Optical brightening of Mrk 421 down to R= 12.1

ATel #15276; Alessandro Marchini (Astronomical Observatory, Department of Physical Sciences, Earth and Environment (DSFTA), University of Siena - Italy); Andrea Lorini, Vincenzo Millucci, Riccardo Paoletti, Leonardo Stiaccini, Stefano Truzzi (DSFTA, University of Siena - Italy); Simone Leonini, Massimo Conti, Paolo Rosi, Luz Marina Tinjaca Ramirez (Montarrenti Observatory, Sovicille (Siena - Italy)); Giacomo Bonnoli (INAF - Brera Astronomical Observatory)
on 13 Mar 2022; 15:06 UT
Credential Certification: Giacomo Bonnoli (giacomo.bonnoli@unisi.it)

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We report that the nearby (z=0.03) TeV-bright blazar Mrk 421 (RA: 11 04 27.31 Dec: +38 12 31.80 J2000.0) has been progressively brightening in the optical band in the last weeks, as indicated by the monitoring performed in the context of our joint program of blazar observations. The program is carried on with the 0.3 m telescope at the Astronomical Observatory of the University of Siena (K54) and with the 0.53 m telescope at Montarrenti Observatory (C88), both in Siena (Italy).

The following table reports our last V and R (Johnson-Cousins) measurements:

Civil Date(UT) Mag (dMag) - Observatory
2022 Mar. 04.96 V= 12.89 (0.01) - Montarrenti
2022 Mar. 05.77 V= 12.83 (0.01) - Siena Univ.
2022 Mar. 08.79 V= 12.71 (0.01) - Siena Univ.
2022 Mar. 09.78 V= 12.64 (0.01) - Siena Univ.
2022 Mar. 10.80 V= 12.60 (0.01) - Siena Univ.
2022 Mar. 12.97 V= 12.48 (0.01) - Siena Univ.

Civil Date(UT) Mag (dMag) - Observatory
2022 Mar. 04.96 R= 12.58 (0.01) - Montarrenti
2022 Mar. 05.77 R= 12.51 (0.01) - Siena Univ.
2022 Mar. 07.79 R= 12.50 (0.01) - Siena Univ.
2022 Mar. 08.79 R= 12.42 (0.01) - Siena Univ.
2022 Mar. 09.78 R= 12.34 (0.01) - Siena Univ.
2022 Mar. 10.80 R= 12.31 (0.01) - Siena Univ.
2022 Mar. 11.88 R= 12.24 (0.01) - Siena Univ.
2022 Mar. 12.97 R= 12.14 (0.01) - Siena Univ.

These measurements were obtained using the photometric sequence made available by the WEBT Collaboration (http://www.oato.inaf.it/blazars/webt/gasp/fc/1101fc.html) and sticking to their prescriptions for the signal aperture to be used in the photometry.
Regarding the last measurement at R=12.14 we stress that similar or lower magnitudes have been reached only a few times along the last 20 years according to the long term monitoring made publicly available by the Tuorla Observatory at https://users.utu.fi/kani/1m/Mkn_421.html and no such values are reported since 2015.

Any enquiry on these observations can be addressed either to Alessandro Marchini (marchini@unisi.it) or to Giacomo Bonnoli (giacomo.bonnoli@inaf.it). We will continue monitoring the source in the following nights. Multi-wavelength follow-up is encouraged.

We acknowledge excellent scientific cooperation with, and valuable support from, the WEBT Collaboration and the Tuorla Observatory.

Brief descriptions of the instrumental setups used for the reported observations are available at the official webpage of the Astronomical Observatory of the University of Siena and on the website of the Unione Astrofili Senesi.