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تسجيل مستخدم جديدExtremely Weak Reflection Features in the X-ray Spectrum of XTE J1118+480: Possible Evidence for X-ray-Emitting Jets?
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J. M. Miller
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We have simultaneously fit Chandra and RXTE spectra of the Galactic black hole XTE J1118+480 with three models for X-ray reflection. We explored a range of accretion disc ionizations (log(xi)=1-4; xi=L_X/nR^{2}) and iron abundances (0.10-1.00). Our fits with the constant density ionized disc models of Ross & Fabian indicate that less than 0.5 per cent (90 per cent confidence upper-limit) of the observed flux is reflected. Fits with the pexrav of model Magdziarz & Zdziarski indicate that the two-dimensional solid angle (Omega/2pi) subtended by the disc relative to a central source of incident hard X-rays is 0.01 +0.06 -0.01. A combination of the high inclination (i=81 degrees), Comptonization, and bulk velocities may each contribute to the low reflection fractions we have measured. The results are also consistent with extended jets being the source of the hard X-ray flux, as the disc would then represent a small solid angle as seen from the emission region.
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Optical spectra were obtained of the optical counterpart of the high latitude soft X-ray transient XTE J1118+480 near its quiescent state with the new 6.5 m MMT and the 4.2 m WHT. The spectrum exhibits broad, double-peaked, emission lines of hydrogen
from an accretion disk superposed with absorption lines of a K7V-M0V secondary star. Cross-correlation of the 27 individual spectra with late-type stellar template spectra reveals a sinusoidal variation in radial velocity with amplitude K = 701 +/- 10 km/s and orbital period P = 0.169930 +/- 0.000004 d. The mass function, 6.1 +/- 0.3 solar masses, is a firm lower limit on the mass of the compact object and strongly implies that it is a black hole. Photometric observations (R-band) with the IAC 0.8 m telescope reveal ellipsoidal light variations of full amplitude 0.2 mag. Modeling gives a large mass ratio (M1/M2 ~ 20) and a high orbital inclination (i = 81 +/- 2 deg). Our combined fits yield a mass of the black hole in the range M1 = 6.0-7.7 solar masses (90% confidence) for plausible secondary star masses of M2 = 0.09-0.5 solar masses. The photometric period measured during the outburst is 0.5% longer than our orbital period and probably reflects superhump modulations as observed in some other soft X-ray transients. The estimated distance is d = 1.9 +/- 0.4 kpc corresponding to a height of 1.7 +/- 0.4 kpc above the Galactic plane. The spectroscopic, photometric, and dynamical results indicate that XTE J1118+480 is the first firmly identified black hole X-ray system in the Galactic halo.
In recent years, an increasing number of proper motions have been measured for Galactic X-ray binaries. When supplemented with accurate determinations of the component masses, orbital period, and donor luminosity and effective temperature, these kine
matical constraints harbor a wealth of information on the systems past evolution. The constraints on compact object progenitors and kicks derived from this are of immense value for understanding compact object formation and exposing common threads and fundamental differences between black hole and neutron star formation. Here, we present the results of such an analysis for the black hole X-ray binary XTE J1118+480. We present results from modeling the mass transfer phase, following the motion in the Galaxy back to the birth site of the black hole, and examining the dynamics of symmetric and asymmetric core-collapses of the black hole progenitor.
We present optical and infrared monitoring of the 2005 outburst of the halo black hole X-ray transient XTE J1118+480. We measured a total outburst amplitude of ~5.7+-0.1 mag in the R band and ~5 mag in the infrared J, H and K_s bands. The hardness ra
tio HR2 (5-12 keV/3-5 keV) from the RXTE/ASM data is 1.53+-0.02 at the peak of the outburst indicating a hard spectrum. Both the shape of the light curve and the ratio L_X (1-10 keV)/L_opt resemble the mini-outbursts observed in GRO J0422+32 and XTE J1859+226. During early decline, we find a 0.02-mag amplitude variation consistent with a superhump modulation, like the one observed during the 2000 outburst. Similarly, XTE J1118+480 displayed a double-humped ellipsoidal modulation distorted by a superhump wave when settled into a near-quiescence level, suggesting that the disk expanded to the 3:1 resonance radius after outburst where it remained until early quiescence. The system reached quiescence at R=19.02+-0.03 about three months after the onset of the outburst. The optical rise preceded the X-ray rise by at most 4 days. The spectral energy distributions (SEDs) at the different epochs during outburst are all quasi-power laws with F_nu proportional to nu^alpha increasing toward the blue. At the peak of the outburst we derived alpha=0.49+-0.04 for the optical data alone and alpha=0.1+-0.1 when fitting solely the infrared. This difference between the optical and the infrared SEDs suggests that the infrared is dominated by a different component (a jet?) whereas the optical is presumably showing the disk evolution.
We discovered the optical counterpart about 13 mag of a soft X-ray transient, XTE J1118+480 on 2000 March 30. We perform astrometry and provide the accurate position as R.A. = 11h18m10s.85, Decl. = +48o0212.9. The outbursting object is identified wit
h a 18.8 mag star in USNO catalog. Our pre-discovery data shows another outburst during 2000 January, again coinciding with an outburst detected in X-rays. Through the CCD time-series photometry, we found the presence of a periodic variation with the amplitude of 0.055 mag and the period of 0.17078(0.00004)d which we consider as promising candidate of orbital period. Because of the high galactic latitude and faint quiescence magnitude of 18.8, XTE J1118+480 is the possible first firmly identified black hole candidate (BHC) X-ray transient in the galactic halo.
Following the recent abundance measurements of Mg, Al, Ca, Fe, and Ni in the black hole X-ray binary XTE J1118+480 using medium-resolution Keck II/ESI spectra of the secondary star (Gonzalez Hernandez et al. 2006), we perform a detailed abundance ana
lysis including the abundances of Si and Ti. These element abundances, higher than solar, indicate that the black hole in this system formed in a supernova event, whose nucleosynthetic products could pollute the atmosphere of the secondary star, providing clues on the possible formation region of the system, either Galactic halo, thick disk, or thin disk. We explore a grid of explosion models with different He core masses, metallicities, and geometries. Metal-poor models associated with a formation scenario in the Galactic halo provide unacceptable fits to the observed abundances, allowing us to reject a halo origin for this X-ray binary. The thick-disk scenario produces better fits, although they require substantial fallback and very efficient mixing processes between the inner layers of the explosion and the ejecta, making quite unlikely an origin in the thick disk. The best agreement between the model predictions and the observed abundances is obtained for metal-rich progenitor models. In particular, non-spherically symmetric models are able to explain, without strong assumptions of extensive fallback and mixing, the observed abundances. Moreover, asymmetric mass ejection in a supernova explosion could account for the required impulse necessary to launch the system from its formation region in the Galactic thin disk to its current halo orbit.
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