The CSS 3C 138: renewed optical activity in 2026 August-September
ATel #18047; V. V. Vlasyuk, O. I. Spiridonova (Special Astrophysical Observatory of the Rus. Ac. Sci.)
on 16 Sep 2026; 00:51 UT
Credential Certification: Alexander Moskvitin (mosk@sao.ru)
Subjects: Optical, Quasar
The compact steep-spectrum (CSS) quasar 3C 138 demonstrate renewed activity in optical range
(R band) since 2026 August, being started from R=16.43 mag level at end of July (MJD
61249). As was reported in ATel #17949, this source was in an elevated gamma-ray emission
state on August 6, 2026, with a daily averaged gamma-ray flux about 2.0 x 10^-6 photons cm^-2
s^-1, which exceeded its average flux in fourth Fermi-LAT source catalog more than 500 times.
Just within 4 days its optical brightness reached R=16.06 +/-0.02 mag level according to our
estimates and faded up to R~16.9 mag at end of September with a mean slope 0.035 mag/day.
After some stabilisation of brightness between August 26 and September 2 the 3C138 started
new phase of re-brightening and reached level R=15.44 +/- 0.02 mag at September 12/13 night
(MJD 61296.0). This value is only ~0.3 mag below 3C138 absolute maximum in a course of last
December flare. The current slope of light curve is about -0.15 mag/day, which is 2 times faster
than slope of 3C138 light curve brightening in 2025 December. Last night data gave some fading
of quasar flux --- up to R=15.60.
All estimates for the 3C 138 re-brightening phase were provided with 0.5-meter SAO RAS
optical reflector. They were calibrated using our own secondary standards in the 3C 138 field.
More accurate data reduction is necessary. We strongly encourage further multi-wavelength
observations.
