65 research outputs found
A Multi-wavelength View of the TeV Blazar Markarian 421: Correlated Variability, Flaring, and Spectral Evolution
We report results from a multi-wavelength monitoring campaign on Mrk 421 over
the period of 2003-2004. The source was observed simultaneously at TeV and
X-ray energies, with supporting observations frequently carried out at optical
and radio wavelengths. The large amount of simultaneous data has allowed us to
examine the variability of Mrk 421 in detail. The variabilities are generally
correlated between the X-ray and gamma-ray bands, although the correlation
appears to be fairly loose. The light curves show the presence of flares with
varying amplitudes on a wide range of timescales both at X-ray and TeV
energies. Of particular interest is the presence of TeV flares that have no
coincident counterparts at longer wavelengths, because the phenomenon seems
difficult to understand in the context of the proposed emission models for TeV
blazars. We have also found that the TeV flux reached its peak days before the
X-ray flux during a giant flare in 2004. Such a difference in the development
of the flare presents a further challenge to the emission models. Mrk 421
varied much less at optical and radio wavelengths. Surprisingly, the normalized
variability amplitude in optical seems to be comparable to that in radio,
perhaps suggesting the presence of different populations of emitting electrons
in the jet. The spectral energy distribution (SED) of Mrk 421 is seen to vary
with flux, with the two characteristic peaks moving toward higher energies at
higher fluxes. We have failed to fit the measured SEDs with a one-zone SSC
model; introducing additional zones greatly improves the fits. We have derived
constraints on the physical properties of the X-ray/gamma-ray flaring regions
from the observed variability (and SED) of the source. The implications of the
results are discussed. (Abridged)Comment: 32 pages, 12 figures, to appear in Ap
Multiwavelength Observations of the Blazar Mrk 421 in December 2002 and January 2003
We report on a multiwavelength campaign on the TeV gamma-ray blazar Markarian
(Mrk) 421 performed during December 2002 and January 2003. These target of
opportunity observations were initiated by the detection of X-ray and TeV
gamma-ray flares with the All Sky Monitor (ASM) on board the Rossi X-ray Timing
Explorer (RXTE) and the 10 m Whipple gamma-ray telescope.The campaign included
observational coverage in the radio (University of Michigan Radio Astronomy
Observatory), optical (Boltwood, La Palma KVA 0.6m, WIYN 0.9m), X-ray (RXTE
pointed telescopes), and TeV gamma-ray (Whipple and HEGRA) bands.
At TeV energies, the observations revealed several flares at intermediate
flux levels, peaking between 1 and 1.5 times the flux from the Crab Nebula.
While the time averaged spectrum can be fitted with a single power law of
photon index Gamma =2.8, we find some evidence for spectral variability.
Confirming earlier results, the campaign reveals a rather loose correlation
between the X-ray and TeV gamma-ray fluxes. In one case, a very strong X-ray
flare is not accompanied by a comparable TeV gamma-ray flare. Although the
source flux was variable in the optical and radio bands, the sparse sampling of
the optical and radio light curves does not allow us to study the correlation
properties in detail.
We present a simple analysis of the data with a synchrotron-self Compton
model, emphasizing that models with very high Doppler factors and low magnetic
fields can describe the data.Comment: Accepted for publication in the Astrophysical Journa
The correlated optical and radio variability of BL Lacertae. WEBT data analysis 1994-2005
Since 1997, BL Lacertae has undergone a phase of high optical activity, with
the occurrence of several prominent outbursts. Starting from 1999, the Whole
Earth Blazar Telescope (WEBT) consortium has organized various multifrequency
campaigns on this blazar, collecting tens of thousands of data points. One of
the main issues in the study of this huge dataset has been the search for
correlations between the optical and radio flux variations, and for possible
periodicities in the light curves. The analysis of the data assembled during
the first four campaigns (comprising also archival data to cover the period
1968-2003) revealed a fair optical-radio correlation in 1994-2003, with a delay
of the hard radio events of ~100 days. Moreover, various statistical methods
suggested the existence of a radio periodicity of ~8 years. In 2004 the WEBT
started a new campaign to extend the dataset to the most recent observing
seasons, in order to possibly confirm and better understand the previous
results. In this campaign we have collected and assembled about 11000 new
optical observations from twenty telescopes, plus near-IR and radio data at
various frequencies. Here, we perform a correlation analysis on the long-term
R-band and radio light curves. In general, we confirm the ~100-day delay of the
hard radio events with respect to the optical ones, even if longer (~200-300
days) time lags are also found in particular periods. The radio
quasi-periodicity is confirmed too, but the "period" seems to progressively
lengthen from 7.4 to 9.3 years in the last three cycles. The optical and radio
behaviour in the last forty years suggests a scenario where geometric effects
play a major role. In particular, the alternation of enhanced and suppressed
optical activity (accompanied by hard and soft radio events, respectively) canComment: 6 pages, 4 figure
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
Multiwavelength Observations of Strong Flares From the TeV-Blazar 1ES 1959+650
Following the detection of strong TeV gamma-ray flares from the BL Lac object
1ES 1959+650 with the Whipple 10 m Cherenkov telescope on May 16 and 17, 2002,
we performed intensive Target of Opportunity (ToO) radio, optical, X-ray and
TeV gamma-ray observations from May 18, 2002 to August 14, 2002. Observations
with the X-ray telescope RXTE and the Whipple and HEGRA gamma-ray telescopes
revealed several strong flares, enabling us to sensitively test the
X-ray/gamma-ray flux correlation properties. Although the X-ray and gamma-ray
fluxes seemed to be correlated in general, we found an ``orphan'' gamma-ray
flare that was not accompanied by an X-ray flare. After describing in detail
the radio (UMRAO, VLA), optical (Boltwood, Abastumani), X-ray (RXTE) and
gamma-ray (Whipple, HEGRA) light curves and Spectral Energy Distributions
(SEDs) we present initial modeling of the SED with a simple Synchrotron
Self-Compton (SSC) model. With the addition of another TeV blazar with good
broadband data, we consider the set of all TeV blazars to begin to look for a
connection of the jet properties to the properties of the central accreting
black hole thought to drive the jet. Remarkably, the temporal and spectral
X-ray and gamma-ray emission characteristics of TeV blazars are very similar,
even though the masses estimates of their central black holes differ by up to
one order of magnitude.Comment: Accepted for Publication in the ApJ on October 2nd, 200
Multifrequency variability of the blazar AO 0235+164. The WEBT campaign in 2004-2005 and long-term SED analysis
A huge multiwavelength campaign targeting the blazar AO 0235+164 was
organized by the Whole Earth Blazar Telescope (WEBT) in 2003-2005 to study the
variability properties of the source. Monitoring observations were carried out
at cm and mm wavelengths, and in the near-IR and optical bands, while three
pointings by the XMM-Newton satellite provided information on the X-ray and UV
emission. We present the data acquired during the second observing season,
2004-2005, by 27 radio-to-optical telescopes. They reveal an increased near-IR
and optical activity with respect to the previous season. Increased variability
is also found at the higher radio frequencies, down to 15 GHz, but not at the
lower ones. The radio (and optical) outburst predicted to peak around
February-March 2004 on the basis of the previously observed 5-6 yr
quasi-periodicity did not occur. The analysis of the optical light curves
reveals now a longer characteristic time scale of 8 yr, which is also present
in the radio data. The spectral energy distributions corresponding to the
XMM-Newton observations performed during the WEBT campaign are compared with
those pertaining to previous pointings of X-ray satellites. Bright, soft X-ray
spectra can be described in terms of an extra component, which appears also
when the source is faint through a hard UV spectrum and a curvature of the
X-ray spectrum. Finally, there might be a correlation between the X-ray and
optical bright states with a long time delay of about 5 yr, which would require
a geometrical interpretation.Comment: 14 pages, 10 figures (8 included in the text and 2 PNG files), in
press for A&
La radioactivité des sels de radium
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Sur les quantités relatives de Radium et d'Uranium contenus dans quelques minéraux
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