Francesco di Mille

A CENSUS OF THE SUPERSOFT X-RAY SOURCES IN M31

We examined X-ray, ultraviolet, and optical archival data of 89 supersoft X-ray sources (SSS) in M31. We studied the timescales of X-ray variability and searched UV and optical counterparts. Almost a third of the SSS are known classical or recurrent novae, and at least half of the others exhibit the same temporal behavior as post-outburst novae. Non-stellar objects among SSS seem to be rare: less than 10% of the classified SSS turned out to be supernova remnants, and only one source has been identified with an active galactic nucleus in the background. Not more than 20% of the SSS that are not coincident with observed novae are persistent or recurrent X-ray sources. A few of these long-lasting sources show characteristics in common with other SSS identified as white dwarf (WD) close binaries in the Magellanic Clouds and in the Galaxy, including variability on timescales of minutes, possibly indicating the spin period of a WD. Such objects are likely to be low-mass X-ray binaries with a massive WD. A third of the non-nova SSS are in regions of recent star formation, often at the position of an O or B star, and we suggest that they may be high-mass X-ray binaries. If these sources host a massive hydrogen-burning WD, as it seems likely, they may become Type Ia supernovae (SNe Ia), constituting the star formation dependent component of the SNe Ia rate.

The 1966-1967 Outburst of V1647 Orionis and the Appearance of McNeil's Nebula

We present the results of an investigation aimed at characterizing previous eruptions of V1647 Orionis, the illuminating star of McNeils Nebula. Photographic plates and films from the Asiago Observatory (1960-1998) and Harvard Observatory (1898-1974) collections were inspected for the nebula. We have determined that, to the plate limits, the known eruption of 1966-1967 is the only one detected during the 38 yr period covered by the Asiago archive. In the Harvard collection, we did not find any additional occurrences in approximately 400 plates of the region taken over the last 100 yr. Based on the Asiago material, we here present a study of the 1966-1967 event. McNeils Nebula and V1647 Ori are clearly visible on 19 plates obtained with the Asiago 67/92 cm Schmidt telescope from 1966 October 22 to 1967 March 4. The object is not seen on plates taken in 1966 March nor in 1967 November, thus setting a minimum duration time for the eruptive event of 5 months and a maximum of 20 months. The 19 plates showing McNeils Nebula were digitized, and a morphological and photometric analysis of the object was undertaken. These data show temporal photometric variability, as well as structural differences between different color bands. We finally compare the 1966-1967 event to the recent 2003-2006 eruption and consider the nature of V1647 Ori within the framework of EX Lupi- and FU Orionis-type eruptions.

X-RAY GRATING OBSERVATIONS OF RECURRENT NOVA T PYXIDIS DURING THE 2011 OUTBURST

The recurrent nova T Pyx was observed with the X-ray gratings of Chandra and XMM-Newton, 210 and 235?days, respectively, after the discovery of the 2011 April 14 outburst. The X-ray spectra show prominent emission lines of C, N, and O, with broadening corresponding to an FWHM of ~2000-3000?km s?1, and line ratios consistent with high-density plasma in collisional ionization equilibrium. On day 210 we also measured soft X-ray continuum emission that appears to be consistent with a white dwarf (WD) atmosphere at a temperature ~420,000?K, partially obscured by anisotropic, optically thick ejecta. The X-ray continuum emission is modulated with the photometric and spectroscopic period observed in quiescence. The continuum at day 235 indicated a WD atmosphere at a consistent effective temperature of 25?days earlier, but with a lower flux. The effective temperature indicates a mass of ~1 M ?. The conclusion of partial WD obscuration is supported by the complex geometry of non-spherically symmetric ejecta confirmed in recent optical spectra obtained with the Southern African Large Telescope in November and December of 2012. These spectra exhibited prominent [O III] nebular lines with velocity structures typical of bipolar ejecta.

FORMATION OF RAMAN SCATTERING WINGS AROUND Hα, Hβ, AND PAα IN ACTIVE GALACTIC NUCLEI

Powered by a supermassive black hole with an accretion disk, the spectra of active galactic nuclei (AGNs) are characterized by prominent emission lines including Balmer lines. The unification schemes of AGNs require the existence of a thick molecular torus that may hide the broad emission line region from the view of observers near the equatorial direction. In this configuration, one may expect that the far UV radiation from the central engine can be Raman scattered by neutral hydrogen to reappear around Balmer and Paschen emission lines which can be identified with broad wings. We produce H

Blue large-amplitude pulsators as a new class of variable stars

\alpha

Raman-scattered O VI λ1032 and He II λ1025 and Bipolar Outflow in the Symbiotic Star V455 Sco

, H

Escape of Resonantly Scattered Ly

\beta

\beta

and Pa

and H

\alpha

\alpha

wings using a Monte Carlo technique to investigate their properties. The neutral scattering region is assumed to be a cylindrical torus specified by the inner and outer radii and the height. While the covering factor of the scattering region affects the overall strengths of the wings, the wing widths are primarily dependent on the neutral hydrogen column density