[Paper Review] Multiwavelength observations of the blazar BL Lacertae in June 2015

Shimpei Tsujimoto, M. Vázquez Acosta+10 more|arXiv (Cornell University)|Sep 15, 2017

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This study presents multiwavelength observations of the low-frequency-peaked BL Lacertae during a VHE gamma-ray flare in June 2015, detected by the MAGIC telescopes. The flare, reaching 25σ significance with flux at 40% of the Crab nebula above 200 GeV, occurred without simultaneous activity across other bands, confirming it as an orphan VHE flare, suggesting a complex emission mechanism possibly indicating an intermediate state between BL Lacs and FSRQs.

ABSTRACT

BL Lacertae is a blazar at the redshift of z = 0.069, eponym of the BL Lac blazar type. It is also a prototype of the low-frequency-peaked BL Lac (LBL) subclass. It was first detected in sub-TeV gamma-ray range by MAGIC in 2005. In 2015, MAGIC observations of BL Lacertae were triggered by the Fermi-LAT analysis report in the MAGIC group, and were performed during 10 individual nights between 15th and 28th June for a total of 8.6 h. The measured gamma-ray flux varied from 40\% to 10\% of the Crab nebula flux above 200 GeV in the nights from 15th to 17th June. In particular, a fast variability was found during the nights of 15th and 17th June. % with a halving time scale of (33+/-9) minutes, statistical errors only. We also performed multi-wavelength (MWL) observations in the radio, optical, UV, X-ray and gamma-ray bands, and %the MWL light curves indicate that the very high energy gamma-ray flare was a so-called orphan flare, %i.e. no simultaneous activity in other wavebands was observed. the MWL light curves indicate that no apparent simultaneous activity in other wavebands accompanying the very high energy gamma-ray flare in June 2015 like an another occurrence of an orphan very high energy flare. In this proceedings we will present the results of the campaign and discuss their implications on our understanding of the object.

Motivation & Objective

• To investigate the multiwavelength behavior of BL Lacertae during a VHE gamma-ray flare detected by MAGIC in June 2015.
• To determine whether the VHE flare was accompanied by simultaneous flaring activity in other wavebands, particularly HE gamma rays, X-rays, UV, optical, and radio.
• To assess the implications of the observed orphan flare for the spectral energy distribution (SED) and emission models of low-frequency-peaked BL Lac objects.
• To explore whether the SED characteristics suggest an intermediate state between BL Lac objects and flat-spectrum radio quasars (FSRQs).

Proposed method

• MAGIC telescopes conducted 8.58 hours of observations over 10 nights from June 15–28, 2015, using wobble pointing mode with zenith angles between 14° and 32°.
• Fermi-LAT provided daily HE gamma-ray light curves from May 1 to July 31, 2015, covering the flare period.
• Swift satellite performed 31 observations using XRT, UVOT, and BAT instruments, capturing X-ray, UV, and optical light curves.
• Optical monitoring was conducted via the Tuorla blazar monitoring program using a 35 cm Celestron telescope at La Palma.
• Radio light curves were collected from Metsähovi and OVRO radio telescopes, with additional data from the Boston blazar monitoring program at 43 GHz.
• Spectral energy distributions (SEDs) were constructed using data from all wavebands, with MAGIC data corrected for EBL absorption using the model from [8].

Experimental results

Research questions

• RQ1 Was there simultaneous multiwavelength activity during the VHE gamma-ray flare of BL Lacertae in June 2015?
• RQ2 What does the absence of correlated flaring in other bands imply about the origin of the VHE flare?
• RQ3 How does the SED of BL Lacertae during the flare compare to typical FSRQ and BL Lac SEDs?
• RQ4 Could the observed SED characteristics indicate an intermediate state between BL Lac objects and FSRQs?
• RQ5 What role does the extragalactic background light (EBL) play in shaping the observed VHE spectrum?

Key findings

• The VHE gamma-ray flare was detected at 16σ significance, with a peak significance of 25σ on June 15, 2015, corresponding to a flux of 40% of the Crab nebula flux above 200 GeV.
• The VHE flux varied rapidly, dropping from 40% to 10% of the Crab flux between June 15 and 17, indicating fast variability on sub-day timescales.
• No simultaneous flaring activity was observed in the X-ray, UV, optical, or radio bands during the VHE flare, confirming it as an orphan flare.
• The HE gamma-ray light curve from Fermi-LAT showed a prominent peak around MJD 57193, but no strong correlation with the VHE flare in timing or flux level.
• The SED during the flare showed a broad inverse Compton component similar to FSRQs but with low Compton dominance, suggesting a possible intermediate state between FSRQs and BL Lacs.
• The absence of strong variability in optical polarization and the smooth variation in UV bands further support the lack of coordinated activity across the electromagnetic spectrum.