A search for electron cyclotron maser emission from compact binaries

Unipolar induction (UI) is a fundamental physical process, which occurs when a conducting body transverses a magnetic field. It has been suggested that UI is operating in RX J0806+15 and RX J1914+24, which are believed to be ultracompact binaries with orbital periods of 5.4 and 9.6 min, respectively. The UI model predicts that those two sources may be electron cyclotron maser sources at radio wavelengths. Other systems in which UI has been predicted to occur are short period extrasolar terrestrial planets with conducting cores. If UI is present, circularly polarized radio emission is predicted to be emitted. We have searched for this predicted radio emission from short period binaries using the Very Large Array (VLA) and Australian Telescope Compact Array (ATCA). In one epoch, we find evidence for a radio source, coincident in position with the optical position of RX J0806+15. Although we cannot completely exclude that this is a chance alignment between the position of RX J0806+15 and an artefact in the data reduction process, the fact that it was detected at a significance level of 5.8σ and found to be transient suggests that it is more likely that RX J0806+15 is a transient radio source. We find an upper limit on the degree of circular polarization to be ∼50 per cent. The inferred brightness temperature exceeds 10¹⁸ K, which is too high for any known incoherent process, but is consistent with maser emission and UI being the driving mechanism. We did not detect radio emission from ES Cet, RX J1914+24 or Gliese 876.     

Detection of the optical counterpart of the proposed double degenerate polar RX J1914+24

We have detected the optical counterpart of the proposed double degenerate polar RX J1914+24. The I -band light curve is modulated on the 9.5-min period seen in X-rays. There is no evidence for any other periods. No significant modulation is seen in J . The infrared colours of RX J1914+24 are not consistent with a main-sequence dwarf secondary star. Our ASCA spectrum of RX J1914+24 is typical of a heavily absorbed polar and our ASCA light curve also shows only the 9.5-min period. We find that the folded I band and X-ray light curves are out of phase. We attribute the I -band flux to the irradiated face of the donor star. The long-term X-ray light curve shows a variation in the observed flux of up to an order of magnitude. These observations strengthen the view that RX J1914+24 is indeed the first double degenerate polar to be detected. In this light, we discuss the synchronizing mechanisms in such a close binary and other system parameters.     

The X-ray spectrum of RX J1914.4+2456 revisited

It has been proposed that RX J1914.4+2456 is a stellar binary system with an orbital period of 9.5 min. As such it shares many similar properties with RX J0806.3+1527 (5.4 min). However, while the X-ray spectrum of RX J0806.3+1527 can be modelled using a simple absorbed blackbody, the X-ray spectrum of RX J1914.4+2456 has proved difficult to fit using a physically plausible model. In this paper, we re-examine the available X-ray spectra of RX J1914.4+2456 taken using XMM–Newton . We find that the X-ray spectra can be fitted using a simple blackbody and an absorption component which has a significant enhancement of neon compared to the solar value. We propose that the material in the interbinary system is significantly enhanced with neon. This makes its intrinsic X-ray spectrum virtually identical to RX J0806.3+1527. We re-access the X-ray luminosity of RX J1914.4+2456 and the implications of these results.     

Optical studies of two Large Magellanic Cloud X-ray transients: RX J0544.1–7100 and RX J0520.5–6932

We report observations that confirm the identities of the optical counterparts to the transient sources RX J0544.1–7100 and RX J0520.5–6932 . The counterparts are suggested to be B-type stars. Optical data from the observations carried out at ESO and SAAO, together with results from the OGLE data base, are presented. In addition, X-ray data from the RXTE All-Sky Monitor are investigated for long-term periodicities. A strong suggestion for a binary period of 24.4 d is seen in RX J0520.5–6932 from the OGLE data.     

Isolated neutron stars: magnetic fields, distances,and spectra

We present timing measurements, astrometry, and high-resolution spectra of a number of nearby, thermally emitting, isolated neutron stars. We use these to infer magnetic field strengths and distances, but also encounter a number of puzzles. We discuss three specific ones in detail: (i) For RX J0720.4-3125 and RX J1308.6+2127, the characteristic ages are in excess of 1 Myr, while their temperatures and kinematic ages indicate that they are much younger; (ii) For RX J1856.5-3754, the brightness temperature for the optical emission is in excess of that measured at X-ray wavelengths for reasonable neutron-star radii; (iii) For RX J0720.4-3125, the spectrum changed from an initially featureless state to one with an absorption feature, yet there was only a relatively small change in T _eff. Furthermore, we attempt to see whether the spectra of all seven sources, in six of which absorption features have now been found, can be understood in the context of strongly magnetised hydrogen atmospheres. We find that the energies of the absorption features can be reproduced, but that it remains puzzling that, for J0720.4-3125 specifically, the spectrum was featureless in one state, and that, generally, the spectra do not have high-energy tails that are harder than the Wien-like ones obseved.      

Accretion column eclipses in the X-ray pulsars GX 1+4 and RX J0812.4−3114

Sharp dips observed in the pulse profiles of three X-ray pulsars (GX 1+4, RX J0812.4−3114 and A 0535+26) have previously been suggested to arise from partial eclipses of the emission region by the accretion column occurring once each rotation period. We present pulse-phase spectroscopy from Rossi X-ray Timing Explorer satellite observations of GX 1+4 and RX J0812.4−3114, which for the first time confirms this interpretation. The dip phase corresponds to the closest approach of the column axis to the line of sight, and the additional optical depth of photons escaping from the column in this direction gives rise to both the decrease in flux and increase in the fitted optical depth measured at this phase. Analysis of the arrival time of individual dips in GX 1+4 provides the first measurement of azimuthal wandering of a neutron star accretion column. The column longitude varies stochastically with a standard deviation ranging between 2° and 6° depending on the source luminosity. Measurements of the phase width of the dip both from mean pulse profiles and from the individual eclipses demonstrate that the dip width is proportional to the flux. The variation is consistent with that expected if the azimuthal extent of the accretion column depends only upon the Keplerian velocity at the inner disc radius, which varies as a consequence of the accretion rate M˙ .     

Discovery of an X-ray-selected radio-loud obscured AGN at z = 1.246

We have discovered an obscured active galaxy at redshift z  = 1.246 identified with the ROSAT X-ray source RX J1011.2+5545. We report on multiwavelength observations of this source and discuss its X-ray, optical and radio properties. This is the first X-ray-selected, obscured active galaxy at high redshift to be shown to be radio-loud, with a radio counterpart exhibiting a classical double-lobe morphology.     

The space density of cataclysmic variables: constraints from the ROSAT North Ecliptic Pole survey

We use the ROSAT North Ecliptic Pole (NEP) survey to construct a small, but purely X-ray flux-limited sample of cataclysmic variable stars (CVs). The sample includes only four systems, two of which (RX J1715.6+6856 and RX J1831.7+6511) are new discoveries. We present time-resolved spectroscopy of the new CVs and measure orbital periods of 1.64 ± 0.02 and 4.01 ± 0.03 h for RX J1715.6+6856 and RX J1831.7+6511, respectively. We also estimate distances for all the CVs in our sample, based mainly on their apparent brightness in the infrared. The space density of the CV population represented by our small sample is  1.1^+2.3_−0.7× 10⁻⁵ pc⁻³  . We can also place upper limits on the space density of any subpopulation of CVs too faint to be included in the NEP survey. In particular, we show that if the overall space density of CVs is as high as  2 × 10⁻⁴ pc⁻³  (as has been predicted theoretically), the vast majority of CVs must be fainter than   L _X≃ 2 × 10²⁹ erg s⁻¹  .     

X-ray and optical-to-infrared follow-up observations of the transient X-ray burster SAX J1810.8–2609

We have performed a ROSAT follow-up observation of the X-ray transient SAX J1810.8–2609 on 1998 March 24 and detected a bright X-ray source (named RX J1810.7–2609) which was not detected during the ROSAT all-sky survey in September 1990. Optical-to-infrared follow-up observations of the 10" radius ROSAT HRI X-ray error box revealed one variable object ( R =19.5±0.5 on March 13, R >21.5 on 1998 August 27) which we tentatively propose as the optical/IR counterpart of RX J1810.7–2609≡SAX J1810.8–2609.     

8个ROSAT X射线源和5G射电源被证认为类星体

１９９５年５月，使用２．１６ｍ望远镜对ＲＯＳＡＴＸ射线源进行光学证认期间，发现８个ＲＯＳＡＴＸ射线源和５Ｇ射电源的光学对应体是以前未发现的类星体，其红移范围从０．３３３至１．０９１。本文给出了这些类星体的多波段特性。     

Measuring proper motions of isolated neutron stars with Chandra

The excellent spatial resolution of the Chandra observatory offers the unprecedented possibility to measure proper motions at X-ray wavelength with relatively high accuracy using as reference the background of extragalactic or remote galactic X-ray sources. We took advantage of this capability to constrain the proper motion of RX J0806.4-4123 and RX J0420.0-5022, two X-ray bright and radio quiet isolated neutron stars (INSs) discovered by ROSAT and lacking an optical counterpart. In this paper, we present results from a preliminary analysis from which we derive 2σ upper limits of 76 mas/yr and 138 mas/yr on the proper motions of RX J0806.4-4123 and RX J0420.0-5022 respectively. We use these values together with those of other ROSAT discovered INSs to constrain the origin, distance and evolutionary status of this particular group of objects. We find that the tangential velocities of radio quiet ROSAT neutron stars are probably consistent with those of ‘normal’ pulsars. Their distribution on the sky and, for those having accurate proper motion vectors, their possible birth places, all point to a local population, probably created in the part of the Gould Belt nearest to the earth.      

The new intermediate polar RX J1238 − 38: a system below the period gap?

We present optical observations of the recently discovered ROSAT source RX J1238 − 38, which is a new member of the intermediate polar class of asynchronous magnetic cataclysmic variables (CVs). Optical photometry reveals two coherent periodicities at 1860 and 2147 s respectively, with similar amplitudes of ∼ 8 per cent. Infrared ( J -band) intensity variations are detected only at the 1860-s period, at an amplitude of ∼ 15 per cent. The initial hypothesis, that these two periods were the spin and synodic (i.e., beat) period respectively, appears not to be supported by the spectroscopic data. The emission lines vary on the longer photometric period, and radial velocity variations are detected at this period and at a longer period of ∼ 5300 s, which we identify as the spin and orbital periods respectively. The most likely explanation for the 1860-s period is that it is the first harmonic of the ω − Ω sideband, leading to an improved determination of the orbital period as 5077 s (= 84 min). If this interpretation is correct, RX J1238 − 38 joins EX Hya as the only other intermediate polar below the 2–3 h period gap, and with an orbital period close to the minimum for CVs with non-degenerate secondaries. The spin-modulated emission-line radial velocities and widths appear to be anticorrelated, with maximum width occurring at maximum blueshift. Such an anticorrelation is expected for aspect changes of accretion curtains. Polarimetric observations of RX J1238 − 38 were inconclusive, although we can put a limit of 0.4 per cent on any variability on the circular polarization, and certainly there is no indication of variations at the photometric or spectroscopic periods.     

X-ray bright sources in the Chandra Small Magellanic Cloud Wing Survey – detection of two new pulsars

We investigate the X-ray and optical properties of a sample of X-ray bright sources from the Small Magellanic Cloud (SMC) Wing Survey. We have detected two new pulsars with pulse periods of 65.8 s (CXOU J010712.6−723533) and 700 s (CXOU J010206.6−714115), and present observations of two previously known pulsars RX J0057.3−7325 (SXP101) and SAX J0103.2−7209 (SXP348). Our analysis has led to three new optical identifications for the detected pulsars. We find long-term optical periods for two of the pulsars, CXOU J010206.6−714115 and SXP101, of 267 and 21.9 d, respectively. Spectral analysis of a subset of the sample shows that the pulsars have harder spectra than the other sources detected. By employing a quantile-based colour–colour analysis we are able to separate the detected pulsars from the rest of the sample. Using archival catalogues we have been able to identify counterparts for the majority of the sources in our sample. Combining this with our results from the temporal analysis of the Chandra data and archival optical data, the X-ray spectral analysis, and by determining the X-ray to optical flux ratios we present preliminary classifications for the sources. In addition to the four detected pulsars, our sample includes two candidate foreground stars, 12 probable active galactic nuclei, and five unclassified sources.     

Periodicity Search in the X-ray Data of RX J0007.0+7302

Some unidentified EGRET sources have been reported to have probable X-ray counterparts. Periodicities in the X-ray data of those sources, if found, may help to strengthen the identification and to reveal their nature. We performed a detailed search of periodicities with a photon-counting method, the H-test, in the XMM and ASCA data of RX J0007.0+7302, which is the most probable X-ray counterpart to the EGRET source 3EG J0010+7309. Although no periods with enough significance were found, a possible one, at 0.1275433± 0.0000001 s (MJD 52327.03399), is quite intriguing based on results of cross-checking the two data sets. We suggest future analysis with other data to search the vicinity of this period.     

Synchrotron emission model of RX J1856.5-3754

A plasma emission model is presented interpreting the observational properties of RX J1856.5-3754. In particular, on the basis of the Vlasov’s kinetic equation we study the process of the quasi-linear diffusion (QLD) developed by means of the cyclotron instability. This mechanism provides simultaneous generation of optical and X-ray emission on the light cylinder scales, in one location of the pulsar magnetosphere. It is assumed that the observed X-ray spectrum of this source is generated via the synchrotron mechanism. A different approach of the synchrotron theory is considered, giving a spectral energy distribution that is in a good agreement with the observational data.     

Constraining the geometry of the neutron star RX J1856.5−3754

RX J1856.5−3754 is one of the brightest, nearby isolated neutron stars (NSs), and considerable observational resources have been devoted to its study. In previous work, we found that our latest models of a magnetic, hydrogen atmosphere match well the entire spectrum, from X-rays to optical (with best-fitting NS radius   R ≈ 14  km, gravitational redshift   z _g∼ 0.2  , and magnetic field   B ≈ 4 × 10¹²  G). A remaining puzzle is the non-detection of rotational modulation of the X-ray emission, despite extensive searches. The situation changed recently with XMM–Newton observations that uncovered 7-s pulsations at the     level. By comparing the predictions of our model (which includes simple dipolar-like surface distributions of magnetic field and temperature) with the observed brightness variations, we are able to constrain the geometry of RX J1856.5−3754, with one angle <6° and the other angle     , though the solutions are not definitive, given the observational and model uncertainties. These angles indicate a close alignment between the rotation and the magnetic axes or between the rotation axis and the observer. We discuss our results in the context of RX J1856.5−3754 being a normal radio pulsar and a candidate for observation by future X-ray polarization missions such as Constellation-X or XEUS .     

Correlated V/R and infrared photometric variations in the Be/X-ray binary LS I +61° 235/RX J0146.9+6121

We report on the long-term variability of the Be/X-ray binary LS I +61° 235/RX J0146.9+6121. New optical spectroscopic and infrared photometric observations confirm the presence of global one-armed oscillations in the circumstellar disc of the Be star, and allow us to derive a V R band quasi-period of 1240±30 d. Pronounced shell events, reminiscent of the spectacular variations in Be stars, are also seen. We have found that the J , H and K infrared photometric bands vary in correlation with the spectroscopic V R variations, implying that the one-armed disc oscillations are prograde. The effect of the oscillations is not only seen in the H α line but is also seen in the He  i λ 6678 and Paschen lines. As these lines are formed at different radii in the equatorial disc of the Be star, such effects confirm the global nature of the perturbation. The Keplerian disc has been found to be denser than the average for a sample of isolated Be stars, which may be indicative of some kind of interaction with the compact companion. Finally, from a Rossi X-ray Timing Explorer observation we derive a spin period of the neutron star of 1404.5±0.5 s.     

The optical counterparts to Be/X-ray binaries in the Magellanic Clouds

The fields of eight X-ray sources in the Magellanic Clouds believed to be Be/X-ray binaries have been searched for possible Be-star counterparts. BVR _c and H α CCD imaging was employed to identify early-type emission stars through colour indices and H α fluxes. Spectroscopy of five sources confirms the presence of H α emission in each case. Based on the positional coincidence of emission-line objects with the X-ray sources, we identify Be-star counterparts to the ROSAT sources RX J0032.9-7348, RX J0049.1-7250, RX J0054.9-7226 and RX J0101.0-7206, and to the recently discovered ASCA source AX J0051-722. We confirm the Be star nature of the counterpart to the HEAO1 source H0544-66. In the field of the ROSAT source RX J0051.8-7231 we find that there are three possible counterparts, each showing evidence for H α emission. We find a close double in the error circle of the EXOSAT source EXO 0531.1-6609, each component of which could be a Be star associated with the X-ray source.     

The identification of the optical/IR counterpart to the ROSAT SMC X-ray transient RX J0117.6−7330

We report observations which identify the optical/IR counterpart to the ROSAT X-ray transient RX J0117.6−7330. The counterpart is suggested to be a B1–B2 star (luminosity class III–V) showing an IR excess and strong Balmer emission lines. The distance derived from reddening and systemic velocity measurements is consistent with the distance derived from X-ray measurements and puts the source in the Small Magellanic Cloud (SMC).     

Discovery of the high-field polar RX J1724.0+4114

We report the discovery of a new AM Herculis binary (polar) as the optical counterpart of the soft X-ray source RX J1724.0+4114 detected during the ROSAT all-sky survey. The magnetic nature of this V  ∼ 17 mag object is confirmed by low-resolution spectroscopy showing strong Balmer and He  II emission lines superimposed on a blue continuum, which is deeply modulated by cyclotron humps. The inferred magnetic field strength is 50 ± 4 MG (or possibly even ≈ 70 MG). Photometric observations spanning ∼ 3 yr reveal a period of 119.9 min, directly below the period gap. The morphology of the optical and X-ray light curves, which do not show eclipses by the secondary star, suggests a self-eclipsing geometry. We derive a lower limit on the distance of d  ≳ 250 pc.