5 Discussion

5.1 Classification

The absorption features detected in the VLT spectrum clearly place XMMU J124825.9+083020 at z=0.43. We identify the source as a BL Lac object, for the following reasons. (1) We detect no emission lines; the upper limit to any emission feature is $\sim$5 Å. (2) The stellar features superposed on the featureless continuum have strengths generally consistent with those of other BL Lac objects. We demonstrate this concretely by comparing a redshifted spectrum of the BL Lac object 0548-322 (Fig. 4). The break contrast at 4000 Å is $\sim$29%, again similar to other BL Lac objects (Laurent-Muehleisen et al. 1998). (3) The optical continuum slope measured in the Magellan spectrum is consistent with those of high-frequency peaked BL Lac objects (e.g., Stickel et al. 1993). And (4) the multiwavelength spectral energy distribution (Fig. 5), although not assembled from simultaneous observations, is highly reminiscent of those of BL Lac objects.

By using two-point spectral indices, namely the radio-to-optical  $\alpha_{{\rm ro}}$ and optical-to-X-ray  $\alpha_{\rm ox}$, it is possible to show that different objects populate different regions of the $\alpha_{{\rm ro}}$- $\alpha_{\rm ox}$ plane (e.g., Brinkmann et al. 1997; Laurent-Muehleisen et al. 1999). Another method has been suggested by Maccacaro et al. (1988), who proposed a nomograph to link the X-ray flux in the energy band 0.3-3.5 keV and the visual magnitude. The values of  $\alpha_{{\rm ro}}$ and  $\alpha_{\rm ox}$ for the present source are 0.42 and 0.95, respectively, thus placing it in the region of X-ray selected BL Lacs (Brinkmann et al. 1997), or high-energy peaked BL Lacs in the $\alpha_{{\rm ro}}-\alpha_{{\rm ox}}$ diagram of Laurent-Muehleisen et al. (1999). The nomograph of Maccacaro et al. (1988) gives a ratio $f_{{\rm x}}/f_{{\rm v}}$ between 1.3 and 3.8 (depending on whether we use the HST optical magnitudes through the F606W or F450W filter, respectively), in the regime of AGNs and BL Lacs.

The spectral indices can be used to deduce some general properties of the dominant radiation mechanism. If $\alpha_{{\rm x}} \leq \alpha_{{\rm r}}$, the source may exhibit relativistic beaming, while if $\alpha_{{\rm x}} > \alpha_{{\rm r}}$, it may fulfill the conditions of the homogeneous synchrotron model (Harris & Krawczynski 2002). In our case, we have $\alpha_{{\rm x}} \approx \alpha_{{\rm r}} \approx 1$, so that we cannot clearly discriminate between these two cases. If the source is a high-frequency peaked BL Lac, however, it is likely that the homogeneous synchrotron model is more applicable. This is confirmed by the $\alpha_{{\rm xox}}$ test of Sambruna et al. (1996): in this case, the difference by $\alpha_{{\rm ox}}-\alpha_{{\rm x}}=\alpha_{{\rm xox}}$ is approximately equal to zero, so avoiding a clear discrimination between the physical mechanism of the source.

The spectral energy distribution (Fig. 5) is comparable to those of BL Lac objects peaked in the $\gamma$-ray domain (e.g., Fossati et al. 1998), but the third EGRET catalog (Hartman et al. 1999) does not have any source within several degrees of XMMU J124825.9+083020.

5.2 Search for ULXs and Contamination with Background Objects

This research is a useful demonstration of just how difficult it is to identify the physical nature of ULXs. It shows the vital importance of redshift determinations. Although we had a lot of photometric data on XMMU J 124825.9+083020, by themselves they were insufficient to clearly establish whether the source belongs to NGC 4698 or is a background object. In the first case, the source would have been something similar to a microquasar (e.g., Mirabel & Rodríguez 1999), probably located in a globular cluster. Indeed, the optical image of the source appeared to be slightly extended (angular extent $\sim$ $3\hbox{$^{\prime\prime}$ }$), and its position with respect to NGC 4698 suggested that it could be a globular cluster, albeit an unusually large one. The possibility of an accreting black hole in a globular cluster is not so remote: Chandra observations of the globular cluster system of NGC 4472 show that about 40% of the bright low mass X-ray binaries are associated with optically identified globular clusters (Kundu et al. 2002). In addition, the X-ray luminosity function shows a break near $3\times 10^{38}$ erg s^-1, suggesting that the brightest X-ray binaries are accreting black holes (Kundu et al. 2002), perhaps microquasars.

Figure 5: The spectral energy distribution of XMMU J124825.9+083020, assembled from data taken with the VLA, DSS, HST, and XMM.

XMM-Newton observations of the Lockman Hole (Hasinger et al. 2001) show that the number of background sources in the energy band 0.5-2 keV with flux greater than $4.0\times 10^{-14}$ erg cm^-2 s^-1 (the best fit value from Table 1) is 15 deg^-2. In the energy band 2-10 keV, there are 40 sources deg^-2 with flux higher than $4.6\times 10^{-14}$ erg cm^-2 s^-1. Assuming the same $\log N - \log S$ relation, and considering that the D₂₅ area of NGC 4698 is about 7.9 arcmin², we expect 0.08 background objects in the 2-10 keV energy band and 0.03 in the energy band 0.5-2 keV. However, despite these low values, we have found, in the present case, that the only ULX is a background AGN.

The above calculations could be underestimated in the present case because the VLT images show an unknown concentration of galaxies north-east of NGC 4698 (see Fig. 1), thus suggesting the possibility of a statistically meaningful excess of background sources. However, no additional X-ray sources is seen in the present XMM-Newton observation. Perhaps, a longer exposure may reveal soft X-ray emission or additional sources, if NGC 4698 lies along the line of sight to a galaxy cluster. The three X-ray sources identified to date, however, have three redshifts: XMMU J124825.9+083020 has z=0.43, NGC 4698 has z=0.0033, and the ROSAT source 1RXS J124828.1+083103 has been recently identified with a Seyfert nucleus at z=0.12 (Xu et al. 2001). Therefore, these three sources are not members of a single cluster.