VLA observations of the 2021 eruption of RS Oph
ATel #14886; Kirill Sokolovsky, Elias Aydi, Laura Chomiuk, Adam Kawash, Jay Strader (MSU), Aliya-Nur Babul, Jennifer Sokoloski (Columbia), Amy Mioduszewski, Justin Linford (NRAO), Koji Mukai (NASA/GSFC), Kwan-Lok Li (NCKU), Tim O'Brien (JBCA/Manchester), Michael Rupen (NRC)
on 31 Aug 2021; 04:55 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)
The 2021 eruption of the recurrent nova RS Oph was discovered on
2021-08-08 by Keith Geary, Alexandre Amorim and Eddy Muyllaert
(AAVSO Alert Notice 752). The pre-discovery observations reported
to the AAVSO by Bin Wang indicate the outburst started shortly
before 2021-08-08 13:47 UT (t0). The eruption has been seen across
the electromagnetic spectrum from radio (ATel #14849) to X-rays
(ATel #14846, #14848, #14850, #14855, #14864, #14872, #14882) and
gamma-rays of GeV (ATel #14834, #14845) and, for the first time,
TeV energies (ATel #14844, #14857). It continues to be intensively
followed spectroscopically in the optical and infrared bands
(ATel #14838, #14840, #14852, #14858, #14860, #14863, #14866,
#14868, #14881, #14883). The search for neutrino emission with
IceCube was negative (ATel #14851).
We used the Karl G. Jansky Very Large Array (VLA) to obtain radio
spectra of RS Oph in the 2.6 - 35 GHz frequency range. The first
observation on 2021-08-13 (t0 + 4.5 d) revealed an inverted
spectrum (positively defined spectral index a = 0.8 +/-0.1) with
flux densities rising from 4.7 mJy at 2.6 GHz to 38 mJy at 35 GHz.
The radio emission became brighter and softer in the following
days reaching 92 mJy at 2.6 GHz, 48 mJy at 35 GHz (a = -0.3 +/-0.1)
by 2021-08-25 (t0 + 16.5 d). The uncertainties are dominated by
the VLA flux density scale calibration accuracy expected to be
~15% at 35 GHz and 5% at lower frequencies. We used 3C286 as
the primary flux density calibrator. Phase-only self-calibration
was applied before measuring the source flux density by fitting
a point source model to the uv-data. The VLA was in its fairly
compact C configuration with the maximum baseline of 3.4 km.
Assuming the Gaia distance of 2.7kpc and the expansion velocity of
3000 km/s (ATel #14881), the observed flux densities correspond to
the brightness temperatures of 10^8 K at 2.6 GHz and 10^6 K at
35 GHz, suggesting that the observed radio emission is largely
nonthermal. The external free-free absorption or the synchrotron
self-absorption within the radio emitting region resulted in
the inverted spectrum shape observed early in the eruption.
A nearly flat spectrum (a = -0.3) together with deviations from
a simple power law fit are observed at later times.
This indicates that while the absorption has mostly cleared,
the emitting region is inhomogeneous or remains partly hidden
behind an inhomogeneous absorbing screen.
The observations reported here were obtained with the National
Radio Astronomy Observatory's Karl G. Jansky Very Large Array.
The National Radio Astronomy Observatory is a facility of
the National Science Foundation operated under cooperative
agreement by Associated Universities, Inc.
The VLA lightcurve and spectra of RS Oph
