[Paper Review] Flaring activity of Mrk 421 in 2012 and 2013: orphan flare and multiwavelength analysis

N. Fraija, J. I. Cabrera+2 more|arXiv (Cornell University)|Aug 6, 2015

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This study analyzes multiwavelength flaring activity in the blazar Mrk 421 during 2012 and 2013 using data from Fermi-LAT, Swift, and ground-based optical telescopes. It identifies a GeV γ-ray orphan flare in 2012 with no associated hard X-ray emission, while the 2013 flare shows correlated variability across GeV γ-rays, X-rays, and optical bands, supporting a one-zone synchrotron self-Compton (SSC) model despite an anti-correlation in the optical R-band peak.

ABSTRACT

The first one started in 2012 July 16 (MJD 56124) and the second one in 2013 April 9 (MJD 56391). The multiwavelength data analysis shows that the $\\gamma$-ray flare observed in 2012 was not detected in the hard-X ray bands. This result is usually interpreted as an "orphan" flare. In 2013, the analysis of the multiwavelength light curves shows that there are two very bright states detected in the optical R-band. The first one in 2013 April 9 (R =11.74 $\\pm$ 0.04) and the second one in May 12 (R =11.62 $\\pm$ 0.04). Also, high activity states were detected in the soft and hard X-rays. A discrete correlation function analysis of this last flare shows a strong correlation between the GeV $\\gamma$-rays and the optical/hard-X ray emission. These results are discussed in terms of the more adequate standard scenarios that could explain the multiwavelength variations displayed by this blazar.

Motivation & Objective

• To investigate the multiwavelength behavior of Mrk 421 during two high-activity states in 2012 and 2013.
• To determine whether the 2012 GeV γ-ray flare, lacking hard X-ray counterparts, qualifies as an 'orphan' flare.
• To assess correlations between GeV γ-rays, X-rays, and optical R-band emissions during the 2013 flare.
• To evaluate the consistency of observed variability with standard one-zone SSC and alternative hadronic models.

Proposed method

• Multiwavelength light curves were constructed using Fermi-LAT (GeV γ-rays, 200 MeV–300 GeV), Swift-XRT (soft X-rays, 0.2–10 keV), and Swift-BAT (hard X-rays, 15–50 keV).
• Optical R-band photometry was collected using a 0.84 m telescope at San Pedro Mártir Observatory, with additional AAVSO data for comparison.
• Discrete correlation function (DCF) analysis was applied to quantify time lags between GeV γ-ray light curves and X-ray/optical light curves.
• The spectral energy distribution (SED) was analyzed to assess consistency with leptonic (SSC) and hadronic (SPB) emission models.
• Flare states were identified based on flux thresholds: γ-ray flux > 1.0 × 10⁻⁶ ph cm⁻² s⁻¹ and R-band magnitude < 12.0.
• Theoretical models, including one-zone SSC and proton-photon interaction scenarios, were evaluated against observational data.

Experimental results

Research questions

• RQ1 Was the 2012 GeV γ-ray flare in Mrk 421 truly orphaned, with no detectable hard X-ray emission?
• RQ2 What is the nature of the multiwavelength correlation during the 2013 flare, particularly between GeV γ-rays and optical/hard X-ray emissions?
• RQ3 Does the observed anti-correlation between the optical R-band peak on May 12 and other bands challenge the standard one-zone SSC model?
• RQ4 Can the 2012 orphan flare be explained by hadronic processes such as neutral pion decay from proton-photon interactions?
• RQ5 What are the implications of the simultaneous variability across γ-ray, X-ray, and optical bands for the physical conditions in the blazar jet?

Key findings

• The 2012 GeV γ-ray flare (MJD 56124) was confirmed as an orphan flare, with no significant hard X-ray emission detected by Swift-BAT, despite strong γ-ray activity.
• The 2013 flare showed two bright optical R-band states: R = 11.74 ± 0.04 on April 9 and R = 11.62 ± 0.04 on May 12, both coinciding with high γ-ray and X-ray activity.
• Discrete correlation function (DCF) analysis revealed no significant time lag between GeV γ-rays and hard X-rays or optical R-band, indicating quasi-simultaneous variability across bands.
• The simultaneous variability across γ-ray, X-ray, and optical bands supports the one-zone synchrotron self-Compton (SSC) model for the 2013 flare.
• The optical R-band peak on May 12 was anti-correlated with γ-ray and X-ray fluxes, posing a challenge to standard SSC models and suggesting complex emission mechanisms.
• The 2012 orphan flare is consistent with hadronic models involving proton-photon interactions, as neutral pion decay can produce γ-rays without X-ray counterparts.