Jennifer Lynn Sokoloski

Shocks in nova outflows – I. Thermal emission

Growing evidence for shocks in nova outflows include (1) mult iple velocity components in the optical spectra; (2) hard X-ray emission starting wee ks to months after the outburst; (3) an early radio flare on timescales of months, in excess of t hat predicted from the freely expanding photo-ionized gas; and, perhaps most dramatically, (4)∼ GeV gamma-ray emission. We present a one dimensional model for the shock interaction between the fast nova outflow and a dense external shell (DES) and its associated th ermal X-ray, optical, and radio emission. The lower velocity DES could represent an earlier stage of mass loss from the white dwarf or ambient material not directly related to the thermonuclear runaway. The forward shock is radiative initially when the density of shocked gas is highest, at which times radio emission originates from the dense cooling layer immediately downstream of the shock. Our predicted radio light curve is characterized by sharper rises to maximum and later peak times at progressively lower frequencies, with a peak brightness temperature that is approximately independent of frequency. We apply our model to the recent gamma-ray producing classical nova V1324 Sco, obtaining an adequate fit to the ear ly radio maximum for reasonable assumptions about the fast nova outflow and assuming the DES possesses a characteristic velocity∼ 10 3 km s −1 and mass∼ few 10 −4 M⊙; the former is consistent with the velocities of narrow line absorption systems observed previously in nova spectra, while the total ejecta mass of the DES and fast outflow is consistent with that inferr ed independently by modeling the late radio peak as uniformly expanding photo-ionized gas. Rapid evolution of the early radio light curves require the DES to possess a steep outer density profile, which may indicate that the onset of mass loss from the white dwarf was rapid, providing indirect evidence that the DES was expelled as the result of the thermonuclear runaway event. Reprocessed X-rays from the shock absorbed by the DES at early times are found to contribute significantly to the optical/UV emission, which we speculate may be responsible for the previously unexplained ‘plateaus’ and secondary maxima in nova optical light curve s.

Symbiotic Stars in X-rays

Until recently, symbiotic binary systems in which a white dwarf accretes from a red giant were thought to be mainly a soft X-ray population. Here we describe the detection with the X-ray Telescope (XRT) on the Swift satellite of nine white dwarf symbiotics that were not previously known to be X-ray sources and one that had previously been detected as a supersoft X-ray source. The nine new X-ray detections were the result of a survey of 41 symbiotic stars, and they increase the number of symbiotic stars known to be X-ray sources by approximately 30%. The Swift/XRT telescope detected all of the new X-ray sources at energies greater than 2 keV. Their X-ray spectra are consistent with thermal emission and fall naturally into three distinct groups. The first group contains those sources with a single, highly absorbed hard component that we identify as probably coming from an accretion-disk boundary layer. The second group is composed of those sources with a single, soft X-ray spectral component that probably originates in a region where low-velocity shocks produce X-ray emission, i.e., a colliding-wind region. The third group consists of those sources with both hard and soft X-ray spectral components. We also find that unlike in the optical, where rapid, stochastic brightness variations from the accretion disk typically are not seen, detectable UV flickering is a common property of symbiotic stars. Supporting our physical interpretation of the two X-ray spectral components, simultaneous Swift UV photometry shows that symbiotic stars with harder X-ray emission tend to have stronger UV flickering, which is usually associated with accretion through a disk. To place these new observations in the context of previous work on X-ray emission from symbiotic stars, we modified and extended the α/β/γ classification scheme for symbiotic-star X-ray spectra that was introduced by Muerset et al. based upon observations with the ROSAT satellite, to include a new δ classification for sources with hard X-ray emission from the innermost accretion region. Because we have identified the elusive accretion component in the emission from a sample of symbiotic stars, our results have implications for the understanding of wind-fed mass transfer in wide binaries, and the accretion rate in one class of candidate progenitors of type Ia supernovae.

THE RECURRENT NOVA T Pyx: DISTANCE AND REMNANT GEOMETRY FROM LIGHT ECHOES

The recurrent nova T Pyxidis (T Pyx) is well known for its small binary separation, its unusually high luminosity in quiescence, and the spectacular Hubble Space Telescope (HST) images of its surrounding remnant. In 2011 April, T Pyx erupted for the first time since 1966. Here we describe HST observations in late 2011 of a transient reflection nebula around the erupting white dwarf. Our observations of this light echo in the pre-existing remnant show that it is dominated by a clumpy ring with a radius of about 5 �� and an inclination of 30 ◦ ‐40 ◦ , with the eastern edge tilted toward the observer. The delay times between the direct optical light from the central source and the scattering of this light from dust in several clumps with the same foreground distance as the central source give a distance to T Pyx of 4.8 ± 0.5 kpc. Given past evidence from two-dimensional optical spectra that the remnant contains a shell-like component, it must actually consist of a ring embedded within a quasi-spherical shell. The large distance of 4.8 kpc supports the contention that T Pyx has an extraordinarily high rate of mass transfer in quiescence, and thus that nova explosions themselves can enhance mass loss from a donor star, and reduce the time between eruptions in a close binary.

EXPANDED VERY LARGE ARRAY NOVA PROJECT OBSERVATIONS OF THE CLASSICAL NOVA V1723 AQUILAE

We present radio light curves and spectra of the classical nova V1723 Aql obtained with the Expanded Very Large Array (EVLA). This is the first paper to showcase results from the EVLA Nova Project, which comprises a team of observers and theorists utilizing the greatly enhanced sensitivity and frequency coverage of EVLA radio observations, along with observations at other wavelengths, to reach a deeper understanding of the energetics, morphology, and temporal characteristics of nova explosions. Our observations of V1723 Aql span 1-37 GHz in frequency, and we report on data from 14 to 175 days following the time of the nova explosion. The broad frequency coverage and frequent monitoring show that the radio behavior of V1723 Aql does not follow the classic Hubble-flow model of homologous spherically expanding thermal ejecta. The spectra are always at least partially optically thin, and the flux rises on faster timescales than can be reproduced with linear expansion. Therefore, any description of the underlying physical processes must go beyond this simple picture. The unusual spectral properties and light curve evolution might be explained by multiple emitting regions or shocked material. Indeed, X-ray observations from Swift reveal that shocks are likely present.