Aims: We present a study of the Be/X-ray binary system V 0332+53 with the main goal of characterizing its behavior mainly during the intermediate-luminosity X-ray event on 2008. In addition, we aim to contribute to the understanding of the global beh
avior of the donor companion by including optical data from our dedicated campaign starting on 2006. Methods: V 0332+53 was observed by RXTE and Swift during the decay of the intermediate-luminosity X-ray outburst of 2008, as well as with Suzaku before the rising of the third normal outburst of the 2010 series. In addition, we present recent data from the Spanish ground-based astronomical observatories of El Teide (Tenerife), Roque de los Muchachos (La Palma), and Sierra Nevada (Granada), and since 2006 from the Turkish TUBITAK National Observatory (Antalya). We have performed temporal analyses to investigate the transient behaviour of this system during several outbursts. Results: Our optical study revealed that continuous mass ejection episodes from the Be star have been taking place since 2006 and another one is currently ongoing. The broad-band 1-60 keV X-ray spectrum of the neutron star during the decay of the 2008 outburst was well fitted with standard phenomenological models, enhanced by an absorption feature of unknown origin at about 10 keV and a narrow iron K-alpha fluorescence line at 6.4 keV. For the first time in V 0332+53 we tentatively see an increase of the cyclotron line energy with increasing flux (although further and more sensitive observations are needed to confirm this). Regarding the fast aperiodic variability, we detect a Quasi-Periodic Oscillation (QPO) at 227+-9 mHz only during the lowest luminosities. The latter fact might indicate that the inner regions surrounding the magnetosphere are more visible during the lowest flux states.
We present recent contemporaneous X-ray and optical observations of the Be/X-ray binary system A,0535+26 with the textit{Fermi}/Gamma-ray Burst Monitor (GBM) and several ground-based observatories. These new observations are put into the context of t
he rich historical data (since $sim$1978) and discussed in terms of the neutron star Be-disk interaction. The Be circumstellar disk was exceptionally large just before the 2009 December giant outburst, which may explain the origin of the unusual recent X-ray activity of this source. We found a peculiar evolution of the pulse profile during this giant outburst, with the two main components evolving in opposite ways with energy. A hard 30-70 mHz X-ray QPO was detected with GBM during this 2009 December giant outburst. It becomes stronger with increasing energy and disappears at energies below 25,keV. In the long-term a strong optical/X-ray correlation was found for this system, however in the medium-term the H$_alpha$ EW and the V-band brightness showed an anti-correlation after $sim$2002 Agust. Each giant X-ray outburst occurred during a decline phase of the optical brightness, while the H$_alpha$ showed a strong emission. In late 2010 and before the 2011 February outburst, rapid V/R variations are observed in the strength of the two peaks of the H$_alpha$ line. These had a period of $sim$,25 days and we suggest the presence of a global one-armed oscillation to explain this scenario. A general pattern might be inferred, where the disk becomes weaker and shows V/R variability beginning $sim$,6 months following a giant outburst.
We compiled X-ray and Optical observations of the accreting X-ray binary system A 0535+262 since its discovery in 1975, that will allow us to shed light on the unpredictable behavior of this binary system. We present the data in terms of the Be-disk
interaction with the neutron star companion. In addition, we show recent results from the continuous monitoring of this source by the Gamma-ray Burst Monitor(GBM), on board the Fermi observatory, since its launch in 2008 June 11.
Recent X-ray observations by Fermi/GBM discovered a new torque reversal of 4U 1626-67 after 18 years of steady spinning down. Using Swift/BAT observations we were able to center this new torque reversal on Feb 4 2008, lasting approximately 150 days.
From 2004 up to the end of 2007, the spin-down rate averaged at a mean rate of ~dnu/dt=-4.8e-13 Hz s-1 until the torque reversal reported here. Since then it has been following a steady spin-up at a mean rate of ~dnu/dt= 4e-13 Hz s-1. The properties of this torque reversal, as well as the lack of correlation between the X-ray flux and the torque applied to the neutron star before this transition, challenges our understanding of the physical mechanisms operating in this system.
After about 18 years of steadily spinning down, the accretion-powered pulsar 4U 1626-67, experienced a torque reversal at the beginning of 2008. For the present study we have used all available Fermi/GBM data since its launch in 2008 June 11 and over
5 yr of hard X-ray Swift/BAT observations (starting from 2004 October up to the present time). This second detected torque reversal is centered near MJD 54500 (2008 Feb 4) and it lasts approximately 150 days. From 2004 up to the end of 2007 4U 1626-67 the spin-down rate decreased at a mean rate of ~ -5.5E-13 Hz s-1 until the source reversed torque again. Since then it has been following a steady spin-up at a mean rate of ~ 5E-13 Hz s-1. In addition, 4U 1626-67 increased its flux simultaneously (a ~2.5 factor). We present detailed long-term timing analysis of this source and a long term spectral hardness ratio study in order to see whether there are spectral changes around this new observed torque reversal.
