Possible Orphan Flare in the flaring gamma-ray blazar VER J0521+211

ATel #13548; Tapio Pursimo (NOT) and Roopesh Ojha (NASA/GSFC/UMBC)
on 10 Mar 2020; 20:56 UT
Credential Certification: Roopesh Ojha (Roopesh.Ojha@gmail.com)

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We report optical photometry of the TeV blazar VER J0521+211 obtained with the 2.56m Nordic Optical Telescope in La Palma, to look for any enhanced optical activity associated with a recent flare reported by the VERITAS Collaboration (ATel#13522) and in its daily averaged gamma-ray flux observed by Fermi-LAT (ATel#13528).

These NOT observations were made using the ALFOSC (Alhambra Faint Object Spectrograph and Camera) instrument at the epoch 2020-03-01T20:50. Standard IRAF data reduction (de-biasing + flat field correction with twilight flats) was carried out and differential photometry was performed. The brightness was calibrated against four Pan-STARRS PS1 field stars near the target. The std-dev of the zero point was < 0.04 mag.

We find that VER J0521+211 has the following magnitudes:
SDSS g 17.25
SDSS r 16.37
SDSS i 15.75
SDSS z 15.34

This represents a dimming of about 0.6, 0.4, and 0.1 magnitudes compared to Pan-STARRS in the g, r and i bands, respectively. It is also fainter than reported on 2020-02-25 (ATel#13523).

The source is still active in the gamma-ray band as can be seen from its Fermi-LAT lightcurve: https://fermi.gsfc.nasa.gov/FTP/glast/data/lat/catalogs/asp/current/lightcurves/VER0521+211_86400_1yr.png

Our results support the possibility of an "orphan" flare that is visible in the gamma-ray/TeV band but not in the optical.