Historic Flaring of BL Lacertae Measured by the Submillimeter Array

ATel #16340; Mark Gurwell (SMA/CfA), Ram Rao (SMA/CfA), SMA Team
on 16 Nov 2023; 20:02 UT
Credential Certification: Mark A. Gurwell (mgurwell@cfa.harvard.edu)

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The well-known active galactic nucleus BL Lacertae (aka BL Lac, 4FGL J2202.7+4216, etc), is a typically bright millimeter/submillimeter (mm/submm) source, and the namesake of a class of AGN characterized by strong relativistic beaming and rapid variability across the EM spectrum. BL Lac is often used for calibration in mm/submm observations, such as at the Submillimeter Array (SMA)*.

In 2023, BL Lac (as measured at the SMA) has varied between ~4 and 10 Jy at 1.3mm, roughly in line with historical variability. However, starting in mid October 2023 the source began a major mm/submm flaring event, and by late October it had exceeded all peak flare flux densities previously measured by the SMA, ~15 Jy. The flaring event has continued; as of 14 November the flux density reached 21 Jy, more than 30% higher than any previous flare by BL Lac in the 20 year light curve measured with the SMA. In the past, these mm/submm flares typically preceded flares seen in the radio. This suggests that radio observations (including VLBI observations) may soon also measure increased activity from BL Lac. Millimeter/submillimeter lightcurves for BL Lac for the periods 2023, 2022-2023, and 2002-2023 can be see at the link below.

The SMA will continue to monitor BL Lac during this historic flaring event (along with many other AGN sources). Current and historical measurements for all sources can be viewed at the Submillimeter Array Observer Center at http://sma1.sma.hawaii.edu/callist/callist.html

*The Submillimeter Array is an eight element interferometric array located near the summit of Maunakea, HI operating in the 1.3mm, 1.1mm, and 870 micron atmospheric bands. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. We recognize that Maunakea is a culturally important site for the indigenous Hawaiian people; we are privileged to study the cosmos from its summit.

See Current and Historical BL Lac Light Curves from SMA here