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Register a new userMultiwavelength Observations of Markarian 421 in 2005 - 2006
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Since September 2005, the Whipple 10m Gamma-ray Telescope has been operated primarily as a blazar monitor. The five Northern Hemisphere blazars that have already been detected at the Whipple Observatory, Markarian 421, H1426+428, Markarian 501, 1ES 1959+650 and 1ES 2344+514, are monitored routinely each night that they are visible. We report on the Markarian 421 observations taken from November 2005 to June 2006 in the gamma-ray, X-ray, optical and radio bands. During this time, Markarian 421 was found to be variable at all wavelengths probed. Both the variability and the correlations among different energy regimes are studied in detail here. A tentative correlation, with large spread, was measured between the X-ray and gamma-ray bands, while no clear correlation was evident among the other energy bands. In addition to this, the well-sampled spectral energy distribution of Markarian 421 (1101+384) is presented for three different activity levels. The observations of the other blazar targets will be reported separately.
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The Major Atmospheric Gamma Imaging Cerenkov (MAGIC) telescope participated in three multiwavelength (MWL) campaigns, observing the blazar Markarian (Mkn) 421 during the nights of 2006 April 28, 29, and 2006 June 14. We analyzed the corresponding MAGIC very-high energy observations during 9 nights from 2006 April 22 to 30 and on 2006 June 14. We inferred light curves with sub-day resolution and night-by-night energy spectra. A strong gamma-ray signal was detected from Mkn 421 on all observation nights. The flux (E > 250 GeV) varied on night-by-night basis between (0.92+-0.11)10^-10 cm^-2 s^-1 (0.57 Crab units) and (3.21+-0.15)10^-10 cm^-2 s^-1 (2.0 Crab units) in 2006 April. There is a clear indication for intra-night variability with a doubling time of 36+-10(stat) minutes on the night of 2006 April 29, establishing once more rapid flux variability for this object. For all individual nights gamma-ray spectra could be inferred, with power-law indices ranging from 1.66 to 2.47. We did not find statistically significant correlations between the spectral index and the flux state for individual nights. During the June 2006 campaign, a flux substantially lower than the one measured by the Whipple 10-m telescope four days later was found. Using a log-parabolic power law fit we deduced for some data sets the location of the spectral peak in the very-high energy regime. Our results confirm the indications of rising peak energy with increasing flux, as expected in leptonic acceleration models.
Markarian 421 was observed for about four days with BeppoSAX and the Whipple Observatory gamma-ray telescope in April 1998. A pronounced, well-defined, flare with hour-scale variability was observed simultaneously in X-rays and very high energy gamma-rays. These data provide the first evidence that the X-ray and TeV intensities are well correlated on time-scales of hours. While the rise of the flare occurred on a similar time-scale in the two wavebands, the decay of the flare was much more rapid in gamma rays, providing the first clear indication that the X-ray and gamma-ray emission may not be completely correlated in Markarian 421.
We report on variability and correlation studies using multiwavelength observations of the blazar Mrk 421 during the month of February, 2010 when an extraordinary flare reaching a level of $sim$27~Crab Units above 1~TeV was measured in very-high-energy (VHE) $gamma$-rays with the VERITAS observatory. This is the highest flux state for Mrk 421 ever observed in VHE $gamma$-rays. Data are analyzed from a coordinated campaign across multiple instruments including VHE $gamma$-ray (VERITAS, MAGIC), high-energy (HE) $gamma$-ray (Fermi-LAT), X-ray (Swift}, RXTE, MAXI), optical (including the GASP-WEBT collaboration and polarization data) and radio (Metsahovi, OVRO, UMRAO). Light curves are produced spanning multiple days before and after the peak of the VHE flare, including over several flare `decline epochs. The main flare statistics allow 2-minute time bins to be constructed in both the VHE and optical bands enabling a cross-correlation analysis that shows evidence for an optical lag of $sim$25-55 minutes, the first time-lagged correlation between these bands reported on such short timescales. Limits on the Doppler factor ($delta gtrsim 33$) and the size of the emission region ($ delta^{-1}R_B lesssim 3.8times 10^{13},,mbox{cm}$) are obtained from the fast variability observed by VERITAS during the main flare. Analysis of 10-minute-binned VHE and X-ray data over the decline epochs shows an extraordinary range of behavior in the flux-flux relationship: from linear to quadratic to lack of correlation to anti-correlation. Taken together, these detailed observations of an unprecedented flare seen in Mrk 421 are difficult to explain by the classic single-zone synchrotron self-Compton model.
We report on TeV gamma-ray observations of the blazar Mrk 421 (redshift of 0.031) with the VERITAS observatory and the Whipple 10m Cherenkov telescope. The excellent sensitivity of VERITAS allowed us to sample the TeV gamma-ray fluxes and energy spectra with unprecedented accuracy where Mrk 421 was detected in each of the pointings. A total of 47.3 hrs of VERITAS and 96 hrs of Whipple 10m data were acquired between January 2006 and June 2008. We present the results of a study of the TeV gamma-ray energy spectra as a function of time, and for different flux levels. On May 2nd and 3rd, 2008, bright TeV gamma-ray flares were detected with fluxes reaching the level of 10 Crab. The TeV gamma-ray data were complemented with radio, optical, and X-ray observations, with flux variability found in all bands except for the radio waveband. The combination of the RXTE and Swift X-ray data reveal spectral hardening with increasing flux levels, often correlated with an increase of the source activity in TeV gamma-rays. Contemporaneous spectral energy distributions were generated for 18 nights, each of which are reasonably described by a one-zone SSC model.
Markarian 421 (Mrk 421) is a high-synchrotron-peaked blazar showing relentless variability across the electromagnetic spectrum from radio to gamma-rays. We use over 7-years of radio and GeV observations to study the correlation and connected variability in radio and GeV bands. Radio data was obtained in a 15GHz band by the OVRO 40-m radio telescope, and GeV data is from Fermi Large Area Telescope. To determine the location of the gamma-ray emission regions in Mrk 421 we correlate GeV and radio light curves. We found that GeV light curve varies independently and accurately leads the variations observed in radio. Using a fast-rise-slow-decay profile derived for shock propagation within a conical jet, we manage to reproduce the radio light curve from GeV variations. The profile rise time is comparable with the Fermi-LAT binning the decay time is about 7.6 days. The best-fit value for the response profile also features a 44 days delay between the GeV and radio, which is compatible with the wide lag range obtained from the correlation. Such a delay corresponds to $10^{17}$ cm/c, which is comparable with the apparent light crossing time of the Mrk 421 radio core. Generally, the observed variability matches the predictions of the leptonic models and suggests that the physical conditions vary in the jet. The emitting region moving downstream the jet, while the environment becomes first transparent to gamma rays and later to the radio.
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