Non-thermal radio emission from colliding flows in classical nova V1723 Aql

Abstract

The importance of shocks in nova explosions has been highlighted by Fermi's discovery of γ-ray producing novae. Over three years of multi-band VLA radio observations of the 2010 nova V1723 Aql show that shocks between fast and slow flows within the ejecta led to the acceleration of particles and the production of synchrotron radiation. Soon after the start of the eruption, shocks in the ejecta produced an unexpected radio flare, resulting in a multi-peaked radio light curve. The emission eventually became consistent with an expanding thermal remnant with mass 2 × 10 −4 M ⊙ and temperature 10 4 K. However, during the first two months, the ∼ >10 6 K brightness temperature at low frequencies was too high to be due to thermal emission from the small amount of X-ray producing shock-heated gas. Radio imaging showed structures with velocities of 400 km s −1 (d/6 kpc) in the plane of the sky, perpendicular to a more elongated 1500 km s −1 (d/6 kpc) flow. The morpho-kinematic structure of the ejecta from V1723 Aql appears similar to nova V959 Mon, where collisions between a slow torus and a faster flow collimated the fast flow and gave rise to γ-ray producing shocks. Optical spectroscopy and X-ray observations of V1723 Aql during the radio flare are consistent with this picture. Our observations support the idea that shocks in novae occur when a fast flow collides with a slow collimating torus. Such shocks could be responsible for hard X-ray emission, γ-ray production, and double-peaked radio light curves from some classical novae.

Shock-powered radio emission from V5589 Sagittarii (Nova Sgr 2012 #1)

“…In the speed of its radio brightening, the radio flare from V5589 Sgr was very similar to the shock-powered early flare in V1723 Aql (Weston et al 2016). …”

“…However, the very flat radio spectra -α = 0.14 ± 0.04 on day 62.3 and α = 0.16 ± 0.04 on day 81.1 -are inconsistent with optically thick thermal emission. Therefore, if the radio-flare emission was thermal, the emitting material most likely had a physical temperature significantly greater than 10 5 K. Unshocked ejecta, however, are typically observed to have T ∼ 10 4 K (Seaquist & Palimaka 1977;Hjellming et al 1979;Nelson et al 2014;Weston et al 2016). Moreover, Cunningham et al (2015) argued on theoretical grounds that photoionziation heating of ejecta by the residual burning of nuclear fuel in a shell on the surface of the WD leads to temperatures of at most a few times 10 4 K, even for photoionization by hot, luminous high-mass WDs (T. Cunningham, private communication; see also Table 2 of Cunningham et al 2015).…”

“…One concern about this proposal is that the radio emission near the peak of the radio flare did not have the spectral index of α = −0.7 typically associated with synchrotron emission (Chevalier 1982;Weiler et al 2002). Weston et al (2016) encountered this same issue when interpreting the early-time flare from the V1723 Aql. They argued that the radio spectrum near the peak of the flare in that source could have been affected by free-free absorption.…”

“…Radio monitoring of novae, especially when combined with observations at other frequencies, provides a powerful tool for understanding the evolution of nova eruptions and examining shocks within the ejecta. High radio brightness temperatures can reveal either non-thermal emission or shock-heated plasma, as in the case of V1723 Aql (Krauss et al 2011;Weston et al 2014Weston et al , 2016. Spatially resolved images of radio synchrotron emission in several novae have been used to trace the location of relativistic particles that have been accelerated in shocks (eg., RS Oph, V959 Mon; O'Brien et al 2006;Sokoloski et al 2008;Rupen et al 2008;.…”

Novae as Tevatrons: prospects for CTA and IceCube

“…Williams & Mason 2010), hard X-ray emission starting weeks to years after the outburst (e.g. Mukai et al 2008;Osborne 2015), and an early sharp maximum in the radio light curve on timescales of months, in ex-E-mail: bmetzger@phys.columbia.edu cess of that expected from freely-expanding photo-ionized ejecta (e.g., Chomiuk et al 2014;Weston et al 2015).…”