Spectral states in Be/X-ray pulsars

In the last quarter of a century, a unified characterization of the spectral evolution of low-mass X-ray binaries, both containing a neutron star and a black hole, was possible. In this context, the notion of source states characterizing the X-ray emission from black-hole binaries and neutron-star low-mass X-ray binaries revealed to be a very useful tool to disentangle the complex spectral and aperiodic phenomenology displayed by those classes of accreting objects. Be/X-ray binaries constitute another major class of transient accreting binaries, for which very little work has been done on the correlated timing and spectral variability. Especially, no definition of source states exists for this class, in spite of their highly variable X-ray emission. When active, Be/X-ray binaries are among the brightest objects in the X-ray sky and are characterized by dramatic variability in brightness on timescales ranging from seconds to years. It is then worth it to ask whether a definition of spectral states is possible for these systems. In this work, we try to address such a question, investigating whether accreting X-ray pulsars display source states and characterizing those states through their spectral properties. Our results show that Be/X-ray pulsars trace two different branches in their hardness-intensity diagram: the horizontal branch, a low-intensity state, and the diagonal branch, a high-intensity state that only appears when the X-ray luminosity exceeds a critical limit. We propose that the two branches are the phenomenological signature of two different accretion modes -- in agreement with recently proposed models -- depending on whether the luminosity of the source is above or below a critical value.Comment: Proceedings of "An INTEGRAL view of the high-energy sky (the first 10 years)" the 9th INTEGRAL Workshop, October 15-19, 2012, Paris, France, in Proceedings of Science (INTEGRAL 2012), Eds. A. Goldwurm, F. Lebrun and C. Winkler, (http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=176), id 02

Disc-loss episode in the Be shell optical counterpart to the high-mass X-ray binary IGR J21343+4738

The main goal of this work is to determine the properties of the optical counterpart to the INTEGRAL source IGR J21343+4738, and study its long-term optical variability. We present optical photometric BVRI and spectroscopic observations covering the wavelength band 4000-7500 A. We find that the optical counterpart to IGR J21343+4738 is a V=14.1 B1IVe shell star located at a distance of ~8.5 kpc. The Halpha line changed from an absorption dominated profile to an emission dominated profile, and then back again into absorption. In addition, fast V/R asymmetries were observed once the disc developed. Although the Balmer lines are the most strongly affected by shell absorption, we find that shell characteristics are also observed in He I lines. The optical spectral variability of IGR J21343+4738 is attributed to the formation of an equatorial disc around the Be star and the development of an enhanced density perturbation that revolves inside the disc. We have witnessed the formation and dissipation of the circumstellar disc. The strong shell profile of the Halpha and He I lines and the fact that no transition from shell phase to a pure emission phase is seen imply that we are seeing the system near edge-on.Comment: accepted for publication in A&

Warped disks during type II outbursts in Be/X-ray binaries: evidence from optical polarimetry

Current models that explain giant (type II) X-ray outbursts in Be/X-ray binaries (BeXB), are based on the idea of highly distorted disks. They are believed to occur when a misaligned and warped disk becomes eccentric, allowing the neutron star to capture a large amount of material. The BeXB 4U 0115+63 underwent two major outbursts in 2015 and 2017. Our aim is to investigate whether the structural changes in the disk expected during type II outbursts can be detected through optical polarimetry. We present the first optical polarimetric observations and new optical spectra of the BeXB 4U 0115+63 covering the period 2013-2017. We study in detail the shape of the H$\alpha$ line profile and the polarization parameters before, during, and after the occurrence of a type II X-ray outburst. We find significant changes in polarization degree and polarization angle and highly distorted line profiles during the 2017 X-ray outburst. The degree of polarization decreased by $\sim$ 1%, while the polarization angle, which is supposed to be related with the disk orientation, first increased by $\sim 10^{\circ}$ in about two months and then decreased by a similar amount and on a similar timescale once the X-ray activity ceased.We interpret the polarimetric and spectroscopic variability as evidence for the presence of a warped disk

Accretion regimes in the X-ray pulsar 4U 1901+03

The source 4U 1901+03 is a high-mass X-ray pulsar than went into outburst in 2003. Observation performed with the Rossi X-ray Timing Explorer showed spectral and timing variability, including the detection of flares, quasi-periodic oscillations, complex changes in the pulse profiles, and pulse phase dependent spectral variability. We re-analysed the data covering the 2003 X-ray outburst and focused on several aspects of the variability that have not been discussed so far. These are the 10 keV feature and the X-ray spectral states and their association with accretion regimes, including the transit to the propeller state at the end of the outburst. We find that 4U 1901+03 went through three accretion regimes over the course of the X-ray outburst. At the peak of the outburst and for a very short time, the X-ray flux may have overcome the critical limit that marks the formation of a radiative shock at a certain distance above the neutron star surface. Most of the time, however, the source is in the subcritical regime. Only at the end of the outburst, when the luminosity decreased below ~10^{36} (d/10 kpc)^2 erg/s, did the source enter the propeller regime. Evidence for the existence of these regimes comes from the pulse profiles, the shape of the hardness-intensity diagram, and the correlation of various spectral parameters with the flux. The 10 keV feature appears to strongly depend on the X-ray flux and on the pulse phase, which opens the possibility to interpret this feature as a cyclotron line.Comment: accepted for publication in A&A. arXiv admin note: text overlap with arXiv:astro-ph/9704084 by other author

The time-lag -- photon-index correlation in GX 339--4

Black-hole transients exhibit a correlation between the time lag of hard photons with respect to softer ones and the photon index of the hard X-ray power law. The correlation is not very tight and therefore it is necessary to examine it source by source. The objective of the present work is to investigate in detail the time-lag -- photon-index correlation in GX 339-4. We have obtained RXTE energy spectra and light curves and have computed the photon index and the time lag of the $9 - 15$ keV photons with respect to the $2 - 6$ keV ones. The observations cover the first stages of the hard state, the pure hard state, and the hard-intermediate state. At low $\Gamma$, the correlation is positive and it becomes negative at large $\Gamma$. By assuming that the hard X-ray power law index $\Gamma$ is produced by inverse Compton scattering of soft disk photons in the jet, we have reproduced the entire correlation by varying two parameters in the jet: the radius of the jet at its base $R_0$ and the Thomson optical depth along the jet $\tau_\parallel$. We have found that, as the luminosity of the source increases, $R_0$ initially increases and then decreases. This behavior is expected in the context of the Cosmic Battery. As a further test of our model, we predict the break frequency in the radio spectrum as a function of the photon index during the rising part of an outburst

Inclination effects on the X-ray emission of Galactic black-hole binaries

Galactic black-hole X-ray binaries (BHBs) emit a compact, optically thick, mildly relativistic radio jet when they are in the hard and hard-intermediate states. In these states, BHBs exhibit a correlation between the time lag of hard photons with respect to softer ones and the photon index of the power law component that characterizes the X-ray spectral continuum above $\sim$ 10 keV. The correlation, however, shows large scatter. Our objective is to investigate the role that the inclination of the system plays on the correlation between the time lag and the photon index. We find that the correlation between the time lag and the photon index is tight in low-inclination systems and becomes weaker in high-inclination systems. The amplitude of the lags is also larger at low and intermediate inclination angles than at high inclination. Our jet model that reproduces the process of Comptonization in an extended jet can account for the observations remarkably well

Aperiodic variability of low-mass X-ray binaries at very low frequencies

We have obtained discrete Fourier power spectra of a sample of persistent low-mass neutron-star X-ray binaries using long-term light curves from the All Sky Monitor on board the Rossi X-ray Timing Explorer. Our aim is to investigate their aperiodic variability at frequencies in the range 1 x 10^{-7}-5 x 10^{-6} Hz and compare their properties with those of the black-hole source Cyg X-1. We find that the classification scheme that divides LMXBs into Z and atoll sources blurs at very low frequencies. Based on the long-term (~ years) pattern of variability and the results of power-law fits (P ~ v^{-a}) to the 1 x 10^{-7}-5 x 10^{-6} Hz power density spectra, low-mass neutron-star binaries fall into three categories. Type I includes all Z sources, except Cyg X-2, and the atoll sources GX9+1 and GX13+1. They show relatively flat power spectra (a < 0.9) and low variability (rms < 20%). Type II systems comprise 4U 1636-53, 4U 1735-44 and GX3+1. They are more variable (20% < rms < 30%) and display steeper power spectra (0.9 < a < 1.2) than Type I sources. Type III systems are the most variable (rms > 30%) and exhibit the steepest power spectra (a > 1.2). The sources 4U 1705-44, GX354-0 and 4U 1820-30 belong to this group. GX9+9 and Cyg X-2 appear as intermediate systems in between Type I and II and Type II and III sources, respectively. We speculate that the differences in these systems may be caused by the presence of different types of mass-donor companions. Other factors, like the size of the accretion disc and/or the presence of weak magnetic fields, are also expected to affect their low-frequency X-ray aperiodic varibility.Comment: 9 pages, 6 figures. To be published in A&

The nature of 1WGA J1958.2+3232: a new intermediate polar

We present low and intermediate resolution spectroscopy of the optical counterpart to the recently discovered pulsating X-ray source 1WGA J1958.2+3232. The presence of strong HI , HeI and HeII emission lines together with the absence of absorption features rules out the possibility that the object is a massive star, as had recently been suggested. The observed X-ray and optical characteristics are consistent with the object being an intermediate polar. The double-peaked structure of the emission lines indicates that an accretion disc is present