P.G. Jonker

Optical spectra of the carbon–oxygen accretion discs in the ultra‐compact X‐ray binaries 4U 0614+09, 4U 1543−624 and 2S 0918−549

We present optical spectra in the range 4600‐8600 ˚ A for three low-mass X-ray binaries which have been suggested to belong to the class of ultra-compact X-ray binaries based on their X-ray spectra. Our spectra show no evidence for hydrogen or helium emission lines, as are seen in classical X-ray binaries. The spectrum of 4U 0614+09 does show emission lines, which we identify with carbon and oxygen lines of C II ,C III ,O II and O III. While the spectra of 4U 1543−624 and 2S 0918−549 have a lower signal-to-noise ratio, and thus are more difficult to interpret, some of the characteristic features of 4U 0614+09 are present in these spectra too, although sometimes they are clearly weaker. We conclude that the optical spectra give further evidence for the ultra-compact nature of these X-ray binaries and for their donor stars being carbon‐oxygen white dwarfs.

Kilohertz quasi-periodic oscillations difference frequency exceeds inferred spin frequency in 4U 1636-53

Recent observations of the low-mass X-ray binary 4U 1636−53 with the Rossi X-ray Timing Explorer show, for the first time, a kilohertz quasi-periodic oscillation (kHz QPO) peak separation that exceeds the neutron star spin frequency as inferred from burst oscillations. This strongly challenges the sonic-point beat-frequency model for the kHz QPOs found in low-mass X-ray binaries. We detect two simultaneous kHz QPOs with a frequency separation of 323.3 ± 4.3 Hz in an average Fourier power spectrum of observations obtained in 2001 September and 2002 January. The lower kHz QPO frequency varied between 644 and 769 Hz. In previous observations of this source the peak separation frequency was ∼250 Hz when the lower kHz QPO frequency was ∼900 Hz. Burst oscillations occur in 4U 1636−53 at ∼581 Hz and also possibly at half that frequency (290.5 Hz). This is the first source where the peak separation frequency is observed to change from less than (half) the burst oscillation frequency to more than that. This observation contradicts all previously formulated implementations of the sonic-point beat-frequency model except those where the disc in 4U 1636−53 switches from prograde to retrograde.

Radio detections of the neutron star X‐ray binaries 4U 1820 − 30 and Ser X‐1 in soft X‐ray states

We present the analysis of simultaneous X-ray (RXTE) and radio (VLA) observations of two atoll-type neutron star X-ray binaries: 4U 1820 − 30 and Ser X-1. Both sources were steadily in the soft (‘banana’) X-ray state during the observations. We have detected the radio counterpart of 4U 1820 − 30 at 4.86 and 8.46 GHz at a flux density of ∼0.1 mJy. This radio source is positionally coincident with the radio pulsar PSR 1820 − 30A. However, the radio emission of the pulsar falls rapidly with frequency (∝ ν −3 ), and we argue that the radio emission of the X-ray binary is dominant above ∼2 GHz. Supporting this interpretation, comparison with previous observations reveals variability at the higher radio frequencies that is likely to be due to the X-ray binary. We have detected for the first time the radio counterpart of Ser X-1 at 8.46 GHz, also at a flux density of ∼0.1 mJy. The position of the radio counterpart has allowed us to identify its optical counterpart unambiguously. We briefly discuss similarities and differences between the disc‐jet coupling in neutron star and black hole X-ray binaries. In particular, we draw attention to the fact that, contrary to other states, neutron star X-ray binaries seem to be more radio-loud than persistent black hole candidates when the emission is ‘quenched’ in the soft state.

The mass of the neutron star in the low‐mass X‐ray binary 2A 1822 − 371

Using phase-resolved spectroscopic observations obtained with the Ultraviolet and Visual Echelle Spectrograph on the ESO Kueyen Very Large Telescope, supplemented by spectroscopic observations obtained with the Boller and Chivens spectrograph on the Walter Baade Magellan Telescope, we have found sinusoidal radial velocity variations with a semi-amplitude of 327 ± 17 km s −1 . From previous observations and from the fact that the epoch of minimum velocity arrived early with respect to the epoch calculated from pulse timing, we know that the companion star is suffering from irradiation. Since we most likely observed primarily the side of the companion star facing the observer at phase ∼0.75, the velocity quoted above is not the true radial velocity semi-amplitude of the companion star. Assuming a uniform contribution to the line profile from this hemisphere yields a radial velocity semi-amplitude of 280 ± 26 km s −1 for a systemic velocity of 54 ± 24 km s −1 ; if the contribution is instead weighted somewhat more towards the side of the companion facing the X-ray source then the true semi-amplitude is larger than this value. Together with the well-constrained inclination (81 ◦ < i < 84 ◦ ) and the mass function determined from pulse-timing analysis (2.03 ± 0.03 × 10 −2 M� ), we derive a lower limit to the mass of the neutron star and to that of the companion star of 0.97 ± 0.24 and 0.33 ± 0.05 M� , respectively (1σ ; including uncertainties in the inclination). We briefly discuss other aspects of the spectrum and the implications of our findings.

Chandra observations of the neutron star soft X-ray transient RX J170930.2 − 263927 returning to quiescence

We present our analysis of Chandra observations obtained when the soft X-ray transient RX J170930.2 - 263927 (XTE J1709 - 267) returned to quiescence after an outburst. Using the type I burst peak luminosity found by Cocchi et al. and the value of NH we derived from our spectral fits, the distance to RX J170930.2 - 263927 can be constrained to 2.5-10 kpc. RX J170930.2 - 263927 is probably associated with the low-metallicity globular cluster NGC 6293, which has a tidal radius of 14.2 arcmin, as the projected distance to the centre of the cluster is approximately 25 pc (9-10 arcmin). If the association is correct, RX J170930.2 - 263927 would be at ~8.5 kpc. We determined that Loutburst/Lquiescence>~ 105 for this outburst. If the quiescent luminosity is caused by cooling of the neutron star core then enhanced core cooling processes were at work, implying a neutron star mass of ~1.7-1.8 Msolar. Combining our Chandra observations with archival ROSAT observations we show that the source very probably exhibits periods of sustained low-level accretion. Variable, low-level activity could provide an alternative explanation for some of the sources in the recently proposed category of faint soft X-ray transients. We found excess emission at ~0.6 keV. If such an excess is a unique feature of ultracompact systems, as was recently proposed, RX J170930.2 - 263927 must have a short orbital period as well. From the constraints on the distance and the non-detection of the optical counterpart with mV <20.5, we conclude that this system must have a low-mass companion.

The faint neutron star soft X-ray transient SAX J1810.8-2609 in quiescence

We present the analysis of a 35-ksec-long Chandra observation of the neutron star soft X-ray transient (SXT) SAX J1810.8-2609. We detect three sources in the field of view. The position of one of them is consistent with the location of the ROSAT error circle of SAX J1810.8-2609. The accurate Chandra position of that source coincides with the position of the proposed optical counterpart, strengthening the identification as the counterpart. We detected the neutron star SXT system in quiescence at an unabsorbed luminosity of similar to1 x 10(32) erg s(-1) (assuming a distance of 4.9 kpc). This luminosity is at the low end of quiescent luminosities found in other neutron star SXTs. This renders support to the existence of a group of faint soft X-ray transients of which the accreting millisecond X-ray pulsar SAX J1808.4-3658 is the most prominent member. The quiescent spectrum of SAX J1810.8-2609 is well-fitted with an absorbed power law with a photon index of 3.3 +/- 0.5. With a value of 3.3 x 10 21 cm 2, the galactic absorption is consistent with the value derived in outburst. Because the spectra of quiescent neutron star SXTs are often fitted with an absorbed blackbody or neutron star atmosphere plus power-law model, we also fitted the spectrum using those fitting functions. Both models provide a good fit to the data. If cooling of the neutron star core and/or crust is responsible for the soft part of the spectrum, the time-averaged mass accretion rate must have been very low (similar to5.7 x 10(13) M-circle dot yr(-1); assuming standard core cooling only) or the neutron star must be massive. We also discuss the possibility that the thermal spectral component in neutron stars in quiescence is produced by residual accretion.

Chandra Observations of the Faintest Low-Mass X-ray Binaries

A group of persistently faint Galactic X-ray sources exist that, based on their location in the Galaxy, high LX/Lopt, association with X-ray bursts, and absence of low-frequency X-ray pulsations, are thought to be low-mass X-ray binaries (LMXBs). We present results from Chandra observations for eight of these systems: 4U 1708-408, 2S 1711-339, KS 1739-304, SLX 1735-269, GRS 1736-297, SLX 1746-331, 1E 1746.7-3224, and 4U 1812-12. Locations for all these sources, excluding GRS 1736-297, SLX 1746-331, and KS 1739-304 (which were not detected), were improved to 06 error circles (90% confidence). Our observations support earlier findings of transient behavior of GRS 1736-297, KS 1739-304, SLX 1746-331, and 2S 1711-339 (which we detect in one of two observations). Energy spectra for 4U 1708-408, 2S 1711-339, SLX 1735-269, 1E 1746.7-3224, and 4U 1812-12 are hard, with power-law indices typically 1.4-2.1, which is consistent with typical faint LMXB spectra.

Wide-Band, Low-Frequency Pulse Profiles of 100 Radio Pulsars with LOFAR

Context. LOFAR offers the unique capability of observing pulsars across the 10−240 MHz frequency range with a fractional bandwidth of roughly 50%. This spectral range is well suited for studying the frequency evolution of pulse profile morphology caused by both intrinsic and extrinsic effects such as changing emission altitude in the pulsar magnetosphere or scatter broadening by the interstellar medium, respectively. Aims. The magnitude of most of these effects increases rapidly towards low frequencies. LOFAR can thus address a number of open questions about the nature of radio pulsar emission and its propagation through the interstellar medium. Methods. We present the average pulse profiles of 100 pulsars observed in the two LOFAR frequency bands: high band (120–167 MHz, 100 profiles) and low band (15–62 MHz, 26 profiles). We compare them with Westerbork Synthesis Radio Telescope (WSRT) and Lovell Telescope observations at higher frequencies (350 and 1400 MHz) to study the profile evolution. The profiles were aligned in absolute phase by folding with a new set of timing solutions from the Lovell Telescope, which we present along with precise dispersion measures obtained with LOFAR. Results. We find that the profile evolution with decreasing radio frequency does not follow a specific trend; depending on the geometry of the pulsar, new components can enter into or be hidden from view. Nonetheless, in general our observations confirm the widening of pulsar profiles at low frequencies, as expected from radius-to-frequency mapping or birefringence theories.

Chandra localisation and optical/NIR follow-up of Galactic X-ray sources

We investigate a sample of eleven Galactic X-ray sources recently discovered with INTEGRAL or RXTE with the goal of identifying their optical and/or near-infrared (NIR) counterpart. For this purpose new Chandra positions of nine objects are presented together with follow-up observations of all the targets in the optical and NIR. For the four sources IGR J16194-2810, IGRJ 16479-4514, IGR J16500-3307 and IGR J19308+530, the Chandra position confirms an existing association with an optical/NIR object, while for two sources (XTE J1716-389 and 18490-0000) it rules out previously proposed counterparts indicating new ones. In the case of IGR J17597-220, a counterpart is selected out of the several possibilities proposed in the literature and we present the first association with an optical/NIR source for J16293-4603 and XTE J1743-363. Moreover, optical/NIR observations are reported for XTE J1710-281 and IGR J17254-3257: we investigate the counterpart to the X-ray sources based on their XMM-Newton positions. We discuss the nature of each system considering its optical/NIR and X-ray properties.

Extended X‐ray emission in the high‐redshift quasar GB 1508+5714 at z= 4.3

We report the discovery of extended X-ray emission around the powerful high-redshift quasar GB 1508+5714 at = 4.3, revealed in a long Chandra ACIS observation. The emission feature is 3-4 arcsec away from the quasar core,which corresponds to a projected distance of about 25 kpc. The X-ray spectrum is best fitted with a power law of photon index 1.92 ′ 0.35 (90 per cent confidence limit). The X-ray flux and luminosity reach 9.2 x 10 - 1 5 erg cm - 2 s - 1 (0.5-8 keV) and 1.6 x 10 4 5 erg s - 1 (2.7-42.4 keV rest frame, Ω Λ = 0.73, Ω m = 0.27, H 0 = 71 km s - 1 Mpc - 1 ), which is about 2 per cent of the total X-ray emission of the quasar. We interpret the X-ray emission as inverse Compton scattering of cosmic microwave background photons. The scattering relativistic electron population could either be a quasi-static diffuse cloud fed by the jet, or an outer extension of the jet with a high bulk Lorentz factor. We argue that the lack of an obvious detection of radio emission from the extended component could be a consequence of Compton losses on the electron population, or of a low magnetic field. Extended X-ray emission produced by inverse Compton scattering may be common around high-redshift radio galaxies and quasars, demonstrating that significant power is injected into their surroundings by powerful jets.