Aitor Ibarra

MAXI J1659-152: the shortest orbital period black-hole transient in outburst

MAXI J1659−152 is a bright X-ray transient black-hole candidate binary system discovered in September 2010. We report here on MAXI, RXTE, Swift, and XMM-Newton observations during its 2010/2011 outburst. We find that during the first one and a half week of the outburst the X-ray light curves display drops in intensity at regular intervals, which we interpret as absorption dips. About three weeks into the outbursts, again drops in intensity are seen. These dips have, however, a spectral behaviour opposite to that of the absorption dips, and are related to fast spectral state changes (hence referred to as transition dips). The absorption dips recur with a period of 2.414  ±  0.005 h, which we interpret as the orbital period of the system. This implies that MAXI J1659−152 is the shortest period black-hole candidate binary known to date. The inclination of the accretion disk with respect to the line of sight is estimated to be 65-80°. We propose the companion to the black-hole candidate to be close to an M5 dwarf star, with a mass and radius of about 0.15-0.25 M⊙ and 0.2-0.25 R⊙, respectively. We derive that the companion had an initial mass of about 1.5 M⊙, which evolved to its current mass in about 5-6 billion years. The system is rather compact (orbital separation of ≳1.33 R⊙), and is located at a distance of 8.6  ±  3.7 kpc, with a height above the Galactic plane of 2.4  ±  1.0 kpc. The characteristics of short orbital period and high Galactic scale height are shared with two other transient black-hole candidate X-ray binaries, i.e., XTE J1118+480 and Swift J1735.5−0127. We suggest that all three are kicked out of the Galactic plane into the halo, rather than being formed in a globular cluster.

Swift observations of the X-ray and UV evolution of V2491 Cyg (Nova Cyg 2008 No. 2)

We present extensive, high-density Swift observations of V2491 Cyg (Nova Cyg 2008 No. 2). Observing the X-ray emission from only one day after the nova discovery, the source is followed through the initial brightening, the super-soft source phase and back to the pre-outburst flux level. The evolution of the spectrum throughout the outburst is demonstrated. The UV and X-ray light curves follow very different paths, although changes occur in them around the same times, indicating a link between the bands. Flickering in the late-time X-ray data indicates the resumption of accretion. We show that if the white dwarf (WD) is magnetic, it would be among the most magnetic known; the lack of a periodic signal in our later data argues against a magnetic WD, however. We also discuss the possibility that V2491 Cyg is a recurrent nova, providing recurrence time-scale estimates.

Pre-nova X-ray observations of V2491 Cygni (Nova Cyg 2008b)

Classical novae are phenomena caused by explosive hydrogen burning onto an accreting white dwarf. Only one classical nova had been identified in X-rays before the actual optical outburst occurred (V2487 Oph). The recently discovered nova, V2491 Cyg, is one of the fastest (He/N) novae observed to date. Using archival ROSAT, XMM-Newton, and Swift data, we show that V2491 Cyg was a persistent X-ray source during its quiescent time before the optical outburst. We present the X-ray spectral characteristics and derive X-ray fluxes. The pre-outburst X-ray emission is variable, and, at least in one observation, it exhibits a soft X-ray source.

Monte Carlo simulations for High Zenigh Angle

With the advent of new generation Imaging Atmospheric Cherenkov Telescopes (IACTs), with improved sensitivity, it will be mandatory to extend observation to High Zenith Angles (HZA). This will allow us to extend by a large factor the observation time as well as the statistics for any gamma-ray source. Here we present some results obtained for several zenith angles and for different energies of the primary particle, obtained with a modification in the CORSIKA code, which will allow us to simulate and study the behavior of atmospheric Extensive Air Showers at HZA. In order to obtain more realistic results, the effect of the atmospheric attenuation, including Rayleigh scattering, Mie scattering and Ozone absorption, is included as well.