Further Fermi-LAT Observations of the Recurrent Nova RS Oph -- Gamma-ray brightening and observed onset

ATel #14845; C. C. Cheung (NRL), T. J. Johnson (GMU, resident at NRL), I. Mereu (INFN Perugia), S. Ciprini (ASI Space Science Data Center, Roma, & INFN Roma Tor Vergata), P. Jean (IRAP, Toulouse), on behalf of the Fermi Large Area Telescope Collaboration
on 10 Aug 2021; 20:05 UT
Credential Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil)

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In ATel #14834, we reported the initial Fermi Large Area Telescope (LAT) gamma-ray detection of RS Oph with a (E >100 MeV) flux of (2.4 +/- 0.8) x 10^-6 photons cm^-2 s^-1 (photon index of 2.3 +/- 0.3) using data in the time interval from 2021 Aug 8, 18:00-24:00 UT (statistical uncertainties only). Preliminary analysis of data from 2021 Aug 9, 00:00 to Aug 10, 01:44 UT in ~6-hr bins shows a steady brightening of the source with peak E > 100 MeV fluxes of (5.5 +/- 0.8) x 10^-6 photons cm^-2 s^-1 and (5.7 +/- 0.8) x 10^-6 photons cm^-2 s^-1 in the last two time bins (Aug 9, 12:00-18:00 and Aug 9, 18:00 - Aug 10, 01:44, respectively) with a hard spectrum (single power-law photon index of 1.9 +/- 0.1 in both bins) and a detection of a high-energy photon with energy E = 12.9 GeV on Aug 9, 21:51:19.9. The VHE detection reported by H.E.S.S. (data from Aug 9, 18:17-22:41 UT; ATel #14844) lies within our last time bin and covers the time of the 12.9 GeV photon detection. The thus-far LAT-observed peak fluxes in RS Oph are 4x greater than the observed 1-day peak in V407 Cyg (Ackermann et al. 2014, Sci, 345, 554; ATel #2487).

Revisiting the analysis of the earlier Aug 8 data reported in ATel #14834 on shorter timescales indicates that the first significant gamma-ray detection (5.6-sigma signficance) lies in the interval spanning Aug 8, 20:00-21:00 (Aug 8.83-8.88) with a (E >100 MeV) flux of (2.1 +/- 0.7) x 10^-6 photons cm^-2 s^-1 (photon index fixed to 2.2). In the preceding time interval with LAT exposure at the nova position (due to Fermi's all-sky observing profile), Aug 8, 16:30-18:00 (Aug 8.69-8.75), the source was not significantly detected (1.6-sigma significance) with a 95% confidence flux upper limit of 1.1 x 10^-6 photons cm^-2 s^-1 (photon index fixed to 2.2). This time range of the first gamma-ray detection is slightly earlier than the epochs of the independent optical nova detections at ~5.0 mag reported in AAVSO Alert Notice 752 (see also, CBAT #5013) on Aug 08.91319 (A. Amorim), Aug 08.92014 (E. Muyllaert), and Aug 08.93056 (K. Geary).

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source, we encourage multi-wavelength observations. For this source the Fermi LAT contact is C.C. Cheung (Teddy.Cheung@nrl.navy.mil).

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.