ray production. Spectral variability of
3C 279 is consistent with variations of the bulk Lorentz factor of the jet,
accompanied by changes in the spectral shape of the electron distribution. Our
modeling results are consistent with the UV spectrum of 3C 279 being dominated
by accretion disk radiation during times of low gamma-ray intensity.Comment: 39 pages including 13 figures; data tables not included (see ApJ web
version or contact author
The WEBT BL Lac Campaign 2000
We present UBVRI light curves of BL Lacertae from May 2000 to January 2001,
obtained by 24 telescopes in 11 countries. More than 15000 observations were
performed in that period, which was the extension of the Whole Earth Blazar
Telescope (WEBT) campaign originally planned for July-August 2000. Rapid flux
oscillations are present all the time, involving variations up to a few tenths
of mag on hour time scales, and witnessing an intense intraday activity of this
source. Colour indexes have been derived by coupling the highest precision B
and R data taken by the same instrument within 20 min and after subtracting the
host galaxy contribution from the fluxes. The 620 indexes obtained show that
the optical spectrum is weakly sensitive to the long-term trend, while it
strictly follows the short-term flux behaviour, becoming bluer when the
brightness increases. Thus, spectral changes are not related to the host galaxy
contribution, but they are an intrinsic feature of fast flares. We suggest that
the achromatic mechanism causing the long-term flux base-level modulation can
be envisaged in a variation of the relativistic Doppler beaming factor, and
that this variation is likely due to a change of the viewing angle. Discrete
correlation function (DCF) analysis reveals the existence of a characteristic
time scale of variability of about 7 h in the light curve of the core WEBT
campaign, while no measurable time delay between variations in the B and R
bands is found.Comment: 14 pages, 8 PostScript figures, 5 JPEG figures, in press for A&
Coordinated Multiwavelength Observations of BL Lacertae in 2000
BL Lacertae was the target of an extensive multiwavelength monitoring
campaign in the second half of 2000. Simultaneous or quasi-simultaneous
observations were taken at radio (UMRAO and Metsaehovi) and optical(WEBT
collaboration) frequencies, in X-rays (BeppoSAX and RXTE), and at VHE
gamma-rays (HEGRA). The WEBT optical campaign achieved an unprecedented time
coverage, virtually continuous over several 10 - 20 hour segments. It revealed
intraday variability on time scales of ~ 1.5 hours and evidence for spectral
hardening associated with increasing optical flux. During the campaign, BL
Lacertae underwent a major transition from a rather quiescent state prior to
September 2000, to a flaring state for the rest of the year. This was also
evident in the X-ray activity of the source. BeppoSAX observations on July
26/27 revealed a rather low X-ray flux and a hard spectrum, while a BeppoSAX
pointing on Oct. 31 - Nov. 2, 2000, indicated significant variability on time
scales of < a few hours, and provided evidence for the synchrotron spectrum
extending out to ~ 10 keV during that time. During the July 26/27 observation,
there is a tantalizing, though not statistically significant, indication of a
time delay of ~ 4 - 5 hr between the BeppoSAX and the R-band light curve. Also,
a low-significance detection of a time delay of 15 d between the 14.5 GHz and
the 22 GHz radio light curves is reported. Several independent methods to
estimate the co-moving magnetic field in the source are presented, suggesting a
value of ~ 2 e_B^{2/7} G, where e_B is the magnetic-field equipartition factor
w.r.t. the electron energy density in the jet.Comment: Accepted for publication in Ap
Optical observations of eight X-ray selected BL Lacertae objects
We present optical data in the Johnson's BV and Cousins' R bands for
eight X-ray selected BL Lacertae objects (BL Lacs)
that have recently been pointed by the Satellite per Astronomia X "Beppo"
(BeppoSAX). The observations were done with the telescope
of the Torino Astronomical Observatory and the observational periods include,
or are close to, the satellite pointings. These data provide optical
information on sources that have been rarely observed in the optical band.
Moreover, they can be compared to the X-ray ones for a better understanding of
the emission properties of these objects.
Variability on short time scales (a few days) was found for MS
0317.0+1834, 1ES 0347-121, and MS 0737.9+7441