A new Fermi-LAT source in the direction of ASASSN-17mt

ATel #10977; Kwan-Lok Li, Laura Chomiuk, and Jay Strader (Michigan State University)
on 16 Nov 2017; 18:12 UT
Credential Certification: K. L. Li (lilirayhk@gmail.com)

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We report a likely Fermi-LAT detection of the optical transient ASASSN-17mt, which was discovered by ASASSN (ATel #10772) and later identified as a Galactic nova (ATel #10785, #10795). Since 2017-10-24UT, a weak gamma-ray signal has been seen in the LAT daily light curve of ASASSN-17mt and the source is most significant between 2017-10-24UT and 2017-11-13UT. Assuming a simple power law spectrum, the photon flux is F(100MeV-300GeV) = (8.0 +/- 3.5) x 10^-8 ph/cm^2/s, with a photon index of 1.9 +/- 0.2 and TS = 38 (equivalent to a ~6 sigma detection). We also performed a gtfindsrc analysis to find the optimized position of the gamma-ray source, which is well consistent with ASASSN-17mt's.

A 3 ksec Swift ToO observation was taken on 2017-11-15UT and a bright X-ray source is detected at the nova position with a count rate of 0.056 +/- 0.005 cts/s. The source can be described by a simple power law with nH = 2.9 (+1.3, -1.0) x 10^22 cm^-2, photon index = 2.8 (+0.8, -0.7), and unabsorbed flux F(0.3-10keV) = 1.9 (+5.4, -1.1) x 10^-11 erg/cm^2/s. In the u-band image taken by UVOT, the nova is detected but overexposed. ASAS-SN Sky Patrol shows V = 10.2/10.5 mag on 2017-11-14/15UT, respectively.

More multi-wavelength follow-up observations are strongly encouraged.

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.

We thanks the Swift team for approving and carrying out the ToO observation. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.

We acknowledge the use of ASAS-SN Sky Patrol public all-sky light curve interface (Kochanek et al. 2017). ASAS-SN is funded in part by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University, NSF grant AST-1515927, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, and the Chinese Academy of Sciences South America Center for Astronomy (CASSACA).