Juan Gutierrez-Soto

Detection of a large Be circumstellar disk during X-ray quiescence of XTE J1946+274

Aims: We present a multiwavelength study of the Be/X-ray binary system XTE J1946+274 with the main goal of better characterizing its behavior during X-ray quiescence. We also aim to shed light on the possible mechanisms which trigger the X-ray activity for this source.Methods: XTE J1946+274 was observed by Chandra-ACIS during quiescence in 2013 March 12. In addition, this source has been monitored from the ground-based astronomical observatories of El Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain) and Sierra Nevada (Granada, Spain) since 2011 September, and from the TUBITAK National Observatory (Antalya, Turkey) since 2005 April. We have performed spectral and photometric temporal analyses in order to investigate the quiescent state and transient behavior of this binary system.Results: Our optical study revealed that a long mass ejection event from the Be star took place in 2006, lasting for about seven years, and another one is currently ongoing. We also found that a large Be circumstellar disk is present during quiescence, although major X-ray activity is not observed. We made an attempt to explain this by assuming the permanently presence of a tilted (sometimes warped) Be decretion disk. The 0.3-10 keV X-ray spectrum of the neutron star during quiescence was well fitted with either an absorbed black-body or an absorbed power-law models. The main parameters obtained for these models were kT = 1.43 +/- 0.17 and Gamma = 0.9 +/- 0.4 (with N-H similar to 2-7 x 10(22) cm(-2)). The 0.3-10 keV flux of the source was similar to 0.8-1 x 10(-12) erg(-1) cm(-2) s(-1). Pulsations were found with P-pulse = 15.757(1) s (epoch MJD 56 363.115) and an rms pulse fraction of 32.1(3)%. The observed X-ray luminosity during quiescent periods was close to that of expected in supersonic propeller regimen.

Short-term variations in Be stars observed by the CoRoT and K epler space missions

The corot and kepler space missions are collecting very high-precision longduration photometric data of many Be stars, allowing us to better understand the origin of their short-term variability and the link between these variations and the Be phenomenon. In this paper, we present a brief summary of the results obtained in the analysis of several Be stars observed with corot in terms of pulsations. In addition, we show that variations of the Be star HD 175869 can be explained as two active regions separated by 150 degrees or as unstable pulsating modes in a star with an extensive mixing in radiative layers corresponding to a core overshooting of 0.35Hp. A preliminary study of the photometric and spectroscopic variability seen in the B1.5IVe star HD 51193 is performed. Currently the kepler satellite is observing the only confirmed Be star in its field of view, namely KIC 6954726. From low-resolution spectra we derived a spectral type of B2.5Ve for this star and we studied the long-term variation of the emission in the Hα line. The 3.5-year kepler light curve will allow us to detect even more close frequencies than with corot and to perform a detailed analysis of the amplitude variations in a Be star.

A Pulsational Study of a Sample of CoRoT Faint Be Stars

We present an intensive study of a sample of 12 Be stars in the first exo-planetary fields of the CoRoT mission. The aim is to understand the short-term variability in Be stars. We use both spectroscopic and photometric data: VLT-GIRAFFE spectra to derive the fundamental parameters and CoRoT light curves to search for low-amplitude frequencies. This allows us to locate the stars in the HR diagram and compare their positions in the instability strip with those of other pulsating B stars. From VLT-GIRAFFE observations we determined the stellar parameters by fitting the observed spectra with non-LTE stellar atmosphere models. The observed spectra were corrected for veiling effects due to circumstellar material and for rotational effects. We estimated the rotational frequency using the corrected fundamental parameters. For the CoRoT light curves, we determined all significant frequencies and compared the main frequencies with the rotational frequency. We also investigated the variation of the frequencies and amplitudes with time and, in particular, during outbursts. This is the first time that a statistical study with space data has been performed for Be stars. Our study suggests that rotational modulation is not the cause of short-term photometric variability in Be stars.

Non-radial pulsations in the Be/X binaries 4U 0115+63 and SAX J2103.5+4545

The discovery of non-radial pulsations (NRP) in the Be/X binaries of the Magellanic Clouds (MC, eg. Fabrycky 2005, Coe et al. 2005, Schmidtke & Cowley 2005) provided a new approach to understand these complex systems, and, at the same time, favoured the synergy between two different fields: stellar pulsations and X-ray binaries. This breakthrough was possible thanks to the MACHO and OGLE surveys. However, in our Galaxy, only two Be/X have been reported to show NRP: GROJ2058+42 (Kiziloglu et al. 2007) and LSI+61 235 (Sarty et al. 2009). Our objective is to study the short-term variability of Galactic Be/X binaries, compare them to the Be/X of the MC and to the isolated Galactic Be observed with CoRoT and Kepler. We present preliminary results of two Be/X stars, namely 4U0115+63 and SAXJ2103.5+4545 showing multiperiodicity and periodicity respectively, most probably produced by non-radial pulsations.

Asteroseismic observations of OB stars

The region of the hot end of the main-sequence is hosting pulsating stars of different types and flavours. Pulsations are not only observed for Slowly pulsating B stars (mid to late B-type stars; high order g-modes) and β Cephei stars (early B-type stars; low order p/g-modes) but are also causing variability in Be stars and OB-supergiants. In this review we give an overview of the asteroseismic observations that are currently available for these types of stars. The first asteroseismic results were solely based on ground-based observations. Recently, the arrival of space-based data gathered by space missions like most, corot and kepler has led to important discoveries for massive stars, highlighting their excellent asteroseismic potential. We show that, despite the unprecedented precision of the space-based data, there is still a clear need for ground-based follow-up observations.