T. J. O’Brien

Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind

RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts have indicated that high-velocity ejecta interact with a preexisting red giant wind, setting up shock systems analogous to those seen in supernova remnants. However, in the previous outburst in 1985, X-ray observations did not commence until 55 days after the initial explosion. Here we report on Swift observations covering the first month of the 2006 outburst with the Burst Alert Telescope (BAT) and X-Ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25 keV band from t = 0 to t ~ 6 days. XRT observations from 0.3 to 10 keV started 3.17 days after outburst. The rapidly evolving XRT spectra clearly show the presence of both line and continuum emission, which can be fitted by thermal emission from hot gas whose characteristic temperature, overlying absorbing column (NH)W, and resulting unabsorbed total flux decline monotonically after the first few days. Derived shock velocities are in good agreement with those found from observations at other wavelengths. Similarly, (NH)W is in accord with that expected from the red giant wind ahead of the forward shock. We confirm the basic models of the 1985 outburst and conclude that standard phase I remnant evolution terminated by t ~ 6 days and the remnant then rapidly evolved to display behavior characteristic of phase III. Around t = 26 days, however, a new, luminous, and highly variable soft X-ray source began to appear, whose origin will be explored in a subsequent paper.

Swift Observations of the 2006 Outburst of the Recurrent Nova RS Ophiuchi. II. One-dimensional Hydrodynamical Models of Wind-driven Shocks

Following the early Swift X-ray observations of the latest outburst of the recurrent nova RS Ophiuchi in 2006 February (Paper I), we present new one-dimensional hydrodynamical models of the system that take into account all three phases of the remnant evolution. The models suggest a novel way of modeling the system by treating the outburst as a sudden increase followed by a decrease in wind mass-loss rate and velocity. The differences between this wind model and previous Primakoff-type simulations are described. A more complex structure, even in one-dimension, is revealed through the presence of both forward and reverse shocks, with a separating contact discontinuity. The effects of radiative cooling are investigated, and key outburst parameters such as mass-loss rate and ejecta velocity and mass are varied. The shock velocities as a function of time are compared to the ones derived in Paper I. We show how the manner in which the matter is ejected controls the evolution of the shock and that for a well-cooled remnant, the shock deceleration rate depends on the amount of energy that is radiated away.

THE DISTANCE TO NOVA V959 MON FROM VLA IMAGING

Determining reliable distances to classical novae is a challenging but crucial step in deriving their ejected masses and explosion energetics. Here we combine radio expansion measurements from the Karl G. Jansky Very Large Array with velocities derived from optical spectra to estimate an expansion parallax for nova V959 Mon, the first nova discovered through its gamma-ray emission. We spatially resolve the nova at frequencies of 4.5-36.5 GHz in nine different imaging epochs. The first five epochs cover the expansion of the ejecta from 2012 October to 2013 January, while the final four epochs span 2014 February to 2014 May. These observations correspond to days 126 through 199 and days 615 through 703 after the first detection of the nova. The images clearly show a non-spherical ejecta geometry. Utilizing ejecta velocities derived from 3D modelling of optical spectroscopy, the radio expansion implies a distance between 0.9 +/- 0.2 and 2.2 +/- 0.4 kpc, with a most probable distance of 1.4 +/- 0.4 kpc. This distance implies a gamma-ray luminosity much less than the prototype gamma-ray-detected nova, V407 Cyg, possibly due to the lack of a red giant companion in the V959 Mon system. V959 Mon also has a much lower gamma-ray luminosity than other classical novae detected in gamma-rays to date, indicating a range of at least a factor of 10 in the gamma-ray luminosities for these explosions.

The 2010 outburst and pre-outburst optical spectrum of the recurrent nova U Scorpii

Aims. We study the evolution of the recurrent nova U Scorpii during its outburst in 2010. Methods. Optical spectroscopic observations of the nova were obtained during 0.83–162.5 days after outburst maximum. Optical linear polarisation observations were made before the onset of the super soft X-ray source (SSS) phase and during the SSS phase. Radio continuum observations were made in the 1280 MHz band during the early decline phase, and in the 610 MHz band at the onset and end of the SSS phase. We also present optical spectra obtained in the pre-outburst quiescence phase. Results. The overall spectral evolution during the 2010 outburst is similar to the previous outbursts. However, the dense temporal coverage, especially during the early phases, reveals several short term variations not reported previously. The early phase emission line widths indicate extremely high velocities for the nova ejecta, ∼10 000 km s −1 . The line profiles are broad, boxy and structured. Narrow P-Cygni absorptions, at ∼500 km s −1 are seen associated with the He I lines, and also the Ca II, N I and hydrogen lines (at wavelengths >8000 A) in the spectrum of day 6.83, obtained immediately after an optical flare. This absorption component could be arising in the plasma emitting the Nitrogen lines seen in X-rays, that probably lies outside the binary orbit. Rapid variations are detected in the strength of the 4660 A N III feature, which shows a significant increase in the line strength on days 8.83 and 13.82, at orbital phases of 0.75 and 0.25, respectively. Instrinsic polarisation is detected just before the onset, and during the SSS phase, with pv ∼ 1.4% during the SSS phase. The flux variations of the N III feature and the intrinsic polarisation are most likely associated with the reforming accretion disc/stream. We estimate the mass of the ejected (hydrogen) shell to be ∼4.6 × 10 −6 M� , for a spherical shell geometry. The non-detection of U Sco in the 1280 MHz and 610 MHz radio bands is consistent with the low mass of the nova ejecta, subgiant nature of the secondary, and the distance to the nova.

Radio Emission from V723 Cas

V723 Cas (Nova Cassiopeiae 1995) is an unusually slow nova discovered on 1995 August 24. MERLIN has been used to follow the evolution of the radio emission from this nova providing ten epochs of data and nine radio maps between 1996 and 2001. The radio light curve shows an initial optically thick rise followed by a turn‐over into optically thin emission and a corresponding decline in flux as the shell expands. The shell is resolved in the radio by 1998 March and subsequently breaks up into several compact components. Later maps display an unusual effect involving a 90 degree change in the preferential emission axis between successive epochs. A simple radiative transfer model is applied to the light curve in order to determine physical parameters for the shell. These results are presented together with a discussion of the radio maps in terms of an expanding non‐uniform shell with varying optical depth.

Outreach from the Jodrell Bank Observatory

The authors describe the various outreach activities at Jodrell Bank Observatory: Visitor Centre (new one being designed), web site, distance learning, school programmes, work with astronomical societies and with other community groups, and finally, interactions with the national and local media.

Novae In External Galaxies From The POINT‐AGAPE Survey And The Liverpool Telescope

We have recently begun a search for Classical Novae in M31 using three years of multicolour data taken by the POINT‐AGAPE microlensing collaboration with the 2.5m Isaac Newton Telescope (INT) on La Palma. This is a pilot program leading to the use of the Liverpool Telescope (LT) to systematically search for and follow novae of all speed classes in external galaxies to distances up to around 5Mpc.

Orientation Effects on Bent Extragalactic Jets

We have investigated how varying several parameters affects the results of a collision between an extragalactic jet and a dense, intergalactic cloud, through a series of hydrodynamic simulations. We have produced synthetic radio images for comparison with observations. These show that a variety of structures may be produced from simple jet-cloud collisions. Moderate Mach numbers and density contrasts are needed to produce observable bends. We investigate the effect of viewing from various angles on the appearance of such sources.