Fermi-LAT detection of a Galactic plane transient consistent with the unassociated gamma-ray source 4FGL J0133.0+5931

ATel #14299; R. Angioni (SSDC/INFN), S. Buson (Univ. Wuerzburg) and C. C. Cheung (NRL) on behalf of the Fermi Large Area Telescope Collaboration
on 2 Jan 2021; 20:41 UT
Credential Certification: Roberto Angioni (r.angioni90@gmail.com)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed transient gamma-ray activity from a source positionally consistent with the unassociated gamma-ray source 4FGL J0133.0+5931, from the second data release of the Fourth Fermi-LAT source catalog (4FGL, The Fermi-LAT collaboration 2020, ApJS, 247, 33; see also 4FGL-DR2, Ballet et al. 2020, arXiv:2005.11208).

Preliminary analysis indicates that this source was in an elevated gamma-ray state on 1 January 2021, with a daily averaged gamma-ray flux (E>100MeV) of (9+/-2) X 10^-7 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 230 relative to the average flux reported in the 4FGL-DR2. This is the highest LAT daily flux ever observed for this source. The corresponding photon index is 2.3+/-0.2, and is identical to the 4FGL-DR2 value. The largest gamma-ray flux was reached in the six-hour interval 12:00 - 18:00 UTC during the same day, with a six-hour-averaged gamma-ray flux of (1.9+/-0.5) X 10^-6 photons cm^-2 s^-1.

The daily-transient detection has best-fit coordinates R.A. = 23.02 deg, Decl. = 59.39 deg (J2000), and a 95% uncertainty radius of 0.20 deg. The best-fit position of 4FGL J0133.0+5931, as reported in the 4FGL-DR2 catalog, corresponds to an ellipse with coordinates R.A. = 23.26 deg, Decl. = 59.53 deg, 95% confidence semi-major and semi-minor axes of 2.9 arcmin and 2.8 arcmin, respectively, and a position angle of 66 deg. The daily-transient localization is larger than the latter and includes the full extent of the 4FGL-DR2 95% localization. A likely counterpart of 4FGL J0133.0+5931 is the radio and X-ray source NVSS J013255+593217, also known as FRC J0132+5932, with coordinates R.A. = 23.230292 deg, Decl. = 59.537778 deg (J2000; Skrutskie et al. 2006, AJ, 131, 1163), and no measured redshift. Within the daily-transient localization there are several other radio objects, including e.g., the flat-spectrum source NVSS J013242+592834, also known as GB6 J0132+5928, with coordinates R.A. = 23.179042 deg, Decl. = 59.476194 (J2000; Condon et al. 1998, AJ, 115, 1693) and no measured redshift. The binary star TYC 3683-985-1 (LS I +59 79) pointed out by Marti et al. (2017, A&A, 598, 81) as being within the 95% localization of the corresponding 3FGL-catalog source (3FGL J0133.3+5930, Acero et al. 2015, ApJS, 218, 23) is now outside of the 4FGL-DR2 uncertainty region.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A Target of Opportunity with the Neil Gherels Swift Observatory has been submitted. We encourage further multifrequency observations of the region, which may help distinguish between possible counterparts. For this source, the Fermi-LAT contact person is Roberto Angioni (roberto.angioni@ssdc.asi.it).

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.