Classical novae in the ASKAP pilot surveys

Abstract

We present a systematic search for radio counterparts of novae using the Australian Square Kilometer Array Pathfinder (ASKAP). Our search used the Rapid ASKAP Continuum Survey, which covered the entire sky south of declination $+41^{\circ}$ ( $\sim$ $34000$ square degrees) at a central frequency of 887.5 MHz, the Variables and Slow Transients Pilot Survey, which covered $\sim$ $5000$ square degrees per epoch (887.5 MHz), and other ASKAP pilot surveys, which covered $\sim$ 200–2000 square degrees with 2–12 h integration times. We crossmatched radio sources found in these surveys over a two–year period, from 2019 April to 2021 August, with 440 previously identified optical novae, and found radio counterparts for four novae: V5668 Sgr, V1369 Cen, YZ Ret, and RR Tel. Follow-up observations with the Australian Telescope Compact Array confirm the ejecta thinning across all observed bands with spectral analysis indicative of synchrotron emission in V1369 Cen and YZ Ret. Our light-curve fit with the Hubble Flow model yields a value of $1.65\pm 0.17 \times 10^{-4} \rm \:M_\odot$ for the mass ejected in V1369 Cen. We also derive a peak surface brightness temperature of $250\pm80$ K for YZ Ret. Using Hubble Flow model simulated radio lightcurves for novae, we demonstrate that with a 5 $\sigma$ sensitivity limit of 1.5 mJy in 15-min survey observations, we can detect radio emission up to a distance of 4 kpc if ejecta mass is in the range $10^{-3}\rm \:M_\odot$ , and upto 1 kpc if ejecta mass is in the range $10^{-5}$ – $10^{-3}\rm \:M_\odot$ . Our study highlights ASKAP’s ability to contribute to future radio observations for novae within a distance of 1 kpc hosted on white dwarfs with masses $0.4$ – $1.25\:\rm M_\odot$ , and within a distance of 4 kpc hosted on white dwarfs with masses $0.4$ – $1.0\:\rm M_\odot$ .

VLBI with SKA: Possible Arrays and Astrometric Science

“…If the values of M ej are 5 × 10 −5 and 5 × 10 −6 M e , the corresponding distance becomes ∼6 and ∼3 kpc, respectively. In these cases, the corresponding mass of the white dwarf is ∼0.4-1.25 M e (Yaron et al 2005;Gulati et al 2023).…”

“…Currently, theory and observations of novae are not overly consistent, especially in terms of the ejecta masses of different subtypes of novae. Radio observations are an ideal tracer of ejecta masses, and have already made significant contributions to the study of novae, such as constraining the expanding ejecta and their morphologies, distances, shell masses, and kinetic energies (see the short review by Gulati et al (2023)). Gulati et al conducted a systematic search for novae with decl.…”

“…The maximum radio flux density of a nova is related to the ejected mass, M ej . At 1 GHz, assuming M ej ∼ 5 × 10 −4 M e , SKA-VLBI may, in theory, be able to locate sources within ∼10 kpc with a precision of better than 10% (based on the astrometry in Table 3) and the maximum flux density, which is associated with the M ej taken from Table 4 in Gulati et al 2023). 14 Note that the assumed maximum flux density is inversely proportional to the square of the distance.…”

The Rapid ASKAP Continuum Survey V: Cataloguing the sky at 1 367.5 MHz and the second data release of RACS-mid

“…Leung et al 2021) and RACS-mid (e.g. Driessen et al 2023;Gulati et al 2023). To help enable further time-domain science, we provide another auxiliary catalogue for Stokes I that is simply the concatenation of source lists from the individual RACS-mid images.…”