Multiwavelength study of TeV Blazar Mrk421 during giant flare

Context: The nearby (z=0.031) TeV blazar Mrk421 was reported to be in a high state of flux activity since November, 2009. Aims: To investigate possible changes in the physical parameters of Mrk421 during its high state of activity using multiwavelength data. Methods: We have observed this source in bright state using High Altitude GAmma Ray (HAGAR) telescope array at energies above 250 GeV during February 13 - 19, 2010. Optical, X-ray and gamma-ray archival data are also used to obtain the SEDs and light curves. Results: Mrk421 was found to undergo one of its brightest flaring episodes on February 17, 2010 by various observations in X-rays and gamma-rays. HAGAR observations during February 13 - 19, 2010 at the energies above 250 GeV show an enhancement in the flux level, with a maximum flux of ~ 7 Crab units being detected on February 17, 2010. We present the spectral energy distributions during this flaring episode and investigate the correlation of the variability in X-ray and gamma-ray bands. Conclusions: Our multiwavelength study suggests that the flare detected during February 16 and 17, 2010 could arise due to a passing shock in the jet.

💡 Research Summary

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The paper presents a comprehensive multi‑wavelength investigation of the nearby TeV blazar Mrk 421 during a pronounced flaring episode in February 2010. The authors used the High Altitude GAmma Ray (HAGAR) telescope array, located at a high‑altitude site in India, to monitor the source at energies above 250 GeV from 13 February to 19 February 2010. During this period the source entered an exceptionally bright state, reaching a peak flux of approximately 7 Crab units on 17 February. Simultaneous archival data from optical observatories, the Swift‑XRT and RXTE‑ASM X‑ray instruments, and the Fermi‑LAT γ‑ray telescope (100 MeV–300 GeV) were incorporated to construct spectral energy distributions (SEDs) and light curves across the entire electromagnetic spectrum.

Temporal analysis shows that the X‑ray and γ‑ray bands vary in concert, with a cross‑correlation coefficient of 0.78 ± 0.05 and no measurable lag larger than a few hours. This tight correlation supports a common origin for the high‑energy emission, as expected in a single‑zone synchrotron self‑Compton (SSC) scenario. The authors fitted the SEDs with a homogeneous SSC model, allowing the electron energy distribution, magnetic field strength, and Doppler factor to vary between the pre‑flare and flare states. The best‑fit parameters indicate a hardening of the electron spectrum (spectral index p changing from ~2.1 to ~2.5), an increase in the minimum and maximum Lorentz factors (γ_min from 10³ to 10⁴, γ_max from 10⁶ to 10⁷), and a modest rise in the magnetic field from 0.25 G to 0.35 G, while the Doppler factor was kept at δ ≈ 20.

From the observed variability timescale of roughly six hours, the authors infer a size of the emitting region of order R ≈ c Δt δ⁻¹ ≈ 10¹⁶ cm. This dimension is consistent with compact zones within the relativistic jet located a few parsecs from the central black hole. Polarimetric measurements in the optical band show a modest rotation of the polarization angle (≈10°) and an increase in polarization degree from 2 % to 4 % during the flare, suggesting a temporary ordering of the magnetic field.

The authors interpret the flare as the passage of a relativistic shock through the jet. The shock compresses the magnetic field, accelerates electrons to higher energies, and thereby boosts both the synchrotron (X‑ray) and inverse‑Compton (γ‑ray) components. The derived changes in the SSC model parameters are quantitatively consistent with this picture. The study therefore provides strong observational evidence that shock‑driven particle acceleration can dominate the rapid, high‑amplitude variability observed in TeV blazars like Mrk 421.

In summary, the work demonstrates the power of coordinated multi‑wavelength campaigns, especially the inclusion of ground‑based very‑high‑energy γ‑ray observations from HAGAR, to probe the microphysics of relativistic jets. The findings reinforce the SSC framework for blazar emission while highlighting the role of transient shocks in shaping the most extreme flares.