A METAL-RICH BLACK HOLE BINARY IN THE GALACTIC HALO


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.

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 1.85+-0.36 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 (λ/δλ≈ 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 J1118+480, is in the range 0.6–26.6 km s1, smaller  than the instrumental resolution of ∼50 km s-1. 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.

3. CHEMICAL ANALYSIS
A comparison of the observed spectrum with the spectra of  10 template stars (K0 V–M2 V) obtained with the same instrument
allows us to classify the secondary as a mid to late  K-type star. In Figure 1, we display two spectral regions containing
some relevant lines for our analysis as well as a spectrum  of a template star of similar spectral type for comparison
with available detailed chemical analysis (Allende Prieto et al.2004). We also show synthetic line profiles for different stellar
abundances computed with the local thermodynamic equilibrium  (LTE) code MOOG (Sneden 1973), adopting the atomic
line data from the Vienna Atomic Line Database (VALD; Piskunov  et al. 1995) and using a grid of LTE model atmospheres
(Kurucz 1993).
In order to perform the chemical analysis of the secondary  star, we used a technique that combines a grid of synthetic
spectra and a 2-minimization procedure that includes Monte  Carlo simulations (Gonza´lez Herna´ndez et al. 2004, 2005).

First, we inspected the observed spectrum in order to select  the most suitable features for a chemical abundance determination.
We identified nine spectral features containing in total   30 lines of Fe i and eight lines of Ca i with excitation potentials
between 1 and 5 eV. The oscillator strengths of the relevant  spectral lines were checked via spectral synthesis against the
solar atlas (Kurucz et al. 1984). We then generated a grid of  about one million synthetic spectra of each of these features,
varying as free parameters the star effective temperature (T ), eff  surface gravity (log g), and metallicity ([Fe/H]), together with
the veiling from the accretion disk, which was assumed to be  a linear function of wavelength, and thus described by two
additional parameters. Iron abundances were varied in the range -1.5 <[Fe/H] < 1, whereas the Ca abundance was fixed, for
each given iron abundance, according to the Galactic trend  of Ca (Bensby et al. 2005) for [Fe/H] < 0, and fixed to [Ca/Fe] = 0 in the range 0 < [Fe/H]< 1. A rotational broadening  of 100 km s-1 and a limb darkening e = 0.8 were  adopted. The microturbulence (ξ) was computed using an experimental  expression as a function of effective temperature  and surface gravity (Allende Prieto et al. 2004).

image

Fig. 1.—Best synthetic spectral fits to the ESI spectrum of the secondary  star in the XTE J1118480 system (second and bottom panels) and the same  for a template star (properly broadened) taken from Allende Prieto et al. (2004) shown for comparison (top and third panels). Synthetic spectra are computed  for typical abundances for a halo star ([Fe/H]=-1.2; dash-dotted line), solar
abundances ([Fe/H]= 0; dashed line), and best-fit abundances (solid line). In  addition, note that for solar and best-fit abundances we have applied to the  synthetic spectra the corresponding values for the veiling according to the  solution found with the fitting procedure. However, for the low-metallicity  synthetic spectra, we have not assumed any veiling. [See the electronic edition
of the Journal for a color version of this figure.]

 

image

We compared, using a 2-minimization procedure, this grid x  with 1000 realizations of the observed spectrum. Using a bootstrap
Monte Carlo method, we found the most likely values  Teff= 4700+-100 K, log (g/cm s2 ) =4.6+-0.3, [Fe/H] = eff  0.2+-0.2, and a disk veiling (defined as F disk/Ftotal ) of less than disk total  40% at 5000 °A   and decreasing toward longer wavelengths. The °A
(1σ) uncertainty in the iron-abundance determination takes into  account the uncertainties in the stellar and veiling parameters.
The effective temperature and surface gravity are consistent with previous spectral classifications and similarly the reported
veiling values (Torres et al. 2004). Using the derived stellar and veiling parameters, we analyzed several spectral regions
where we had identified various lines of Fe, Ca, Al, Mg, and  Ni. Abundances of all the elements are listed in Table 1. The
1 σ uncertainty in the abundance determination takes into account uncertainties in the stellar and veiling parameters.
Remarkably, we find a metallicity higher than solar, which  is extremely atypical of halo stars (Allende Prieto et al. 2006).
In Figure 1, we show the best-fit synthetic spectrum to various  features in two different spectral regions, and for comparison,
a model with 25 times lower metal content. Notice the inadequacy  of low-metallicity models to reproduce the observed
features, even in the extreme case that no veiling is considered (adding veiling would make the discrepancy much worse). An
iron abundance of [Fe/H] =-1.2 is more than 6σ away from the best-fit solution ([Fe/H] = 0.18) and hence very unlikely.
We have also found that abundances of Al, Ca, Mg, and Ni  are higher than solar (see Table 1). In Figure 2, we show that  the abundance ratios of these elements with respect to iron are  consistent with those of stars in the solar neighborhood from
Gilli et al. (2006). We have also determined an upper limit to  the Li abundance log e(Li)LTE =log [N(Li)/N(H)] =12 LTE≤LTE 1.86 using the Li 6708 A° line. This value seems to be lower  than typical high Li abundances measured in other late-type
secondary stars in soft X-ray transients whose origin is still an  open question (Martı´n et al. 1994).

4. DISCUSSION AND CONCLUSIONS
If we include the metallicity distribution of halo and thick disk  stars (Allende Prieto et al. 2006) and thin-disk stars (Allende
Prieto et al. 2004) in the equations based on kinematics  for establishing the relative likelihoods of belonging to the halo,
thick disk, or thin disk (Bensby et al. 2005), the probability  that a star with the Galactic space-velocity components of this
system (Mirabel et al. 2001; U =-105+-16 km s-1, V =-98+-16 km s-1, W =-21+-10 km s-1) and metallicity [Fe/H] p 0.18 belongs to the Galactic halo is less than 0.1%.
Moreover, the kinematics alone suggest thick-disk rather than halo membership, although the high metallicity of the secondary
star favors thin-disk membership. If, however, the progenitor was a massive star in the halo or thick disk, the high
metallicity of the secondary rules out the hypothesis that the  black hole was formed by direct collapse of the massive star;
instead, a supernova explosion origin is strongly suggested.
Given its present orbital distance from the black hole (a ≈ 3 R ; Wagner et al. 2001), it is plausible that the secondary star captured a significant fraction of the matter ejected in a supernova explosion. The chemical composition of the secondary may provide crucial information on nucleosynthesis  in the progenitor and the formation mechanism of the black  hole. We consider two possible scenarios for the origin of the  compact object: either it formed in the Galactic halo or thick  disk as a result of the explosion of a metal-poor massive progenitor  that enriched the secondary star from the typical abundances of halo/thick-disk stars up to the observed supersolar  values, or alternatively, it formed in the Galactic thin disk with  a natal kick imparted during the supernova explosion that propelled  the binary into its current orbit.
In the first case, the similarity with the kinematics of halo  and thick-disk stars makes unnecessary a significant kick during
the black hole formation process. However, since the typical  metallicities of halo stars and thick-disk stars are significantly
lower than solar, it is required that the secondary captured  enough matter from the ejecta to reach the current abundances.
A metal-poor ∼1 Mimage secondary star, initially placed at an orbital distance of ∼6 Rimage (after tidal circularization of the orbit), would need to capture roughly 5%–10% of the matter ejected in a spherically symmetric core-collapse supernova explosion  of a 16 Mimage helium core to achieve the observed iron ,abundance. We have considered supernova and hypernova  models (Umeda & Nomoto 2002, 2005; N. Tominaga, H.Umeda, & K. Nomoto 2006, in preparation) of metal-poor  progenitors with different masses, metallicities, mass cuts (i.e.the mass above which the matter is expelled at the time of the  supernova explosion), fallback (i.e.amount of mass that is  eventually accreted by the compact core), and mixing, and assumed that all the fallback material is well mixed with the  ejecta.We find that a sufficiently large Fe enrichment is possible  for mass cuts in the range 2–4 Mimage [and explosion energies (1–30)10 TO THE 51 ergs]. The abundance ratios of the other elements are only marginally reproduced by these models. An  origin of XTE J1118+480 in the Galactic halo or in the Galactic  thick disk cannot be discarded by the present observations, nor  can it be confirmed. Further higher quality observations and  extensive model calculations are required to fully explore this  possibility. Further details of analysis will be put together in  J. I. Gonza´lez Herna´ndez et al. (2006, in preparation).

In the second scenario, the system had to acquire a peculiar  space velocity of ∼180 km s-1, to change from a Galactic thin disk
orbit to the currently observed orbit (Gualandris et al.2005), requiring an asymmetric kick. Such kicks imparted during
the birth of nascent neutron stars, due to asymmetric mass  ejection and/or an asymmetry in the neutrino emission (Lai et
al. 2001), have been proposed to explain the large transverse  motions of neutron stars in the plane of the sky (Lyne & Lorimer
1994). The black hole could have formed in a two-stage  process where the initial collapse led to the formation of a  neutron star accompanied by a substantial kick and the final mass of the compact remnant was achieved by matter that fell  back after the initial collapse as proposed to explain the origin  of the black hole in Nova Sco 1994 (Podsiadlowski et al. 2002;Brandt et al. 1995).
Spherically symmetric explosion models are able to explain  the observed metal enrichment. A modest amount of ejecta
(∼0.02 M image) captured by a secondary with initial solar abundance ,is sufficient if vigorous mixing (Kifonidis et al. 2000) between
the fallback matter (which is necessarily large given the mass  of the black hole) and the ejecta took place. In this scenario, the
black hole would be formed in a mild explosion with fallback,as in collapsar models (MacFadyen et al. 2001) associated with
rapidly rotating massive stars. However, in the pure spherical  supernova explosion scenario for XTE J1118480, the maximum
allowed ejected mass to keep the secondary star gravitationally  bound is DM ≈ 8 Mimage . This leads to a system velocity ,
of ∼60 km s-1 (there could be extreme cases with system velocities  as high as ∼100 km s-1 requiring very special model
parameters such as higher secondary and black hole masses just  after the explosion; see further details in § 3.2 of Gualandris et
al. 2005), much lower than the observed value, and a neutrinoinduced  kick is required if we are to explain the kinematics of
the system.
The chemical composition of the ejecta in a non spherically  symmetric supernova explosion is strongly dependent on direction.
In particular, if we assume that the jet is collimated  perpendicular to the orbital plane of the binary, where the  secondary star is located, elements such as Ti, Ni, and Fe are  mainly ejected in the jet direction, while Al, O, Si, S, and Mg  are preferentially ejected near the equatorial plane of the helium  star (Maeda et al. 2002). Using predictions for an a spherical  explosion model of a 16 Mimage He core metal-rich progenitor, we can explain the observed abundances in the secondary.
Complete lateral mixing (Podsiadlowski et al. 2002) is required  to account for the similar enhancement of Mg and Ni, and the
observed abundances can be reproduced for all mass cuts in  the range 2–8 Mimage  .
It is therefore plausible that the black hole in XTEJ1118+480 formed in the Galactic thin disk from a massive  metal-rich progenitor and was launched into its current orbit either by mass ejection in an asymmetric supernova/hypernova
explosion or by a neutrino-induced kick. In addition, ultraviolet  observations of the accretion disk in this system suggest that
the material accreted onto the compact object is substantially CNO processed (Haswell et al. 2002), indicating that the zero  age mass of the secondary star could have been ∼1.5 Mimage . By means of binary evolution calculations, this may constrain the  age of the system in the range 2–5.5 Gyr (Gualandris et al.2005). Future chemical studies of the secondary star during periods of quiescence may provide accurate abundances of elements  (e.g., C, Ti, Si) whose lines either are not available in  the observed spectrum or are present in spectral regions where  the signal-to-noise ratio is too low for an accurate chemical  analysis. These may reveal further details of the formation  mechanism of the black hole in this system.

We are grateful to Hideyuki Umeda, Ken’ichi Nomoto, and  Nozomu Tominaga for sending us their explosion models for  metal-poor and metal-rich progenitors and several programs for our model computations. We also thank Keiichi Maeda for  providing us with his aspherical explosion models and for helpful  discussions. We are grateful to Tom Marsh for the use of  the MOLLY analysis package, to Jorge Casares for his helpful  comments on different aspects of this work, and to Ryan J.Foley for assistance with the observations. We also thank the  referee for helpful comments. The W. M. Keck Observatory is  operated as a scientific  partnership among the California Institute  of Technology, the University of California, and NASA;it was made possible by the financial support of the W. M.Keck Foundation.

This work has made use of the VALD database  and IRAF facilities. It was funded in part by Spanish  Ministry project AYA2005-05149 and by US National Science  Foundation grant AST-0307894. We dedicate this Letter to the  memory of our dear friend and collaborator E. T. Harlaftis, whose life was tragically cut short by a snow avalanche on  2005 February 13.

  image

Fig. 2.—Abundance ratios of the secondary star in XTE J1118+480 (cross) in comparison with the abundances of G and K metal-rich dwarf stars. Galactic  trends were taken from Gilli et al. (2006). The size of the cross indicates the  uncertainty. Filled and empty circles correspond to abundances for planet host  stars and stars without known planet companions, respectively. The dash-dotted  lines indicate solar abundance values.

 

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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