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

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&

Spectroscopy of the brightest optical counterparts of X-ray sources in the direction of M~31 and M~33

Recent surveys of the Local Group spiral Galaxies M31 and M33 with XMM-Newton yielded a large number of X-ray sources. As part of the effort to identify and classify the objects responsible for this X-ray emission, we have obtained optical spectra of the brightest optical counterparts of the identified X-ray sources, using the 1.3m Skinakas Telescope. Most of these objects are foreground star candidates. The purpose of the present study is to confirm this identification and to explore the compatibility between the optical spectral classification and the observed X-ray properties of the sources. We have obtained optical spectra for the 14 brightest optical counterparts of X-ray sources identified by XMM-Newton in the direction of M31 and for 21 optical counterparts in the direction of M33, using the 1.3m Skinakas telescope in Crete, Greece. All of the M31 sources and all but one of the M33 sources were confirmed to be foreground stars, of spectral types between A and M. One of the stars is a late M dwarf with H-alpha emission, a flare star, also displaying strong X-ray variability. One of the M~33 sources (lying within the D25 ellipse) corresponds to a previously known background galaxy, LEDA 5899.Comment: 9 pages, 12 figures, accepted in A&

Bound-state dark matter with Majorana neutrinos

We propose a simple scenario in which dark matter (DM) emerges as a stable neutral hadronic thermal relics, its stability following from an exact $\operatorname{U}(1)_D$ symmetry. Neutrinos pick up radiatively induced Majorana masses from the exchange of colored DM constituents. There is a common origin for both dark matter and neutrino mass, with a lower bound for neutrinoless double beta decay. Direct DM searches at nuclear recoil experiments will test the proposal, which may also lead to other phenomenological signals at future hadron collider and lepton flavour violation experiments.Comment: 9 pages, 4 figures. arXiv admin note: text overlap with arXiv:1803.0852

A quantitative explanation of the radio--X-ray correlation in black-hole X-ray binaries

The observed correlation between the radio and X-ray fluxes in the hard state of black-hole X-ray binaries (BHXRBs) has been around for more than two decades. It is currently accepted that the hard X-rays in BHXRBs come from Comptonization in the corona and the radio emission from the relativistic jet (Lorentz $\gamma >> 1$), which is a narrow structure of a few $R_g=GM/c^2$ at its base. The relativistic jet and the corona, however, are separate entities with hardly any communication between them, apart from the fact that both are fed from the accreting matter. It is also widely accepted that the accretion flow around black holes in BHXRBs consists of an outer thin disk and an inner hot flow. From this hot inner flow, an outflow emanates in the hard and hard-intermediate states of the source. By considering Compton up-scattering of soft disk photons in the outflow (i.e., in the outflowing corona, which is a wider structure, tens to hundreds of $R_g$ at its base, with low Lorentz gamma) as the mechanism that produces the hard X-ray spectrum, we have been able to explain quantitatively a number of observed correlations. Here, we demonstrate that this outflowing corona can also explain quantitatively the observed radio - X-ray correlation. In addition, we make the following theoretical predictions for GX 339-4: 1) the radio flux in the hard and hard-intermediate states should be a bell-shaped curve as a function of the photon-number spectral index Gamma, 2) the radio - X-ray correlation should break down when the source moves from the hard to the hard-intermediate state and instead the radio flux should first increase sharply in the hard-intermediate state and then decrease also sharply, in a very narrow range of the X-ray flux, and 3) the X-ray polarization will be parallel to the outflow in the hard state and perpendicular to it in the hard-intermediate one.Comment: Accepted for publication in A&

Energy and time-lag spectra of black-hole systems in the low state

Most, probably all, accreting binaries that are believed to contain a black-hole emit radio waves when they are in the low/hard state. Whenever this radio emission has been resolved, a jet-like structure has become apparent. We propose that Compton upscattering of low-energy photons in the jet can explain both the energy spectra and the time lags versus Fourier frequency observed in the low/hard state of black-hole systems. The soft photons originate in the inner part of the accretion disk. We have performed Monte Carlo simulations of Compton upscattering in a jet and have found that for a rather wide range of values of the parameters we can obtain power-law high-energy X-ray spectra with photon-number index in the range 1.5 - 2 and power-law time lags versus Fourier frequency with index ~ 0.7. The black-hole source Cyg X-1 in the low/hard state is well described by our model.Comment: 4 pages 3 figures, to be published in A&A Letter

A jet model for black-hole X-ray sources

A jet model for Galactic black-hole X-ray binaries will be presented that appears to explain several observational characteristics. In particular, it explains the energy spectrum from radio to hard X-rays, the time-lags as a function of Fourier frequency, the increase of the variability amplitude (QPO and high frequency) with increasing photon energy, and the narrowing of the autocorrelation function with increasing photon energy. On the other hand, there are additional observational constraints that no model has tried to explain yet. It is important that we all try to address these constraints if we are to make any progress in understanding black-hole X-ray source

Spectroscopy of the bright optical counterparts of X-ray sources in the direction of M 31. II

A recent survey of the Local Group spiral galaxy M 31 with XMM-Newton yielded a large number of X-ray sources. This is the second in a series of papers with the aim of identifying the optical counterparts of these X-ray sources. We have obtained optical spectra for 21 bright optical counterparts of 20 X-ray sources in the direction of M 31, using the 1.3-m Skinakas telescope in Crete, Greece. For 17 of the 20 X-ray sources, we have identified the optical counterpart as a normal late type star (of type F or later) in the foreground (i.e. in the Milky Way). For two more sources there were two possible optical counterparts in each case, while two more objects have X-ray properties that are not compatible with the spectral characteristics of late type non-flaring stars.Comment: Accepted for publication by Astronomy & Astrophysics (7 pages, 8 figures, and 2 tables