Optical variability in gamma-ray blazar VER J0521+211

ATel #13727; Eleonora Caruso (NYU Shanghai), David M. Russell, Payaswini Saikia, Maria Cristina Baglio, D. M. Bramich (NYU Abu Dhabi)
on 11 May 2020; 10:10 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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VER J0521+211 (2FGL J0521.7+2113) was first detected as a very high energy (VHE; E>100 GeV) gamma-ray source in 2009 by VERITAS (ATel #2260). It has shown gamma-ray flares in 2009, 2013 and 2020 detected by VERITAS and Fermi LAT (ATel #2309, #5472, #13522, #13528). The source is classified as a blazar (intermediate BL Lac object) and has been associated with the radio-loud active galaxy RGB J0521.8+2112 = MG2 J052147+2112 = TXS 0518+211 (ATel #2260, #5519, #5520, #13522).

The most recent activity was detected in February 2020 as a VHE gamma-ray flare by VERITAS, and also enhanced X-ray flux from the Neil Gehrels Swift Observatory (Swift) XRT (ATel #13522). ATel #13523 reported on an optical observation with the 60cm telescope of Belogradchik Observatory (Bulgaria) at the time of the gamma-ray flare, which showed no optical brightening, and encouraged further multiwavelength monitoring. Follow-up optical data taken with 2.56m Nordic Optical Telescope on La Palma (Spain) showed that the source was still faint, and a possible orphan gamma ray flare, not visible in the optical, was proposed for the flaring source (ATel #13548). They found that VER 0521+211 had SDSS optical magnitudes on 2020 March 1 of g' = 17.25, i' = 15.75. When compared with the Pan-STARRS magnitudes, these values were fainter than the catalogue values, as well as fainter than those observed on February 26th in ATel #13523. Specifically this represented a dimming of about 0.6 and 0.1 magnitudes compared to Pan-STARRS in the g' and i' bands, respectively. The source was also fainter than reported on February 25 (ATel#13523), when the outburst began.

We report on optical monitoring of VER J0521+211 with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes, between March 21st (MJD 58929) and April 5th (MJD 58944). For the analysis of the LCO data, we are making use of a new real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019 and Pirbhoy et al. 2020, ATel #13451 for details). We measure a position for the source of RA = 05:21:45.97, Dec = +21:12:51.5 (J2000) which agrees with GAIA DR2.

The optical magnitudes in g' and i' bands are consistent with a fading, and reached their faintest values on 2020 March 24, with magnitudes of g' = 17.61+/-0.01 and i' = 15.99+/-0.02. The source faded by 1 mag in g' and 0.3 mag in i' bands between March 1st (MJD58909) and March 24th (MJD58932), before re-brightening slightly, peaking on April 3rd (MJD 58942) with g' = 17.33+/-0.02 and i' = 15.85+/-0.02. It can be noticed that the updated Fermi-LAT lightcurve of blazar VER 0521+211 shows a new high value of gamma-rays detected a few days earlier than the peak value in the optical. The small optical flare occurs near the start of the second period of gamma-ray flaring, which suggests a possible correlation between the two. It is unclear if there could have been a similar, weak optical activity associated with the first high energy flare seen by VERITAS, FERMI LAT and Swift XRT in February, due to poor optical coverage at the time.

Our light curves, also comparing to the periods of gamma-ray and X-ray flaring, are linked below (we take data from the Swift XRT lightcurve; Stroh & Falcone 2013, ApJS, 207, 28). VER 0521+211 is no longer visible from the ground for optical telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

LCO light curve of blazar VER 0521+211