P. Reig

Bright radio emission from an ultraluminous stellar-mass microquasar in M 31

A subset of ultraluminous X-ray sources (those with luminosities of less than 1040 erg s−1; ref. 1) are thought to be powered by the accretion of gas onto black holes with masses of ∼5–20, probably by means of an accretion disk. The X-ray and radio emission are coupled in such Galactic sources; the radio emission originates in a relativistic jet thought to be launched from the innermost regions near the black hole, with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way, and the absorption of soft X-rays in the interstellar medium hinders the determination of the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source in the nearby galaxy M 31, whose peak luminosity exceeded 1039 erg s−1. The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a black hole of stellar mass. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission.

Phase Lag Variability Associated with the 0.5-10 HZ Quasi-Periodic Oscillations in GRS 1915+105

We have used Rossi X-Ray Timing Explorer data to measure the lags between soft (2-5 keV) and hard (5-13 keV) photons and to study the aperiodic variability of the superluminal black hole candidate GRS 1915+105 during low-flux states. The power density spectra exhibit quasi-periodic oscillations (QPO) whose frequency increases with increasing count rate and varies in the frequency range 0.6-8 Hz. A correlation between the QPO frequency and the phase lag spectra is reported for the first time. This correlation is found for both the phase lag continuum and the phase lag at the QPO frequency. We find that as the QPO frequency moves to higher values the phase lags reverse sign from positive to negative. The absolute value of the lag always increases with photon energy. The negative (soft) lags are associated with a softer energy spectrum, whereas the positive (hard) lags are seen when the source is harder. We describe a possible scenario that may account for the change in the sign of the lags.

Discovery of two new persistent Be/X-ray pulsar systems

We present RXTE observations of two recently identified massive X-ray binaries. RX J0440.9+4431/BSD 24-491 and RX J1037.5-564/LS 1698 are confirmed as accreting Be/X-ray systems following the discovery of X-ray pulsations, with barycentric pulse periods of 202.5

Orbital x-ray variability of the microquasar LS 5039

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RoboPol: First season rotations of optical polarization plane in blazars

0.5 s and 860

On the radio emitting high mass X-ray binary LS 5039

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Accreting magnetars: a new type of high-mass X-ray binaries?

2 s respectively. The X-ray spectral analysis shows that the energy spectra of the pulsars can be represented by a power-law, modified at low energy by an absorption component and at high energy by a cut-off. Very weak Fe lines may be present. Both sources appear to display a low cut-off energy when compared to typical X-ray pulsars, low X-ray variability (factor of < 10), and no dependence of the X-ray spectrum with energy. Given the similarity of these X-ray properties with those of the other persistent BeXRB pulsars, 4U0352+309/X Per and RX J0146.9+6121/LS I +61 235, we suggest that RX J0440.9+4431/BSD 24-491 and RX J1037.5-564/LS 1698 are also members of this subclass.

Patterns of variability in Be/X-ray pulsars during giant outbursts

The properties of the orbit and the donor star in the high-mass X-ray binary microquasar LS 5039 indicate that accretion processes should mainly occur via a radiatively driven wind. In such a scenario, significant X-ray variability would be expected due to the eccentricity of the orbit. The source has been observed at X-rays by several missions, although with a poor coverage that prevents reaching any conclusion about orbital variability. Therefore, we conducted RXTE observations of the microquasar system LS 5039 covering a full orbital period of 4 days. Individual observations are well fitted with an absorbed power law plus a Gaussian at 6.7 keV, to account for iron-line emission that is probably a diffuse background feature. In addition, we have taken into account that the continuum is also affected by significant diffuse background contamination. Our results show moderate power-law flux variations on timescales of days, as well as the presence of miniflares on shorter timescales. The new orbital ephemerides of the system recently obtained by Casares et al. have allowed us to show, for the first time, that an increase of emission is seen close to the periastron passage, as expected in an accretion scenario. Moreover, the detected orbital variability is a factor of ~4 smaller than the one expected by using a simple wind accretion model, and we suggest that an accretion disk around the compact object could be responsible for this discrepancy. On the other hand, significant changes in the photon index are also observed, clearly anticorrelated with the flux variations. We interpret the overall X-ray spectral characteristics of LS 5039 in the context of X-ray radiation produced by inverse Compton and/or synchrotron processes in the jet of this microquasar.

A jet model for Galactic black-hole X-ray sources: some constraining correlations

We present first results on polarization swings in optical emission of blazars obtained by RoboPol, a monitoring programme of an unbiased sample of gamma-ray bright blazars specially designed for effective detection of such events. A possible connection of polarization swing events with periods of high activity in gamma-rays is investigated using the data set obtained during the first season of operation. It was found that the brightest gamma-ray flares tend to be located closer in time to rotation events, which may be an indication of two separate mechanisms responsible for the rotations. Blazars with detected rotations during non-rotating periods have significantly larger amplitude and faster variations of polarization angle than blazars without rotations. Our simulations show that the full set of observed rotations is not a likely outcome (probability ≤1.5 × 10^(−2)) of a random walk of the polarization vector simulated by a multicell model. Furthermore, it is highly unlikely (∼5 × 10^(−5)) that none of our rotations is physically connected with an increase in gamma-ray activity.

RoboPol: optical polarization-plane rotations and flaring activity in blazars

We present new optical { near{IR spectroscopic and photometric observations of the newly discovered galactic microquasar LS 5039, which indicate a classication for the mass donor in the system of O6.5V((f)). Optical spectroscopy and photometry shows no variability over a timescale of years, and we nd no evidence of modulation by, or emission from the compact companion in these data. However signicant photometric variability (0:4 mag) is present in the H and K bands between 1995{2000. Such variability has been observed in other radio bright X-ray binaries where it has been attributed to synchrotron emission from the jet. However, given the non{thermal spectral index of the radio emission in LS 5039 this explanation appears unlikely, predicting a near{IR flux3 orders of magnitude too small to contribute signicantly at such wavelengths. Nightly optical photometry over a 21 day period between 2000 May{June reveals variability at a level of a few hundredths of a magnitude, with no periodicity or long term trend visible. Likewise, while the radio lightcurves show moderate variability (20 per cent of the mean flux density) we nd no evidence of periodic modulation { Monte Carlo simulations constrain any such periodic variability to <4 per cent modulation at 2.25 GHz. The dierences in behaviour between LS 5039 and Cygnus X-1 { the most closely related radio emitting High Mass X-ray Binary { are likely to be a result of the weaker stellar wind and probable greater orbital separation of LS 5039 compared to Cyg X-1.