XTE J1118480: A METAL-RICH BLACK HOLE BINARY IN THE GALACTIC HALO (A)


Emilios T. Harlaftis,5,6

Jonay I. Gonza´lez Herna´ndez,1,2,3 Rafael Rebolo,1,4 Garik Israelian,1 Alexei V. Filippenko,7 and Ryan Chornock7
Received 2006 April 3; accepted 2006 April 26; published 2006 May 25

ABSTRACT
We present medium-resolution optical spectra of the secondary star in the high Galactic latitude black hole  X-ray binary XTE J1118480 and determine the abundance of Mg, Al, Ca, Fe, and Ni in its atmosphere. For  all the elements investigated, we find supersolar abundances; thus, we reject the hypothesis that the black hole  came from the direct collapse of an ancient massive halo star. The compact primary most likely formed in a  supernova event of a massive star whose nucleosynthetic products polluted the secondary star. The observed element abundances and their ratios can be explained using a variety of supernova models with a wide range of  metallicities. While an explosive origin in the Galactic halo or thick disk cannot be discarded, a metal-rich
progenitor is clearly favored by the observed abundance pattern. This suggests that the black hole was produced  in the Galactic thin disk with a violent natal kick, propelling the X-ray binary to its current location and orbit.
Subject headings: black hole physics—stars: abundances—stars: evolution—stars: individual (XTE J1118480)—supernovae: general—X-rays: binaries

1. INTRODUCTION
The low-mass X-ray binary XTE J1118480 was discovered  with the all-sky monitor aboard the Rossi X-Ray Timing Explorer
on UT 2000 March 29 (Remillard et al. 2000). Throughout  its outburst, the source remained in the low/hard state, one
of the characteristic spectral states of an accreting black hole  binary (McClintock et al. 2003). The system consists of a black
hole with a mass estimated in the range 6–8 M and a late, type secondary star of 0.1–0.5 M (Wagner et al. 2001). ,
The extraordinarily high Galactic latitude (b ≈ 62.3), together  with its distance of 1.850.36 kpc (Wagner et al. 2001), places
the system at a height of ∼1.6 kpc above the Galactic plane. In  addition, an accurate measurement of its proper motion coupled
with its distance provides space-velocity components U, V that  seem consistent with those of some old halo globular clusters
(Mirabel et al. 2001). If the system formed in the Galactic halo,the black hole could be either the remnant of a supernova in the
very early Galaxy or the result of a direct collapse of an ancient  massive star. However, the galactocentric orbit crossed the Galactic  plane many times in the past, and an alternative possibility  is that the system formed in the Galactic disk and was launched  into its present orbit as a consequence of the “kick” acquired in  the supernova explosion of a massive star (Gualandris et al.  2005). The metallicity of the secondary star may provide a key  to distinguish among these two possible birthplaces, giving important  clues to the formation of the black hole and the properties  of the supernova explosion, such as symmetry, released energy, and characteristics of the ejected matter (Israelian et al. 1999;Podsiadlowski et al. 2002).

2. OBSERVATIONS
We obtained 74 medium-resolution spectra (l/dl ≈ 6000) of  the secondary star in XTE J1118480, in quiescence, on UT
2004 February 14, using the 10 m Keck II telescope, equipped  with the Echellette Spectrograph and Imager (ESI; Sheinis et
al. 2002). The exposure time was fixed at 300 s to minimize  the effects of orbital smearing, which, for the orbital parameters
of XTE J1118480, is in the range 0.6–26.6 km s1, smaller  than the instrumental resolution of ∼50 km s1. Each individual
spectrum was corrected for the radial velocity of the star, and  the spectra were combined in order to improve the signal-tonoise
ratio. After binning in wavelength in steps of 0.3 , the °A  final spectrum had an average signal-to-noise ratio of 80 in the
continuum. The data cover the spectral range 4000–9000 °A  and clearly show the characteristic emission lines of accreting
low-mass X-ray binaries (Balmer series and Ca ii near-infrared  triplet, He i l5876, He i l6678), superimposed on the typical
photospheric spectrum of a late-type star.

(TO BE CONTINUED)

notes

1 Instituto de Astrofı´sica de Canarias, E-38205 La Laguna, Tenerife, Spain;jonay@iac.es, rrl@iac.es, gil@iac.es.
2 CIFIST Marie Curie Excellence Team.
3 Observatoire de Paris-Meudon, GEPI, 5 Place Jules Janssen, 92195 Meudon Cedex, France.
4 Consejo Superior de Investigaciones Cientı´ficas, Spain.
5 Institute of Space Applications and Remote Sensing, National Observatory  of Athens, P.O. Box 20048, Athens 118 10, Greece.
6 In memoriam.
7 Department of Astronomy, University of California at Berkeley, 601 Campbell  Hall, Berkeley, CA 94720-3411; alex@astro.berkeley.edu, chornock@astro.berkeley.edu.

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