VLA radio detection of the recurrent nova U Sco following its 2022 eruption
ATel #15502; Kirill Sokolovsky, Elias Aydi, Laura Chomiuk, Adam Kawash, Jay Strader (MSU), Jennifer Sokoloski (Columbia), Justin Linford (NRAO), Koji Mukai (NASA/GSFC), Kwan-Lok Li (NCKU), Tim O'Brien (JBCA/Manchester), Michael Rupen (NRC)
on 6 Jul 2022; 07:38 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)
The 2022 eruption of the recurrent nova U Sco was discovered on
t0= 2022-06-06.720 UT by Masayuki Moriyama (vsnet-alert 26798).
It has been extensively followed photometrically, spectroscopically
(ATel #15417, #15420, #15422, #15423, #15451, #15499) and in X-rays
(ATel #15436, #15438, #15465). There was virtually no Fermi/LAT
exposure at the U Sco position around the time of the eruption.
The low-frequency radio observation of U Sco with uGMRT
on 2022-06-13.85 (t0 + 7.13d) resulted in the 3-sigma upper limit
on the nova flux density at 1.4 GHz of 0.15 mJy (ATel #15449).
We observed U Sco with the Karl G. Jansky Very Large Array (VLA) on
2022-06-25.22 (t0 + 18.5d) in a wide range of frequencies between
2.6 and 34.9 GHz. The VLA was in its extended A configuration
resulting in the naturally weighted beam sizes from 0.78"x1.45" at
PA=23.1 deg. to 0.06"x0.10" at PA=14.4 deg. at 2.6 and 34.9 GHz,
respectively. U Sco was clearly detected at 7.0 GHz and above with
the following flux densities and 1-sigma uncertainties (flux
densities rose with increasing frequency, with a spectral index
of +1.1):
f(GHz) S(mJy) err(mJy)
2.6 ..... 0.036
3.4 ..... 0.022
5.1 ..... 0.023
7.0 0.061 0.019
13.7 0.211 0.027
16.5 0.216 0.032
31.1 0.424 0.081
34.9 0.452 0.099
We relied on the phase transfer from the calibrator -
no self-calibration was applied before measuring flux density due
to the relatively low signal-to-noise on target.
We used the 34.9 GHz data to measure the position of U Sco with
respect to the nearby phase calibrator J1619-1817. The calibrator
VLBI position was taken from Leonid Petrov's Radio Fundamental
Catalogue version rfc_2021d. The measured position of the U Sco
radio emission peak is
16:22:30.778 -17:52:43.35 +/-0.02" J2000 epoch 2022.48
It is consistent with the position derived from the simultaneously
obtained lower-frequency VLA data and with the Gaia position
corrected for the proper motion. The reported VLA position
uncertainty is that of fitting a point source model to the uv-data
in difmap. We neglect contribution of the phase transfer
uncertainty to the VLA astrometry of U Sco as the phase calibrator
is just 0.87 deg. away from the target.
VLA spectrum of U Sco obtained on 2022-06-25.22 UT
