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[OIII] 5007 Emission from the Black-Hole X-ray Binary in a NGC 4472 Globular Cluster

We present the discovery of [OIII] 5007 emission associated with the black hole X-ray binary recently identified in a globular cluster in the Virgo elliptical galaxy NGC 4472. This object is the first confirmed black-hole X-ray binary in a globular cluster. The identification of [OIII] 5007 emission from the black-hole hosting globular cluster is based on two independent fiber spectra obtained at the VLT with FLAMES, which cover a wavelength range of 5000-5800 Angstrom at a spectral resolution of about 6000. In each of these spectra we find an emission line at 5031.2 Angstrom with an uncertainty of several tenths of an Angstrom. These are consistent with [OIII] 5007 emission at the 1475 +/- 7 km/s radial velocity of the globular cluster previously determined from an analysis of its absorption lines. This agreement within the small uncertainties argues strongly in favor of the interpretation of the line as [OIII] 5007 emission from the black-hole hosting globular cluster. We also find that the emission line most likely has a velocity width of several hundred km/s. Such a velocity width rules out a planetary nebula explanation for the [OIII] 5007 emission and implicates the black hole as the source of the power driving the nebular emission.


introduction
presented the discovery of a black-hole X-ray binary in a globular cluster in the Virgo elliptical NGC 4472. This is the first unambiguous black-hole X-ray binary in any globular cluster. This result is based on a 100 ks XMM-Newton observation of the Virgo elliptical NGC 4472 and its globular clusters. Specifically, MKZR07 identified an X-ray source in a spectroscopically confirmed globular cluster of the Virgo elliptical galaxy NGC 4472 with an X-ray luminosity of 4 × 10 39 ergs s −1 for which the observed count rate varied by a factor of seven over a few hours. The high X-ray luminosity, about ten times L Edd for a neutron star, rules out any single object other than a black hole in the old stellar population of the globular cluster (MKZR07). However, a high X-ray luminosity alone does not guarantee a black hole source in extragalactic globular clusters, as multiple neutron stars are a possibility in these systems which are unresolved in X-ray data. Variability has long been recognized as a key for distinguishing a black hole X-ray binary from multiple neutron stars in extragalactic globular clusters (e.g. Kalogera, King, & Rasio 2004). Therefore, the clear variability observed during the XMM observation unambiguously indicates that the high X-ray luminosity in this source originates from a black hole (MKZR07).
This object is the first and currently only confirmed black-hole X-ray binary in a globular cluster. As such it opens up new possibilities to address questions about the evolution of black holes in dense stellar systems. It also provides a unique opportunity to study in a nearby system how the energy from a highly luminous accreting black hole is fed back into its surrounding medium in a dense stellar system. To begin considering these questions, this letter presents initial optical spectroscopy of this blackhole X-ray binary and the globular cluster in which it is found. We present the observations in Section 2, describe the results of their analysis in section 3, and discuss their implications in Section 4.
2. observations As part of a study of the kinematics of the globular cluster system of Virgo elliptical galaxy NGC 4472, we used the FLAMES multi-fiber spectrograph (Pasquini et al. 2004) on the VLT/UT2 to obtain spectra of globular clusters over a wide area around NGC 4472 ). The globular cluster targets were selected from the multicolor, wide-field imaging study of Rhode & Zepf (2001, hereafter RZ01). RZ01 used BV R MOSIAC images from the Mayall 4-m to identify candidate globular clusters over an approximately 35 ′ × 35 ′ field of view. The spectroscopic data were obtained with FLAMES in its GI-RAFFE/MEDUSA mode, which offers 130 fibers over a 25' diameter field of view. By observing through four fiber se-tups, each slightly offset in a different direction from the center of the galaxy, we were able to obtain spectra for nearly 400 globular clusters over a large range of distances from the center of the galaxy, with good azimuthal coverage, and many overlaps between pointings to provide checks on the accuracy of the radial velocities of the clusters.
The observations were carried out in service mode at the VLT/UT2 with two fiber setups observed in Jan 2004 and two observed in Apr 2005. All of the data were obtained with grating LR4, which covers the range 5015Å to 5831Å with a resolving power λ/∆(λ) of 6000. This range covers many stellar absorption features including strong Mg and Fe lines and is therefore excellent for determining the radial velocity of globular clusters. The fairly high spectral resolution also results in small velocity uncertainties (see Bergond et al. 2006). For studying possible emission lines from the black-hole X-ray binary, the narrow wavelength range is imperfect. However, it does provide a first look at the spectrum of the unique source, and the spectral resolution corresponding to a velocity width of about 50 km s −1 is useful for assessing whether any of the lines are broader than this value.
The bright X-ray source XMMU 122939.7+075333 identified in the XMM and Chandra images is located at the position of the globular cluster candidate RZ 2109 in the Rhode & Zepf 2001 cluster sample. The astrometric matching of these catalogs was described in MKZR07, with the agreement good to < 0.4 ′′ , a level similar to the positional uncertainties in the catalogs themselves. As discussed in MKZR07, this excellent agreement gives a negligible probability (< 10 −6 ) that the X-ray source and globular cluster are not associated with one another. This globular cluster was observed in two of the FLAMES fiber setups described above. One of these was observed for a total of 4.3 hours in Jan 04, and the other for 3.2 hours in Apr 05. The reduction of the FLAMES multifiber spectra was carried out in the same way as our previously published analysis of similar data for globular clusters around NGC 3379 (Bergond et al. 2006).
Our standard analysis of the radial velocity of RZ 2109 from the cross-correlation of its absorption lines with stellar templates gives a radial velocity of 1477 ± 7 km s −1 for the Jan 04 observations, and 1460 ± 19 km s −1 for the Apr 05 observations. We adopt a weighted average of 1475 ± 7 km s −1 as the best estimate of the globular cluster radial velocity (in MKZR07 we used the 1477 km s −1 of the observation with the smallest error). The agreement between the two gives high confidence in this radial velocity, and its value unambiguously identifies the object as member of the NGC 4472 globular cluster system, which has a mean radial velocity of 1018 km s −1 and a velocity dispersion of about 300 km s −1 (e.g. Zepf et al. 2000).

evidence for [oiii] 5007 emission
To test for the presence of emission lines in the optical spectrum of XMMU 122939.7+075333, we visually inspected the two independent FLAMES spectra of the object described above. As shown in Figure 1, both spectra show evidence for an emission line at a wavelength corresponding to [OIII] 5007 at the radial velocity observed for the host globular cluster. Although the line is only seen at modest signal-to-noise in each spectrum, the fact that it is observed in the same place in the two independent spectra, and that it is located precisely at the wavelength of [OIII] 5007 redshifted by the radial velocity of the cluster strongly supports this identification of the line. More specifically, we find the line center to be 5031.5Å in Jan 04 spectrum, while in Apr 05 spectrum, we find a line center of 5030.8Å with an uncertainty of several-tenths of an angstrom, based on both gaussian fits and simple mean wavelengths of the continuum subtracted emission line. We adopt 5031.2 ± 0.3Å as the best estimate of the emission line center. Based on a zero redshift wavelength of 5006.8Å for the [OIII] line, we then find a radial velocity of the line of about 1462 km s −1 with an uncertainty of about 18 km s −1 , in excellent agreement with 1475 km s −1 found for the stellar absorption lines of the host globular cluster.
A second key feature of the line seen in each spectrum is that it appears to be broader than the spectral resolution of the data. Specifically, standard gaussian fits to the line give a FWHM of 4 − 6Å in both independent spectra. This is much larger than the 0.9Å (4.5 pixels) FWHM of the effective instrumental resolution.  (Acker et al. 1992 and references therein), and therefore the observed velocity width of several hundred km s −1 eliminates the possibility that the optical emission is from a PNe.
The luminosity of the emission line can also be estimated from the spectra. Although flux standards were not observed as part of this program, the target globular cluster has a known magnitude of V=21.0 (RZ01). As a result, we can fix the flux in the continuum to be that for a globular cluster with V=21.0, and use that flux calibration to estimate the observed flux in the [OIII] 5007 line. We can also check that the detailed flux distribution across the V band agrees with that expected from models of stellar populations (e.g. Bruzual & Charlot 2003) like that of the globular cluster and its known colors. It is worth noting that the uncertainty in the determination of the total counts in the emission line is substantial, as described below, and thus the uncertainty in this flux calibration contributes negligibly to the error budget of the line luminosity determination.
It is in this way that the flux calibration was estimated for the two spectra shown in Figure 1. An independent check on this calibration can be made by comparing the observed signal to noise to that expected given the cluster's known magnitude and the observing conditions, based on the well characterized throughput of the FLAMES instrument. We find good agreement between these, providing further evidence that the flux estimate is reasonable, and that the uncertainty in the line luminosity is dominated by the measurement of the line itself.
The [OIII] line luminosity can then be determined by measuring the total line flux in the calibrated spectrum, and adopting a distance to NGC 4472 of 16 Mpc (Macri et al. 1999). The most uncertain part of this procedure is determining the total fux in the line, and in particular setting the level of the continuum and the range of wavelengths over which to determine the line flux. Our best estimate from both of our spectra in Figure 1 is that L([OIII] 5007) is about 4 × 10 35 ergs s −1 . However, if the line is broader than we are able to clearly discern in our data, we estimate this value could be roughly twice as large and we also find it might be possible to to fit the line with a flux that is smaller by 50%. Therefore, we attribute a factor of two uncertainty to our L([OIII] 5007) ≃ 4 × 10 35 ergs s −1 estimate.

discussion
We have shown that optical spectroscopy reveals [OIII] 5007 emission associated with the black-hole X-ray binary system in a globular cluster around the elliptical galaxy NGC 4472. This [OIII] 5007 emission clearly establishes an interaction between the X-ray binary and material around the black hole system. As such it also provides a potentially valuable testbed for understanding the relation between the binary evolution and accretion history of black holes and their interaction with the surrounding interstellar medium. Our spectra have also been able to establish the general velocity width of the [OIII] 5007 line. The several hundred km s −1 width of the line both demonstrates the emission is associated with the black-hole X-ray binary since no other cluster source could produce it, and also shows the material may be driven out of the globular, as the velocity width is much larger than the escape velocity of globular clusters.
[OIII] 5007 can be produced in principle by either collisional or photoionization. We consider each of these possibilities and their implications for the mass of the black hole. In the collisional case, a strong wind is generated by the black-hole X-ray binary, which then drives a shock front into the interstellar medium of the globular cluster. We can write standard solutions for the radius R of this shock front at time t as R(t) = 28[(Ṁ /10 −6 M ⊙ /yr)(v 0 /20000 km s −1 ) 2 (0.1cm −3 /n 0 )] 1/5 (t/10 5 yr) 3/5 pc (e.g. Castor, McCray, & Weaver 1975), and the current velocity as V (t) = 3/5R(t)/t, whereṀ is the mass loss rate in the wind, v 0 the initial velocity, and n 0 the density of the interstellar medium. We now consider what likely values for n 0 ,Ṁ , v 0 , and t are and how these might be constrained by current and future observations. The density of the interstellar medium of globular clusters, n 0 , is only well-known in a few cases. The best constraint is probably for 47 Tuc, for which the gradient in pulsar dispersion measurements with radial position in the cluster gives a free-electron density of 0.067 ± 0.015 cm −3 (Freire et al. 2003). Similar studies of pulsar dispersion measurements give a similar or slightly higher density in M15 (Freire et al. 2001), and a recent study may have detected somewhat more neutral hydrogen in M15 (van Loon et al. 2006). Most other studies of globular clusters have only been able to place upper limits which are typically well above the adopted value. That there would be some interstellar medium in a globular cluster seems natural, as mass loss from the cluster stars provides a continual source of interstellar material. Even if many processes cause clusters to also lose ISM, the velocities with which this mass is driven out would have to be extraordinarily high or else the steady state density of the ISM will be generally similar to the fiducial value adopted here (Pfahl & Rappaport 2001).
The radius and current velocity of the shocked region also depend on the kinetic luminosity in the wind,Ṁ v 2 0 . In the context of super-Eddington accretion onto a stellar mass black hole, this should be close to the Eddington luminosity of the source, and should be independent of the actual accretion rate, providing it is well above the accretion rate require to yield the Eddington luminosity for an accretion efficiency of 0.1c 2 (Begelman, King & Pringle 2006). The observed X-ray luminosity of L X = 4 × 10 39 ergs/sec and and inner disk radius of ∼ 4000 km inferred from the X-ray data (MKZR07) can be well fit by this model with a 10 M ⊙ black hole accreting at about 40 times the rate required to give the Eddington luminosity.
In this case, the predicted initial wind velocity v 0 is 20000 km/sec andṀ is 8 × 10 −6 M ⊙ . These then give a timescale t of about 2 × 10 5 years for the bubble to expand to the point where the one-dimensional velocity of the outflow is about 150 km/sec. The corresponding size of the bubble is about 40 pc, which is within the tidal radius expected for a fairly massive outer globular cluster like RZ 2109. Taken at face value, a model in whichṀ is constant over the above timescale loses somewhat more mass than the 1 M ⊙ upper limit for a donor star in the globular cluster. However, the detailed application of this model involves many uncertainties including the level of isotropy of the accretion flow, uncertainties in the radii inferred from X-ray spectroscopy, and inhomogeneity in the ISM along with the effect of input of additional material from ongoing stellar mass loss in the globular cluster. It would then seem a wind-driven system with a constantṀ remains feasible.
Alternatively, if the system experiences periods of quiescence, the total mass lost and kinetic power are less than calculated assuming these are constant at the above values. This is because the timescale is the time since the object first became an energetic binary, whileṀ and thus the kinetic luminosity are the time averaged values since this time, and are thus reduced directly by the duty cycle. In either case, the calculation suggests that a scenario in which a super-Eddington stellar mass black hole driving a strong wind through the interstellar medium of the globular cluster may be able to produce the observed [OIII] 5007 emission line. The X-ray data also allow the possibility that the accretor is an intermediate-mass black hole (IMBH) emitting at sub-Eddington luminosities (MKZR07). However, a strong wind is not expected in the IMBH sub-Eddington case (Proga 2007 references therein), so if the [OIII] 5007 emission is from shocks, then it would seem the black hole has a stellar mass.
We now consider the possibility that the [OIII] 5007 emission line is photoionized. Because the velocity width of the line is significantly greater than the virial velocity of the globular cluster, if the line is photoionized, it must be produced near the black hole. For a 10 3 M ⊙ black hole, the velocity width indicates the line must come from about ∼ 7 × 10 14 cm away from the black hole. This may be possible, although it may prove difficult to have enough mass at the right distance from the black hole to produce the [OIII] 5007 line luminosity at the observed velocities. For completeness we note that for a stellar mass black hole the observed [OIII] 5007 seems unlikely to be due to photoionization. The broad lines observed in Galactic black-hole X-ray binaries are typically permitted lines such as the Balmer lines and He II 4686, as might be expected due to the high densities at the small distances from the central source required to match the velocities. In contrast, as described above a shock is expected if the accretor is a stellar mass black hole and can produce roughly the observed [OIII] 5007 emission. A final alternative is it may be possible for a stellar mass black hole to drive a wind as described earlier, but for photoionization of the wind material by the central source to play a significant role in producing [OIII] 5007.
This analysis shows that undertanding the physical process which produces the [OIII] 5007 emission can place constraints on the mass of the black hole. Determining the mass of the black hole has important implications beyond understanding this individual system. Specifically, it would seem very unlikely to find a stellar mass black-hole X-ray binary in a globular cluster with an intermediate mass black hole. This is because most black holes are believed to be either dynamically ejected (Kulkarni, Hut, & McMillan 1993, Sigurdsson & Hernquist 1993 or merge to form a IMBH (Miller & Hamilton 2002). Any surviving stellar mass black hole in a cluster with an IMBH would have to reside in the outer regions of the cluster to avoid either of these fates. However, a location in the outer region of a globular cluster with its low stellar density is not conducive to the stellar dynamical interactions that make close, accreting binaries with black hole primaries in globular clusters. Therefore, it appears that if it can be shown the black hole must have a stellar mass, then this globular cluster would be very unlikely to host an IMBH. This would then demonstrate that the M BH − σ relation for larger mass stellar systems does not hold for all stellar systems at globular cluster masses.
There are several future observations that will help constrain the physical parameters of this interesting system. One of these is expanding the wavelength coverage beyond the narrow region currently available. This will provide line ratio diagnostics such as [SII]/Hα and [OIII]/Hβ to determine the relative importance of collisional and photoionization processes. As described above, this will have immediate implications for the mass of the accreting black hole, as the currently observed [OIII] 5007 would seem to require photoionization for an IMBH, while shock ionization works well in the case of a stellar mass black hole. Wider wavelength coverage would also lead to constraints on the density structure in the cluster interstellar medium, and possibly assessing the total power and lifetime of the system.
A key parameter that may be constrained by future observations is the radius at which the [OIII] 5007 emission originates. We note that at the 16 Mpc distance of NGC 4472, 1 ′′ corresponds to 77 pc, so images with much higher spatial resolution than this, such as from HST, may be able to resolve the nebulae and determine its size if it originates in shocks. Correspondingly, a tight upper limit would suggest photoionization, as the spatial scale in this case is orders of magnitude smaller and unresolvable. Moreover, if the line emission originates in shocks, resolving it would allow the size and velocity width to be combined to directly give the timescale, and the total kinetic luminosity would then follow. This would also provide a constraint on whether the accretion has been constant or intermittent, since it is the time averaged kinetic luminosity that matters for the shock radius while the timescale that matters is the total lifetime of the system. SEZ and AK acknowledge support for this work from XMM grant NNG04GF54G, NSF award AST-0406891 (SEZ), and NASA-LTSA grant NAG5-12975 (AK), GB is supported at the IAA/CSIC by an I3P contract (I3P-PC2005-F) funded by the European Social Fund, with additional support by DGI grant AYA 2005-07516-C02-01 and the Junta de Andalucía, and KLR acknowledges support from a NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-0302095. The authors acknowledge useful discussions with Phil Charles, Mike Eracleous, Andrew King, Dave Russell, and Mark Voit. We also thank the referee for a prompt and effective report. Fig. 1.-Plots of the optical spectra of the globular cluster shown to host a black hole X-ray binary by MKZR07. Both of the two independent fiber spectra from FLAMES on the VLT show emission with a central wavelength of 5031.2Å with a uncertainty of several tenths of an angstrom, and a line width of about 4 − 6Å . The observed wavelength strongly supports the identification of this line as [OIII] 5007 from the globular cluster, which has a radial velocity of the 1475 km s −1 determined from its stellar absorption lines. The dotted line denotes the wavelength expected at [OIII] 5007 at this redshift. The 4 − 6Å width of the [OIII] 5007 line is clearly broader than the 0.9Å spectral resolution of the data. This width corresponds to several hundred km s −1 , much larger than any planetary nebulae, and implicates the black-hole X-ray binary as the source of the energy driving this emission. The plotted data have been smoothed by the instrumental resolution, and the calibration of the flux given on the y-axis is described in the text.