Showing papers by "M. H. van Kerkwijk published in 2001"
TL;DR: In this paper, the radial velocities of HD 77581, the B-supergiant companion of the X-ray pulsar Vela X-1, were measured using high-resolution optical spectra obtained in a nine-month campaign.
Abstract: We measured the radial-velocity curve of HD 77581, the B-supergiant companion of the X-ray pulsar Vela X-1, using 183 high-resolution optical spectra obtained in a nine-month campaign. We derive radial-velocity amplitudes for dierent lines and wavelength regions, and nd all are consistent with each other, as well as with values found in previous analyses. We show that one apparent exception, an anomalously low value derived from ultra-violet spectra obtained with the International Ultraviolet Explorer, was due to an error in the analysis procedures. We re-analyse all IUE spectra, and combine the resulting velocities with the ones derived from the new optical spectra presented here, as well as those derived from optical spectra published earlier. As in previous analyses, the radial velocities show strong deviations from those expected for a pure Keplerian orbit, with root- mean-square amplitudes of 7k m s 1 for strong lines of Si iv and N iii near 4100 A, and up to20 km s 1 for weaker lines of N ii and Al iii near 5700 A. The deviations likely are related to the pronounced line-prole variations seen in our spectra. Our hope was that the deviations would average out when a sucient number of spectra were added together. It turns out, however, that systematic deviations as a function of orbital phase are present as well, at the 3 km s 1 level, with the largest deviations occurring near inferior conjunction of the neutron star and near the phase of maximum approaching velocity. While the former might be due to a photo-ionisation wake, for which we observe direct evidence in the proles of H and H, the latter has no straightforward explanation. As a result, our best estimate of the radial-velocity amplitude, Kopt =2 1:7 1: 6k m s 1 , has an uncertainty not much reduced to that found in previous analyses, in which the influence of the systematic, phase-locked deviations had not been taken into account. Combining our velocity amplitude with the accurate orbital elements of the X-ray pulsar, we infer Mns sin 3 i =1 :78 0:15 M ??? .
141 citations
TL;DR: In this article, the authors present spectroscopy and Hα imaging of a faint nebula surrounding the X-ray bright, nearby neutron star RX J1856.5-3754.
Abstract: We present spectroscopy and Hα imaging of a faint nebula surrounding the X-ray bright, nearby neutron star RX J1856.5-3754 . The nebula shows no strong lines other than the hydrogen Balmer lines and has a cometary-like morphology, with the apex being approximately 1" ahead of the neutron star, and the tail extending up to at least 25" behind it. We find that the current observations can be satisfactorily accounted for by two different models. In the first, the nebula is similar to "Balmer-dominated" cometary nebulae seen around several radio pulsars, and is due to a bow shock in the ambient gas arising from the supersonic motion of a neutron star with a relativistic wind. In this case, the emission arises from shocked ambient gas; we find that the observations require an ambient neutral hydrogen number density n_(H^0) ≃ 0.8 cm^(-3) and a rotational energy loss Ė ≃ 6 × 10^(31) erg s^(-1). In the second model, the nebula is an ionisation nebula, but of a type not observed before (though expected to exist), in which the ionisation and heating are very rapid compared to recombination and cooling. Because of the hard ionising photons, the plasma is heated up to ~70 000 K and the emission is dominated by collisional excitation. The cometary morphology arises naturally as a consequence of the lack of emission from the plasma near and behind the neutron star (which is ionised completely) and of thermal expansion. We confirm this using a detailed hydrodynamical simulation. We find that to reproduce the observations for this case, the neutral hydrogen number density should be n_(H^0) ≃ 3 cm^(-3) and the extreme ultraviolet flux of the neutron star should be slightly in excess, by a factor ~1.7, over what is expected from a black-body fit to the optical and X-ray fluxes of the source. For this case, the rotational energy loss is less than 2 × 10^(32) erg s^(-1). Independent of the model, we find that RX J1856.5-3754 is not kept hot by accretion. If it is young and cooling, the lack of pulsations at X-ray wavelengths is puzzling. Using phenomenological arguments, we suggest that RX J1856.5-3754 may have a relatively weak, few 10^(11) G, magnetic field. If so, it would be ironic that the two brightest nearby neutron stars, RX J1856.5-3754 and RX J0720.4-3125 , may well represent the extreme ends of the neutron star magnetic field distribution, one a weak field neutron star and another a magnetar.
95 citations
TL;DR: In this article, an asymmetry in the optical light curve of the pulsar PSR J2051-0827 has been observed and it is shown that the system is only mildly inclined (i ~ 40°) and the unilluminated side of the companion has a temperature likely less than 3000 K.
Abstract: Hubble Space Telescope observations of the companion to the eclipsing millisecond pulsar PSR J2051-0827 have revealed its "dark" side. The R magnitude at minimum is ~26, while the difference between the side heated by the impinging pulsar radiation and the unirradiated side is ~3.3 mag. For the first time, these data show that there is an asymmetry in the optical light curve of the companion. Furthermore, significant variability is seen in the companion brightness measured after optical maximum on successive orbits. The data are modeled by a gravitationally distorted low-mass secondary star that is irradiated by the pulsar wind. These model fits indicate that the system is only mildly inclined (i ~ 40°) and the unilluminated side of the companion has a temperature likely less than 3000 K.
72 citations
TL;DR: In this article, the authors present near-infrared and optical observations of the anomalous X-ray pulsar 1E 2259+586 taken with the Keck telescope and derive a subarcsecond Chandra position and tie it to our optical reference frame using other stars in the field.
Abstract: We present near-infrared and optical observations of the field of the anomalous X-ray pulsar 1E 2259+586 taken with the Keck telescope. We derive a subarcsecond Chandra position and tie it to our optical reference frame using other stars in the field. We find a very faint source, Ks = 21.7 ± 0.2 mag, with a position coincident with the Chandra position. We argue that this is the counterpart. In the J, I, and R bands, we derive (2 σ) limits of 23.8, 25.6, and 26.4 mag, respectively. As with 4U 0142+61, for which a counterpart has previously been found, our results are inconsistent with models in which the source is powered by accretion from a disk but may be consistent with the magnetar model.
71 citations
TL;DR: In this paper, the authors present near-infrared and optical observations of the field of the Anomalous X-ray Pulsar 1E 2259+58.6 taken with the Keck telescope.
Abstract: We present near-infrared and optical observations of the field of the Anomalous X-ray Pulsar 1E 2259+58.6 taken with the Keck telescope. We derive a subarcsecond Chandra position and tie it to our optical reference frame using other stars in the field. We find a very faint source, Ks = 21.7\pm0.2 mag, with a position coincident with the Chandra position. We argue that this is the counterpart. In the J, I, and R bands, we derive (2 sigma) limits of 23.8, 25.6 and 26.4 mag, respectively. As with 4U 0142+61, for which a counterpart has previously been found, our results are inconsistent with models in which the source is powered by accretion from a disk, but may be consistent with the magnetar model.
68 citations
TL;DR: In this paper, the authors present spectroscopy and imaging with the Very Large Telescope (VLT) of the neutron star RX J1856, and they find that the entire optical to UV spectral energy distribution is well described by a slightly reddened Rayleigh-Jeans tail (f_λ = (2.96 ± 0.06) x 10^(-19) (λ/5000˚A)^(−4) 10^((−0.4)(0.12± 0.05)
Abstract: We present spectroscopy and imaging with the Very Large Telescope (VLT) of the neutron star RX J1856.5-3754. Little is known about the nature of this source other than that it is a nearby hot neutron star. Our VLT spectrum does not show any strong emission or absorption features. With considerable care to photometric calibration, we obtain photometric measurements over the optical and ultra-violet (UV) using our VLT observations and a detailed analysis of archival Hubble Space Telescope data. We find that the entire optical to UV spectral energy distribution is well described by a slightly reddened Rayleigh-Jeans tail (f_λ = (2.96 ± 0.06) x 10^(-19) (λ/5000˚A)^(−4) 10^((−0.4)(0.12±0.05)(A_λ/A_V −1.138)) erg s^(−1) cm^(−2) ˚A^(−1), where A_λ/A_V, where is the reddening curve; implied V = 25.58 ± 0.02). The reddening is consistent with the interstellar absorption inferred from X-ray spectroscopy. The simplest explanation for this Rayleigh-Jeans emission is that the optical-UV radiation arises from thermal emission from the surface of the neutron star. The high degree to which the data conform to the Rayleigh-Jeans tail significantly limits contributions from other sources of emission. In particular, our observations are inconsistent with the presence of an accretion disk and also strongly constrain the amount of magnetospheric emission from this enigmatic neutron star.
68 citations
TL;DR: In this paper, an optical counterpart to a transient in a globular cluster is securely identified, based on a short target-of-opportunity observation with the Chandra X-Ray Observatory and is associated with the brightest of 24 X-ray sources detected during quiescence in 2000 July with Chandra.
Abstract: After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in 2001 August, as noted with the Rossi X-Ray Timing ExplorerAll-Sky Monitor. We carried out a short target-of-opportunity observation with the Chandra X-Ray Observatoryand are able to associate the transient with the brightest of 24 X-ray sources detected during quiescence in 2000 July with Chandra. Furthermore, we securely identify the optical counterpart and determine that the 1998 X-ray outburst in NGC 6440 was from the same object. This is the first time that an optical counterpart to a transient in a globular cluster is securely identified. Since the transient is a type I X-ray burster, it is established that the compact accretor is a neutron star. Thus, this transient provides an ideal case to study the quiescent emission in the optical and X-ray of a transiently accreting neutron star while knowing the distance and reddening accurately. One model that fits the quiescent spectrum is an absorbed power-law plus neutron star hydrogen atmosphere model. We find an intrinsic neutron star radius of km and an unabsorbed bolometric luminosity for the neutron star atmosphere of 31 1712 ergs s that is consistent with predictions for a cooling neutron star.
61 citations
TL;DR: In this paper, the authors present spectroscopy and imaging with the Very Large Telescope (VLT) of the neutron star RX J1856, showing that the entire optical to UV spectral energy distribution is well described by a slightly reddened Rayleigh-Jeans tail.
Abstract: We present spectroscopy and imaging with the Very Large Telescope (VLT) of the neutron star RX J1856.5-2754. Little is known about the nature of this source other than that it is a nearby hot neutron star. Our VLT spectrum does not show any strong emission or absorption features. With considerable care to photometric calibration, we obtain photometric measurements over the optical and ultra-violet (UV) using our VLT observations and a detailed analysis of archival {\em Hubble Space Telescope} data. We find that the entire optical to UV spectral energy distribution is well described by a slightly reddened Rayleigh-Jeans tail ($f_\lambda=(2.96\pm0.06)\times10^{-19} (\lambda/5000\un{\AA})^{-4} 10^{-0.4(0.12\pm0.05)(A_\lambda/A_V-1.138)} \un{erg}\un{s^{-1}}\un{cm^{-2}}\un{\AA^{-1}}$, where $A_\lambda/A_V$ is the reddening curve; implied $V=25.58\pm0.02$). The reddening is consistent with the interstellar absorption inferred from X-ray spectroscopy. The simplest explanation for this Rayleigh-Jeans emission is that the optical-UV radiation arises from thermal emission from the surface of the neutron star. The high degree to which the data conform to the Rayleigh-Jeans tail significantly limits contributions from other sources of emission. In particular, our observations are inconsistent with the presence of an accretion disk and also strongly constrain the amount of magnetospheric emission from this enigmatic neutron star.
60 citations
TL;DR: In this article, the authors show that deep optical observations of SGR 0526-66, the only SGR with small enough optical extinction for meaningful observations, show no evidence of an optical counterpart.
Abstract: Soft γ-ray repeaters (SGRs) are among the most enigmatic sources known today. Exhibiting huge X-ray and γ-ray bursts and flares, as well as soft quiescent X-ray emission, their energy source remains a mystery. Just as mysterious are the anomalous X-ray pulsars (AXPs), which share many of the same characteristics. Thanks to recent Chandra X-Ray Observatory observations, SGR 0526-66, the first SGR, now appears to be a transition object bridging the two classes, and therefore observations of it have implications for both SGRs and AXPs. The two most popular current models for their persistent emission are accretion of a fossil disk and decay of an enormous (~1015 G) magnetic field in a magnetar. We show how deep optical observations of SGR 0526-66, the only SGR with small enough optical extinction for meaningful observations, show no evidence of an optical counterpart. These observation place strong new constraints on both accretion disk and magnetar models and suggest that the spectral energy distribution may peak in the hard UV. Almost all accretion disks are excluded by the optical data, and a magnetar would require a ~1015-1016 G field.
50 citations
TL;DR: In this paper, an optical counterpart to a transient in a globular cluster is securely identified, which is the first time that the optical counterpart of an X-ray burster has been identified.
Abstract: After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in August 2001, as noted with the RXTE All-Sky Monitor. We carried out a short target of opportunity observation with the Chandra X-ray Observatory and are able to associate the transient with the brightest of 24 X-ray sources detected during quiescence in July 2000 with Chandra. Furthermore, we securely identify the optical counterpart and determine that the 1998 X-ray outburst in NGC 6440 was from the same object. This is the first time that an optical counterpart to a transient in a globular cluster is securely identified. Since the transient is a type I X-ray burster, it is established that the compact accretor is a neutron star. Thus, this transient provides an ideal case to study the quiescent emission in the optical and X-ray of a transiently accreting neutron star while knowing the distance and reddening accurately. One model that fits the quiescent spectrum is an absorbed power law plus neutron star hydrogen atmosphere model. We find an intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is consistent with predictions for a cooling neutron star.
47 citations
TL;DR: In this article, the authors show that deep optical observations of SGR 0526-66, the only SGR with small enough optical extinction for meaningful observations, show no evidence of an optical counterpart.
Abstract: Soft Gamma-ray Repeaters (SGRs) are among the most enigmatic sources known today. Exhibiting huge X- and Gamma-ray bursts and flares, as well as soft quiescent X-ray emission, their energy source remains a mystery. Just as mysterious are the Anomalous X-ray pulsars (AXPs), which share many of the same characteristics. Thanks to recent Chandra observations, SGR 0526-66, the first SGR, now appears to be a transition object bridging the two classes, and therefore observations of it have implications for both SGRs and AXPs. The two most popular current models for their persistent emission are accretion of a fossil disk or decay of an enormous (~10^15 G) magnetic field in a magnetar. We show how deep optical observations of SGR 0526-66, the only SGR with small enough optical extinction for meaningful observations, show no evidence of an optical counterpart. These observation place strong new constraints on both accretion disk and magnetar models, and suggest that the spectral energy distribution may peak in the hard-UV. Almost all accretion disks are excluded by the optical data, and a magnetar would require a ~10^15-10^16 G field.
TL;DR: In this article, the radio error box of the soft X-ray tran- sient 4U 1630 47 during its 1998 outburst was investigated and a variable source at K = 1 6:1 mag located inside the error circle was identified.
Abstract: We present K band photometry of the region including the radio error box of the soft X-ray tran- sient 4U 1630 47 during its 1998 outburst. We detect a variable source at K =1 6:1 mag located inside the radio error circle which we identify as the counterpart to the X-ray source. We discuss the properties of the source, and conclude that it is most likely a black-hole X-ray binary similar to 4U 1543 47, GRO J1655 40 or SAX J1819.3 2525, containing a relatively early-type secondary.
TL;DR: In this paper, the radial velocities of the B-supergiant companion of the X-ray pulsar Vela X-1 were measured using high-resolution optical spectra obtained in a nine-month campaign.
Abstract: We measured the radial-velocity curve of HD77581, the B-supergiant companion of the X-ray pulsar Vela X-1, using 183 high-resolution optical spectra obtained in a nine-month campaign. We derive radial-velocity amplitudes for different lines and wavelength regions, and find all are consistent with each other, as well as with values found in previous analyses. We show that one apparent exception, an anomalously low value derived from ultra-violet spectra obtained with the IUE, was due to an error in the analysis procedures. We re-analyse all IUE spectra, and combine the resulting velocities with the ones derived from the new optical spectra presented here, as well as those derived from optical spectra published earlier. As in previous analyses, the radial velocities show strong deviations from those expected for a pure Keplerian orbit. The deviations likely are related to the pronounced line-profile variations seen in our spectra. It turns out that systematic deviations as a function of orbital phase are present. Our best estimate of the radial-velocity amplitude, Kopt = 21.7 +/- 1.6 km/s, has an uncertainty not much reduced to that found in previous analyses, in which the systematic deviations had not been taken into account. Combining our velocity amplitude with the accurate orbital elements of the X-ray pulsar, we infer M_ns sin^3i = 1.78 +/- 0.15 Msun.
TL;DR: In this article, K band photometry of the region including the radio error box of the soft X-ray transient 4U 1630-47 during its 1998 outburst is presented.
Abstract: We present K band photometry of the region including the radio error box of the soft X-ray transient 4U 1630-47 during its 1998 outburst. We detect a variable source at K=16.1 mag located inside the radio error circle which we identify as the counterpart to the X-ray source. We discuss the properties of the source, and conclude that it is most likely a black-hole X-ray binary similar to 4U 1543-47, GRO J1655-40 or SAX J1819.3-2525, containing a relatively early-type secondary.
TL;DR: In this paper, the authors used the same HST data and found the radius of RX J1856.5-3754 is R{infty}=15+/-6 km, which is smaller than the radii calculated from all proposed equations of state (EOS) of dense baryonic matter.
Abstract: RX J1856.5-3754, a bright soft X-ray source believed to be the nearest thermally emitting neutron star, has commanded and continues to command intense interest from X-ray missions. One of the main goals is to determine the radius of this neutron star. An integral part of the determination is an accurate parallax. Walter (2001) analyzed Hubble Space Telescope (HST) data and derived a parallax, pi=16.5+/-2.3 mas. Combining this distance with the angular radius derived from blackbody fits to observations of RX J1856.5-3754 with ROSAT, EUVE, HST, Pons et al. (2001) derived an observed radius ("radiation radius"), R_{infty}= 7 km. This value is smaller than the radii calculated from all proposed equations-of-state (EOS) of dense baryonic matter (Haensel 2001). Here, we have analyzed the same HST data and find pi=7+/-2 mas. We have verified our result using a number of different, independent techniques, and find the result to be robust. Adopting our parallax, the radius of RX J1856.5-3754 is R_{infty}=15+/-6 km. This radius falls squarely in the range of radii, 12--16 km, expected from calculations of neutron star structure for different equations of state. With additional HST observations, the parallax estimate can be improved to the point by which the inferred radius can constrain the choice of EOS.
TL;DR: In this article, HST/STIS, Keck J-and K_s-band, and VLA 332-MHz and 1400-MHz images of the region around the Soft Gamma-ray Repeater SGR 1900+14 were presented.
Abstract: We present HST/STIS, Keck J- and K_s-band, and VLA 332-MHz and 1400-MHz images of the region around the Soft Gamma-ray Repeater SGR 1900+14. No non-stellar sources were detected at the position of SGR 1900+14, giving 3-sigma limits of S_{332 MHz} 29.0 mag, J > 22.8 mag, K_s > 20.8 mag (point sources), and S_{332 MHz} < 6.1 mJy arcmin^{-2}, S_{1.4 GHz} < 6.2 mJy arcmin^{-2} (extended emission). Given the very high extinction, the STIS and J-band non-detections are not constraining, but the K_s-band limit rules out basic accretion disk models for SGR 1900+14, and is almost comparable in depth to the K_s-band detection for the anomalous X-ray pulsar 4U 0142+61. Finally, we report the detection in this field of three new candidate supernova remnants, SNRs G043.5+00.6, G042.0-00.1, and G041.5+00.4.