Erik Kuulkers

Photospheric radius expansion X-ray bursts as standard candles

We examined the maximum bolometric peak luminosities during type I X-ray bursts from the persistent or transient luminous X-ray sources in globular clusters. We show that for about two thirds of the sources the maximum peak luminosities during photospheric radius expansion X-ray bursts extend to a critical value of 3:790:1510 38 erg s 1 , assuming the total X-ray burst emission is entirely due to black-body radiation and the recorded maximum luminosity is the actual peak luminosity. This empirical critical luminosity is consistent with the Eddington luminosity limit for hydrogen poor material. Since the critical luminosity is more or less always reached during photospheric radius expansion X-ray bursts (except for one source), such bursts may be regarded as empirical standard candles. However, because significant deviations do occur, our standard candle is only accurate to within 15%. We re-evaluated the distances to the twelve globular clusters in which the X-ray bursters reside.

A black hole in the superluminal source sax j1819.3-2525 (v4641 sgr)

Spectroscopic observations of the fast X-ray transient and superluminal jet source SAX J1819.3-2525 (V4641 Sgr) reveal a best-fitting period of Pspect = 2.81678 ± 0.00056 days and a semiamplitude of K2 = 211.0 ± 3.1 km s-1. The optical mass function is f(M) = 2.74 ± 0.12 M☉. We find a photometric period of Pphoto = 2.81730 ± 0.00001 days using a light curve measured from photographic plates. The folded light curve resembles an ellipsoidal light curve with two maxima of roughly equal height and two minima of unequal depth per orbital cycle. The secondary star is a late B-type star that has evolved off the main sequence. Using a moderate resolution spectrum (R = 7000) we measure Teff = 10500 ± 200 K, log g = 3.5 ± 0.1, and Vrot sin i = 123 ± 4 km s-1 (1 σ errors). Assuming synchronous rotation, our measured value of the projected rotational velocity implies a mass ratio of Q ≡ M1/M2 = 1.50 ± 0.08 (1 σ). The lack of X-ray eclipses implies an upper limit to the inclination of i ≤ 707. On the other hand, the large amplitude of the folded light curve (≈0.5 mag) implies a large inclination (i 60°). Using the above mass function, mass ratio, and inclination range, the mass of the compact object is in the range 8.73 ≤ M1 ≤ 11.70 M☉ and the mass of the secondary star is in the range 5.49 ≤ M2 ≤ 8.14 M☉ (90% confidence). The mass of the compact object is well above the maximum mass of a stable neutron star, and we conclude that V4641 Sgr contains a black hole. The B-star secondary is by far the most massive, the hottest, and the most luminous secondary of the dynamically confirmed black hole X-ray transients. We find that the α-process elements nitrogen, oxygen, calcium, magnesium, and titanium may be overabundant in the secondary star by factors of 2-10 times with respect to the Sun. Finally, assuming E(B-V) = 0.32 ± 0.10, we find a distance 7.40 ≤ d ≤ 12.31 kpc (90% confidence). This large distance and the high proper motions observed for the radio counterpart make V4641 Sgr possibly the most superluminal galactic source known, with an apparent expansion velocity of 9.5c and a bulk Lorentz factor of Γ 9.5, assuming that the jets were ejected during one of the bright X-ray flares observed with the Rossi X-ray Timing Explorer.

X-ray bursts at extreme mass accretion rates from GX 17+2

We report on ten type I X-ray bursts originating from GX 17+2 in data obtained with the RXTE/PCA in 1996-2000. Three bursts were short in duration (∼10 s), whereas the others lasted for ∼6-25 min. All bursts showed spectral softening during their decay. There is no evidence for high-frequency (>100 Hz) oscillations at any phase of the bursts. We see no correlations of the burst properties with respect to the persistent X-ray spectral properties, suggesting that in GX 17+2 the properties of the bursts do not correlate with inferred mass accretion rate. The presence of short bursts in GX 17+2 (and similar bright X-ray sources) is not accounted for in the current X-ray bursts theories at the high mass accretion rates encountered in this source. We obtain satisfactory results if we model the burst emission with a black body, after subtraction of the persistent pre-burst emission. The two- component spectral model does not fit the total burst emission whenever there is a black-body component present in the persistent emission. We conclude that in those cases the black-body contribution from the persistent emission is also present during the burst. This implies that, contrary to previous suggestions, the burst emission does not arise from the same site as the persistent black-body emission. The black-body component of the persistent emission is consistent with being produced in an expanded boundary layer, as indicated by recent theoretical work. Five of the long bursts showed evidence of radius expansion of the neutron star photosphere (independent of the spectral analysis method used), presumably due to the burst luminosity reaching the Eddington value. When the burst luminosity is close to the Eddington value, slight deviations from pure black-body radiation are seen at energies below � 10 keV. Similar deviations have been seen during (long) X-ray bursts from other sources; they can not be explained by spectral hardening models. The total persistent flux just before and after the radius expansion bursts is inferred to be up to a factor of 2 higher than the net peak flux of the burst. If both the burst and persistent emission are radiated isotropically, this would imply that the persistent emission is up to a factor of 2 higher than the Eddington luminosity. This is unlikely and we suggest that the persistent luminosity is close to the Eddington luminosity and that the burst emission is (highly) anisotropic (ξ ∼ 2). Assuming that the net burst peak fluxes equal the Eddington limit, applying standard burst parameters (1.4 Mneutron star, cosmic composition, electron scattering opacity appropriate for high temperatures), and taking into account gravitational redshift and spectral hardening, we derive a distance to GX 17+2 of ∼8 kpc, with an uncertainty of up to ∼30%.

RXTE Observations of the Neutron Star Low-Mass X-Ray Binary GX 17+2: Correlated X-Ray Spectral and Timing Behavior

We have analyzed ~600 ks of Rossi X-Ray Timing Explorer data of the neutron star low-mass X-ray binary and Z source GX 17+2. A study was performed of the properties of the noise components and quasi-periodic oscillations (QPOs) as a function of the broadband spectral properties, with the main goal to study the relation between the frequencies of the horizontal branch (HBO) and upper kHz QPOs. It was found that when the upper kHz QPO frequency is below 1030 Hz these frequencies correlate, whereas above 1030 Hz they anticorrelate. GX 17+2 is the first source in which this is observed. We also found that the frequency difference of the high-frequency QPOs was not constant and that the quality factors (Q-values) of the HBO, its second harmonic, and the kHz QPOs are similar and vary almost hand in hand by a factor of more than 3. Observations of the normal branch oscillations during two type I X-ray bursts showed that their absolute amplitude decreased as the flux from the neutron star became stronger. We discuss these and other findings in terms of models that have been proposed for these phenomena. We also compare the behavior of GX 17+2 and other Z sources with that of black hole sources and consider the possibility that the mass accretion rate might not be the driving force behind all spectral and variability changes.

Interstellar X-ray absorption spectroscopy of oxygen, neon, and iron with the CHANDRA LETGS Spectrum of X0614+091

We —nd resolved interstellar O K, Ne K, and Fe L absorption spectra in the Chandra X-Ray Observatory Low-Energy Transmission Grating Spectrometer (LETGS) spectrum of the low-mass X-ray binary X0614]091. We measure the column densities in O and Ne and —nd direct spectroscopic constraints on the chemical state of the interstellar O. These measurements probably probe a low-density line of sight through the Galaxy, and we discuss the results in the context of our knowledge of the properties of interstellar matter in regions between the spiral arms.

BeppoSAX Wide Field Cameras observations of six type I X-ray bursters

We have discovered three certain (SAX J1324.5 6313, 2S 1711 339 and SAX J1828.5 1037) and two likely (SAX J1818.7+1424 and SAX J2224.9+5421) new thermonuclear X-ray burst sources with the BeppoSAX Wide Field Cameras, and observed a second burst ever from a sixth one (2S 0918 549). Four of them (excluding 2S 1711 339 and 2S 0918 549) are newly detected X-ray sources from which we observed single bursts, but no persistent emission. We ob- serve the first 11 bursts ever from 2S 1711 339; persistent flux was detected during the first ten bursts, but not around the last burst. A single burst was recently detected from 2S 0918 549 by Jonker et al. (2001); we observe a second burst showing radius expansion, from which a distance of 4.2 kpc is derived. According to theory, bursts from very low flux levels should last>100 s. Such is indeed the case for the last burst from 2S 1711 339, the single burst from SAX J1828.5 1037 and the two bursts from 2S 0918 549, but not for the bursts from SAX J1324.5 6313, SAX J1818.7+1424 and SAX J2224.9+5421. The bursts from the latter sources all last20 s. We suggest that SAX J1324.5 6313, SAX J1818.7+1424, SAX J1828.5 1037 and SAX J2224.9+5421 are members of the recently proposed class of bursters with distinctively low persistent flux levels, and show that the galactic distribution of this class is compatible with that of the standard low-mass X-ray binaries.

BeppoSAX Measurements of the Bright Gamma-Ray Burst 010222

We analyze the BeppoSAX measurements of the prompt and afterglow emission of the γ-ray burst GRB 010222. Among 45 GRBs detected with the Wide Field Cameras on BeppoSAX, the 40-700 keV fluence of (9.3 ± 0.3) × 10-5 ergs cm-2 is only surpassed by GRB 990123. In terms of the isotropic 20-2000 keV energy output of 7.8 × 1053 ergs, it ranks third of all GRBs with measured distances. Since this burst is so bright, the data provide complete and valuable coverage up to 65 hr after the event, except for a gap between 3.5 and 8.0 hr. The 2-10 keV flux history shows clear signs of a break, which is consistent with a break seen in the optical, and provides supporting evidence for the achromatic nature of the break. An explanation for the break in the context of a collimated expansion is not straightforward. Rather, a model is favored whereby the fireball is braked to the nonrelativistic regime quickly (within a fraction of day) by a dense (~106 cm-3) circumburst medium. This implies that, after a mild beaming correction, GRB 010222 may be the most energetic burst observed thus far. The X-ray decay index after the break is 1.33 ± 0.04, the spectral index 0.97 ± 0.05. The decay is, with unprecedented accuracy, identical to that observed in the optical.

A measurement of the broadband spectrum of XTE J1118+480 with BeppoSAX and its astrophysical implications

We report on results of a Target-of-Opportunity observation of the X-ray transient XTE J1118+480 performed on 2000 April 14-15 with the Narrow Field Instruments (0.1-200 keV) of the BeppoSAX satellite. The measured spectrum is a power law with a photon index of ~1.7 modified by an ultrasoft X-ray excess and a high-energy cutoff above ~100 keV. The soft excess is consistent with a blackbody with a temperature of ~40 eV and a low flux, while the cutoff power law is well fitted by thermal Comptonization in a plasma with an electron temperature of ~102 keV and an optical depth of order unity. Consistent with the weakness of the blackbody, Compton reflection is weak. Although the data are consistent with various geometries of the hot and cold phases of the accreting gas, we conclude that a hot accretion disk is the most plausible model. The Eddington ratio implied by recent estimates of the mass and the distance is ~10-3, which may indicate that advection is probably not the dominant cooling mechanism. We finally suggest that the reflecting medium has a low metallicity, consistent with the location of the system in the halo.

The Power Spectral Properties of the Z Source GX 340+0

We present an analysis of ~390 ks of data of the Z source GX 340+0 taken during 24 observations with the Rossi X-Ray Timing Explorer satellite. We report the discovery of a new broad component in the power spectra. The frequency of this component varied between 9 and 14 Hz and remained close to half that of the horizontal-branch quasi-periodic oscillations (HBOs). Its rms amplitude was consistent with being constant around ~5%, while its FWHM increased with frequency from 7 to 18 Hz. If this sub-HBO component is the fundamental frequency, then the HBO and its second harmonic are the second and fourth harmonic component, while the third harmonic was not detected. This is similar to what was recently found for the black hole candidate XTE J1550-564. The profiles of both the horizontal- and the normal-branch quasi-periodic oscillation peaks were asymmetric when they were strongest. We describe this in terms of a shoulder component at the high-frequency side of the quasi-periodic oscillation peak, of which the rms amplitudes were approximately constant at ~4% and ~3%, respectively. The peak separation between the twin kilohertz quasi-periodic oscillations was consistent with being constant at 339±8 Hz, but a trend similar to that seen in, e.g., Sco X-1 could not be excluded. We discuss our results within the framework of the various models that have been proposed for the kilohertz QPOs and low-frequency peaks.

On the spectral evolution of Cygnus X-2 along its color-color diagram

We report on the results of a broad band (0.1{200 keV) spectral study of Cyg X{2 using two BeppoSAX observations taken in 1996 and 1997, respectively, for a total eective on-source time of100 ks. The color-color (CD) and hardness-intensity (HID) diagrams show that the source was in the horizontal branch (HB) and normal branch (NB) during the 1996 and 1997 observation, respectively. Five spectra were selected around dierent positions of the source in the CD/HID, two in the HB and three in the NB. These spectra are t to a model consisting of a disk blackbody, a Comptonization component, and two Gaussian emission lines at 1k eV and 6.6 keV, respectively. The addition of a hard power-law tail with photon index2, contributing1.5% of the source luminosity, improves the t of the spectra in the HB. We interpret the soft component as the emission from the inner accretion disk, with inner temperature, kTin, varying between0.8 and1.7 keV and inner radius, Rin, varying between26 and11 km (assuming an inclination angle of the system of 60). The Comptonization component is probably emitted by hot plasma (electron temperature kTe varying between 3a nd20 keV, optical depth 11 0:4, seed-photon temperature kTW 1 2:4 keV) surrounding the NS. The changes in the parameters of the blackbody component indicate that the inner rim of the disk approaches the NS surface when the source moves from the HB to the NB, i.e. as the (inferred) mass accretion rate increases. The parameters of the Comptonized component also change signicantly when the source moves from the HB to the NB. We discuss possible scenarios which can explain these changes.