XMM-Newton detects the beginning of the X-ray decline of SN 1995N

We present the results of a new XMM-Newton observation of the interacting supernova 1995N, performed on July 27, 2003. We find that the 0.2-10.0 keV flux has dropt at a level of 1.44e-13 erg cm^-2 s^-1, about one order of magnitude lower than that of a previous ASCA observation performed on January 1998. The X-ray spectral analysis shows statistically significant evidence for the presence of two distinct components, that can be modeled with emission from optically thin, thermal plasmas at different temperatures. From these temperatures we derive that the exponent of the ejecta density distribution is n ~ 6.5.Comment: 3 pages, 2 figures. To appear in proceedings of the International Conference "1604-2004: Supernovae as Cosmological Lighthouses" (Padova, Italy, June 16-19, 2004), eds. M. Turatto, W. Shea, S. Benetti and L. Zampieri, ASP conference Serie

Native Language Identification on Text and Speech

This paper presents an ensemble system combining the output of multiple SVM classifiers to native language identification (NLI). The system was submitted to the NLI Shared Task 2017 fusion track which featured students essays and spoken responses in form of audio transcriptions and iVectors by non-native English speakers of eleven native languages. Our system competed in the challenge under the team name ZCD and was based on an ensemble of SVM classifiers trained on character n-grams achieving 83.58% accuracy and ranking 3rd in the shared task.Comment: Proceedings of the Workshop on Innovative Use of NLP for Building Educational Applications (BEA

Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1

NGC 55 ULX1 is a bright Ultraluminous X-ray source located 1.78 Mpc away. We analysed a sample of 20 Swift observations, taken between 2013 April and August, and two Chandra observations taken in 2001 September and 2004 June. We found only marginal hints of a limited number of dips in the light curve, previously reported to occur in this source, although the uncertainties due to the low counting statistics of the data are large. The Chandra and Swift spectra showed clearly spectral variability which resembles those observed in other ULXs. We can account for this spectral variability in terms of changes in both the normalization and intrinsic column density of a two-components model consisting of a blackbody (for the soft component) and a multicolour accretion disc (for the hard component). We discuss the possibility that strong outflows ejected by the disc are in part responsible for such spectral changes.Comment: 9 pages, 6 figure; accepted to be published on MNRA

VLT Observations of the Ultraluminous X-ray Source NGC 1313 X-2

We present archive ESO VLT photometric and spectroscopic data of the Ultraluminous X-ray source NGC 1313 X-2. The superb quality of the VLT images reveals that two distinct objects, with R magnitudes 23.7 and 23.6, are visible inside the Chandra error box. The two objects, separated by 0.75 arcsec, were unresolved in our previous ESO 3.6 m+EFOSC image. We show that both are stars in NGC 1313, the first a B0-O9 main sequence star of ~20 Msun, while the second a G supergiant of ~10 Msun. Irrespectively of which of the two objects the actual counterpart is, this implies that NGC 1313 X-2 is a high mass X-ray binary with a very massive donor.Comment: 7 pages, 4 figures, accepted for publication in ApJ Letter

Ultraluminous X-ray Sources forming in low metallicity natal environments

In the last few years multiwavelength observations have boosted our understanding of Ultraluminous X-ray Sources (ULXs). Yet, the most fundamental questions on ULXs still remain to be definitively answered: do they contain stellar or intermediate mass black holes? How do they form? We investigate the possibility that the black holes hosted in ULXs originate from massive (40-120 $M_\odot$) stars in low metallicity natal environments. Such black holes have a typical mass in the range $\sim 30-90 M_\odot$ and may account for the properties of bright (above $\sim 10^{40}$ erg s$^{-1}$) ULXs. More than $\sim 10^5$ massive black holes might have been generated in this way in the metal poor Cartwheel galaxy during the last $10^7$ years and might power most of the ULXs observed in it. Support to our interpretation comes from NGC 1313 X-2, the first ULX with a tentative identification of the orbital period in the optical band, for which binary evolution calculations show that the system is most likely made by a massive donor dumping matter on a $50-100 M_\odot$ black hole.Comment: 4 pages. To appear in the Proceedings of the Conference "X-Ray Astronomy 2009: Present Status, Multiwavelength Approach and Future Perspectives", Bologna, Italy, September 2009, Eds. A. Comastri, M. Cappi, L. Angelini, 2010 AIP (in press)

The Ultraluminous X-ray Sources NGC 1313 X-1 and X-2

We present a detailed analysis of XMM archival data of two Ultraluminous X-ray Sources (ULXs) in the nearby spiral galaxy NGC 1313: NGC 1313 X-1 and X-2. The spectral continuum of these sources was modeled with a soft thermal component plus a power-law. If the soft component originates from an accretion disk, the inferred mass of the compact remnant is > 100 M_sun, making it an Intermediate Mass Black Hole (IMBH). A detailed analysis of the residuals of the XMM EPIC-pn spectrum shows some evidence for the presence of an Oxygen emission line in NGC 1313 X-1. The simultaneous presence of an excess in emission, although at a much reduced significance level, at different energies in the X-ray spectra of NGC 1313 X-1 and X-2 is suggestive of typical emission lines from young supernova remnants. An optical counterpart for NGC 1313 X-2 was also identified. On an ESO 3.6 m image, the Chandra error box embraces a R \~ 23 mag stellar-like object and excludes a previously proposed optical counterpart.Comment: 7 pages, 2 figures, to be published in Advances in Space Researc