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The HELLAS2XMM survey: II. Multiwavelength observations of P3: an X-ray bright, optically inactive galaxy

Recent X-ray surveys have clearly demonstrated that a population of optically dull, X-ray bright galaxies is emerging at 2-10 keV fluxes of the order of 10^{-14} erg cm^{-2} s^{-1}. Although they might constitute an important fraction of the sources responsible for the hard X-ray background, their nature is still unknown. With the aim to better understand the physical mechanisms responsible for the observed properties, we have started an extensive program of multiwavelength follow-up observations of hard X-ray, optically quiet galaxies discovered with XMM-Newton. Here we report the results of what can be considered the first example of this class of objects: CXOUJ031238.9-765134, originally discovered by Chandra, and optically identified by Fiore et al. (2000) with an apparently normal early-type galaxy at z=0.159, usually known as"FIORE P3". The analysis of the broad-band energy distribution suggests the presence of a heavily obscured active nucleus.


INTRODUCTION
The discrete sources responsible for a large fraction (>75 %) of the X-ray background emission below 8 keV have been detected by Chandra deep surveys down to extremely faint fluxes S 2−8keV < 10 −15 erg cm −2 s −1 (Mushotzky et al. 2000;Giacconi et al. 2001;Hornschmeier et al. 2001;Brandt et al. 2001;Tozzi et al. 2001). The unprecedented arcsec Chandra spatial resolution allows to unambiguously identify the optical counterparts of the X-ray sources. Extensive programs of optical identifications showed that about half of them are associated with optically bright objects at redshifts < 1.5 while the other half appears to be a mixture of optically faint (I > 23.5) galaxies and Active Galactic Nuclei, presumably at high redshift. The source breakdown of the optically bright popu-lation is intriguing. Contrary to the situation for the faint soft X-ray selected ROSAT sources (Schmidt et al. 1998), only a minority (< 20%) of them are associated with broad line objects and, even more surprisingly, some 50% of the spectroscopically identified sources do not show obvious signatures of AGN activity in their optical spectra, which are instead typical of early-type "normal" galaxies Giacconi et al. 2001;Barger et al. 2001). These findings seem to support previous claims (Griffiths et al. 1995) based on lower spatial resolution ROSAT observations.
The large X-ray-to-optical flux ratio, which exceeds by more than one order of magnitude the average value of earlytype galaxies of similar optical luminosity (Fabbiano, Kim, & Trinchieri 1992), and the hard X-ray spectra, determined from the analysis of X-ray colors, both suggest that (obscured) AGN activity is taking place in their nuclei. In principle X-ray spectroscopy could provide a stringent test on their nature. Unfortunately, all the sources are detected with a number of photons which is too small to apply conventional X-ray spectral fitting techniques and to constrain the absorbing column density. The lack of optical emission lines could be also explained if the nuclear light is overshined by either the stellar continuum or a non-thermal component, or if they are not efficiently produced.
A much better understanding of the sources powering Xray bright, optically quiet galaxies can be achieved only by means of multiwavelength observations. For this reason we have started an extensive program of multifrequency followup observations of hard X-ray selected sources serendipitously discovered in XMM-Newton fields over an area of about 3 deg 2 (the XMM High Energy Large Area Survey: HELLAS2XMM; see Baldi et al. 2002).
In this paper we present a multiwavelength study of what can be considered the first example of these objects: the Xray source CXOUJ 031238.9-765134, optically identified as a bright (R=18.0), "normal" early-type galaxy at z=0.159 by Fiore et al. (2000, hereinafter F00) in their pilot study of serendipitous X-ray sources in two shallow Chandra fields.

OBSERVATIONS
The field surrounding the radio loud quasar PKS 0312-76 has been observed by both Chandra and XMM-Newton during their Calibration and Performance Phases. The Chandra source CXOUJ 031238.9-765134 (hereinafter P3, being the third source catalogued by F00 in the PKS field) was also detected by XMM with a number of photons sufficient for a rough spectral analysis. Here we present the X-ray spectral analysis combined with deep radio observations and near-infrared and optical spectroscopy.

X-ray data
The analysis of Chandra data has been already reported by F00 based on preliminary calibration data and detection techniques. An additional observation of the PKS 0312-76 field was retrieved from the archive, combined with the first one and analyzed using version 2.2 of the CXC software. The periods of high background were filtered out leaving about 24.7 ksec of useful data. The WAVDETECT algorithm (Freeman et al. 2002) was run on the cleaned image conservatively setting the falsepositive probability threshold to 10 −7 . The source was clearly detected in both the soft (0.5-2 keV) and hard (2-8 keV) bands with 40±6 and 22±5 net counts, respectively. Although the counting statistics do not allow a proper spectral analysis, the hard to soft band ratio of 0.55±0.15 implies a hard power-law spectrum (Γ ≃ 1.4) assuming the Galactic absorption column density (N H = 8 × 10 20 cm −2 ; Dickey & Lockman 1990). The 2-10 keV Chandra flux is 2.5 × 10 −14 erg cm −2 s −1 , consistent with that reported by F00.
Our target was clearly detected also in the 25 ksec XMM-Newton PV observation of the same field. Even though the XMM positional accuracy is not as good as the Chandra one, the two X-ray centroids agree within about 2 arcsec.
The XMM and Chandra contours are overlaid on a R-band optical image in Fig. 1. It can be seen that the elongated XMM contours are probably due to the presence of a faint X-ray source which is clearly resolved by Chandra at a distance of ∼ 6 arcsec from P3. This object appears to be coincident with an extremely faint R > 24 optical counterpart. Its X-ray flux is only about 10-15 % of P3, while its hardness ratio suggests a spectral shape similar to P3.
The XMM data were processed using version 5.0 of the Science Analysis System (SAS). The event files were cleaned up from hot pixels and soft proton flares (see Baldi et al. 2002 for details). The hot pixels were removed using both the XMM-SAS and the IRAF task cosmicrays, while for the proton flares a count rate threshold was applied removing all the time intervals with a count rate greater than 0.15 c/s in the 10-12.4 keV energy range for the two MOS and greater than 0.35 c/s in the 10-13 keV band for the pn unit. The resulting exposure times are 24.7, 26.5 and 26.1 ksec in the pn, MOS1 and MOS2 detectors, respectively. The source is clearly detected in both the soft (0.5-2 keV) and hard (2-10 keV) bands with an approximately equal number of net counts (about 100 for each band combining the PN and MOS data). Source and background spectra were extracted from the cleaned events using an extraction radius of about 26" for the pn, 35" for MOS1 and 17" for MOS2 (the source is nearby a CCD gap). Background spectra were extracted by nearby source-free regions with radii between 1 and 1.5 arcminutes depending on the detector. The PN and MOS spectra were rebinned with at least 20 counts per channel and fitted simultaneously using XSPEC 11.0 leaving the relative normalizations free to vary. The latest version of response and effective area files was used. The Galactic column density in the direction of P3 is included in the spectral fits. The limited counting statistics of the observed spectra allow us to fit only simple models. Acceptable fits are obtained with both a power law with photon index Γ = 1.10±0.35 and with a thermal model with an extremely high temperature kT > 9 keV, confirming the presence of the hard spectrum inferred from the Chandra hardness ratio. A good description of the data is also obtained fixing the power-law slope at Γ=1.8 (an average value for AGN; e.g., Nandra et al. 1997); this gives an absorption column density N H = 8±5 × 10 21 cm −2 at the source frame (errors are at the 90% confidence level for one parameter). The 2-10 keV flux is in the range 2.5-3.0 × 10 −14 erg s −1 cm −2 , depending on the XMM detector, and is in good agreement with the Chandra value. The absorption-corrected 2-10 keV luminosity is about 3 ×10 42 erg s −1 . The good agreement between the power-law slope and the hard X-ray flux measured by Chandra and the XMM-Newton suggests the lack of significant flux and spectral variability on a time scale of about 7 months. Taking into account the present uncertainties in the adopted spectral models and the Chandra vs. XMM-Newton cross calibrations (Snowden et al. 2002), the maximum fractional variability allowed by the data is of the order of 30%. Our results are in good agreement with those recently obtained by Lumb, Guainazzi, & Gondoin (2001) from the analysis of the same XMM-Newton data. Our analysis, however, relies on a more detailed evaluation of the instrumental and cosmic backgrounds at the source position.

The Optical spectrum
The spectroscopic observations have been performed with the ESO 3.6m telescope equipped with EFOSC2 (Patat 1999) during two different observing runs. On the night of 2000 January 4 we used the grism #6 and a slit-width of 2 arcsec, whereas the grism #13 with a 1.5 arcsec slit was used in the night of 2000 December 26. Both configurations 2 yield a similar resolution of about 26Å but with the latter set-up the spectrum extends more toward the red up to 9000Å . The spectroscopic data have been reduced separately using standard IRAF routines. Bias exposures taken on each night were stacked, checked for consistency with the overscan regions of spectroscopic frames and subtracted out. The bias-subtracted frames were then flat-fielded in a standard manner using internal lamp flats obtained during the same run. The sky background was removed by fitting a third-order polynomial along the spatial direction in source free regions. The 2-dimensional spectra of P3 have been checked by eye and extracted via optimal averaging (Horne 1986). In both the observing runs the wavelength calibration was made using the HeAr arc lamp, and using the spectroscopic standard star LTT4364 (Hamuy et al., 1994) for flux calibration. The slit was oriented with different position angles during the two observations: in one case we set the angle in order to include in the slit both the principal target and a nearby fainter object (turned out to be a background galaxy). The other position angle has been chosen to include a bright star in the slit in order to perform an optimal subtraction of the atmospheric telluric features. Due to the different slit orientations, the non-photometric conditions of the nights and the relatively narrow slits we used, the present spectra cannot be considered spectrophotometric. The comparison of spectra taken one year apart shows good agreement, compatible with the precision of the flux calibration (∼ 15%). In Fig. 2 we show the average of the two spectra, with the principal identified absorption lines labeled. We also mark the [OII] emission line at 3727Å and the expected position of the Hα features. Unfortunately, at the redshift of P3 the Hα line position coincides with the strongest atmospheric telluric band in the optical. We did our best in order to remove the telluric features, but we cannot absolutely exclude the presence of a weak feature, either in emission or in absorption. The average redshift is z = 0.1595±0.0007, measured from the position of the principal absorption lines and by crosscorrelation with an early-type galaxy template.

The near-infrared spectrum
Near Infrared spectroscopic observations of the P3 galaxy were performed at the ESO Very Large Telescope (VLT). The data were collected on 2000 November 10 and 11 under good seeing (0.7 to 0.9 arcsec) and photometric conditions. The galaxy has been observed with the 1 ′′ slit in two different filters, SH (1.42-1.83 µm) and SK (1.84-2.56 µm), available for the Low Resolution grating (LR) in the Short Wavelength (SW) configuration of ISAAC (Moorwood et al. 1999). The pixel size is 0.147 arcsec/pixel along the slit and the resolving powers were 500 and 450 in the SH and SK band, respectively. The target was observed several times at different positions along the slit (nodding mode). Each single exposure was 200 s long, resulting in a total exposure time of 2 hours in each filter. The data were reduced using routines from the ECLIPSE (Devillard 1998), IRAF and MIDAS packages. Since we observed in nodding mode, the dark, bias, and sky contributions were removed by subtracting the pairs of offset frames. The data were then flat-fielded using a master flat field image. Wavelength calibration and correction for distortions along the slit were performed using ArXe arc exposures, while the distortion along the spatial direction was corrected using the star-trace exposures provided by ESO. The individual frames were then aligned and combined. The telluric absorption features were removed using the spectroscopic standard Hip33590 (A0V) after deleting from the standard spectrum the intrinsic stellar features. To obtain the intrinsic shape of the continuum of the galaxy and to perform the flux calibration we have used a blackbody curve with the appropriate temperature to fit the intrinsic continuum of the standard. In order to correct for the slit losses we have finally normalized the spectra to the magnitudes in the H and Ks bands (H=15.7, K ′ =14.9) obtained by the acquisition images. The final spectra in both the SH and SK filters are shown in Fig. 3. The infrared spectrum is consistent, within the precision of the flux calibration (∼ 15%), with a featureless continuum. There is no evidence of significant emission lines and in particular of Pα. Imaging observations in the L band (3.8µm) have been performed at the VLT with the ISAAC camera along with the NIR infrared spectroscopy. About half of the total exposure time of 30 minutes was lost due to technical problems. The images have been acquired combining chopping and telescope nodding in opposite directions to allow a better sky subtraction (see ISAAC User Manual). All of the chopped images, i.e. background-subtracted frames with a positive and negative image, have been flat fielded and then combined together with a median filter to obtain the final image. The data have been calibrated using the IR photometric standard HD219761 from Van der Bliek, Manfroid, & Bouchet (1996) list. The source is not detected at the limiting magnitude L = 13.4 (3σ upper limit).

Radio data
Radio observations of the PKS 0312-76 field were performed at 5 GHz with the Australia Telescope Compact Array (ATCA) in the 6-km configuration (maximum baseline length), with a resolution of 2 × 2 arcsec 2 . The data were collected in a run of 12-hours on 2000 September 27. In order to improve the sensitivity by a factor of √ 2, we used both ATCA receivers at 5000 MHz, each with a bandpass of 128 MHz. The optimal positioning of the bands was obtained by centering one frequency at 4800 MHz  and the other at 5824 MHz (5760-5888 MHz). The primary flux density calibrator was PKS 1934−638, while the source PKS 0230−790 was used as a phase and secondary amplitude calibrator. The field was observed in the mosaic mode by cycling through a grid of 5 pointings on the sky, one of which was centered on the position of P3. The grid of the pointings in the mosaic was designed to yield an uniform noise over the area covered by the Chandra data. With the mosaic technique, images obtained with single pointings are combined together into a large image (mosaic) of the entire observed region.
The data were analyzed with the software package MIRIAD. Each bandpass (4800 and 5824 MHz) was calibrated and cleaned separately to produce two individual images that we combined together into a single mosaic at the end of the reduction phase. Self-calibration was used to make additional correction to the antenna gains and to improve the image quality. The final map has an uniform noise of 50 µJy (1σ) over an area with a semicircular shape (due to the odd numbers of pointings) with a radius of about 10 arcmin, surrounded by an area where the noise increases with the distance from the center.
The P3 galaxy is well inside the area with an uniform noise of 50 µJy. We searched for a possible radio source within a box of 20 × 20 arcsec centered on the Chandra position. No sources were found down to 100 µJy (2σ). Therefore we can conclude that the P3 galaxy has not a radio counterpart in our map and that its 3σ radio upper limit at 5312 MHz is 0.15 mJy.

DISCUSSION
Although the overall spectral energy distribution (hereinafter SED) is dominated by the optical-infrared light of the host galaxy, the X-ray flux level is almost two orders of magnitude higher than that expected on the basis of the L X -L B correlation of early-type galaxies (Fabbiano et al. 1992).
The relatively high X-ray luminosity and the X-ray spectral properties strongly suggest nuclear activity in the central region of P3. Based on the Chandra detection and the optical spectrum F00 suggested three different possibilities, namely: a radiatively inefficient advection dominated accretion flow (ADAF), a BL Lac object or a hidden, possibly obscured, Seyfert-like AGN. The multiwavelength observations presented in this paper allow us to investigate in more detail these possibilities.
The ASCA detection of relatively luminous (L 2−10keV ≃ 10 40 − 10 42 erg s −1 ) extremely hard power-law components ( α = 0.2) in the nuclei of six nearby elliptical galaxies which are known to host a massive black hole (Allen, Di Matteo, & Fabian 2000) was consistent, given the observed radio emission, with an ADAF model in which a significant fraction of energy is removed from the flow by winds. This possibility however appears to be inconsistent with recent Chandra observations of three of these galaxies (Loewenstein et al. 2001): most of the spatially unresolved hard X-ray emission detected by ASCA originates in X-ray binaries. The lack of significant hard X-ray emission at the position of the optical nuclei allowed to place upper limits of about 1-2 order of magnitude lower than the luminosities estimated by Allen et al. (2000). Although ADAF models with strong outflows or convection might still be accommodated with the Chandra observations, the parameters computed by Allen et al. (2000) to fit the ASCA data need to be reconsidered. A detailed comparison of ADAF models to the present data is beyond the scope of this paper.
Here we limit ourselves to compare the observed fluxes in the radio and X-ray band with the model spectra of Quataert & Narayan (1999) for several values of the physical parameters involved in the ADAF models with winds as most of the bolometric luminosity predicted by these models is produced in the hard X-ray and radio bands. For the range of model parameters computed by Quataert & Narayan (1999) the X-ray flux density is about 1-2 order of magnitude greater than the ra- dio flux density (see their Fig. 7). The observed lower limit on the X-ray to radio flux density of P3 (> 10 3 ) is clearly inconsistent with ADAF model predictions. The observed SED is consistent with an early-type galaxy template normalized to match the optical and infrared spectrum plus the average SED of radio-quiet AGN (Elvis et al. 1994) rescaled to the observed X-ray flux (Fig. 4, left panel). We note that the average SED of radio quiet quasars has been obtained from a sample of highluminosity quasars and thus is unlikely to be appropriate for low-luminosity AGN (hereinafter LLAGN) which are known to be characterized by a quite different broad-band spectrum (Ho 1999). In particular, low-luminosity objects lack an ultraviolet excess and have a more pronounced contribution from X-rays to the overall energy output. The relative ratio between X-ray and optical flux is usually parameterized by the two point spectral index α ox (the slope obtained connecting the fluxes at 2500Å and 2 keV) which is significantly lower for lowluminosity AGN (between 0.5 and 1.0) with respect to higherluminosity Seyferts and quasars, for which α ox ≃ 1.5 (e.g., Brandt, Laor, & Wills 2000). The calculation of α ox for P3 is not straightforward as the 2500Å flux density obtained extrapolating the observed optical spectrum is likely to be dominated by the host galaxy. As a consequence, the derived value of α ox = 0.92 should be considered an upper limit on the "nuclear" optical to X-ray flux ratio. Prompted by the similarities between LLAGN and the P3 multiwavelength data, the broad-band SED of the four LLAGN hosted by an elliptical galaxy in the Ho (1999) sample have been compared with that of P3 and normalized to the observed X-ray flux (Fig. 4, right panel). Although the comparison sample is very small, we note that the average flux in the radio band of low-luminosity AGN greatly exceeds the observed upper limit. The general trend of increasing Xray variability for decreasing X-ray luminosities (Nandra et al. 1997) would also be inconsistent with the lack of strong X-ray variability.
The BL Lac hypothesis might be tenable despite the presence of a large Calcium break (F00) and the low radio flux density. Indeed, it has been proposed (Giommi & Padovani 1994) that BL Lac objects discovered in radio and X-ray surveys are the two extremes of a single population differing by the frequency of the high-energy cut-off in their energy distribution which is related to the maximum energy of the synchrotron emitting electrons. Moreover, there is increasing evidence that the SED of BL Lac objects and Flat Spectrum Radio Quasars (Blazars) can be unified in a spectral sequence determined by the total luminosity Ghisellini et al. 1998). The objects populating the low-luminosity extreme of the sequence peak at high frequency (in the X-ray band) and are called HBL. The upper limit on the 5 GHz radio luminosity (< 10 39 erg s −1 ), on the radio to X-ray spectral index (α rx < 0.6, F ν ∝ ν −α ) and the flat slope in the X-ray band would be consistent with the presence of a rather extreme member of the HBL class hosted by the P3 galaxy. The lack of X-ray variability, however, may constitute a problem with this interpretation.
An alternative explanation would be the presence of a hidden AGN. There are several examples of (obscured) AGN discovered only by means of X-ray observations and not recognized as such by optical spectroscopy (see Matt 2001 and references therein). In order to test whether this is the case for P3, we have computed the expected optical magnitude in the R band assuming an average value of the optical to X-ray flux ratio typical of hard X-ray selected unobscured quasars which turned out to be R=19.4. For a "standard" Galactic extinction curve (Savage & Mathis 1979) the best-fit N H value obtained from the analysis described in Sect. 2.1 corresponds to A R = 2.6. Extensive simulations have been carried out, using the IRAF program artdata, to estimate if a nuclear point-like source with R=22 would have been detected in the P3 nucleus. The results indicate that a nuclear source fainter than R ≈21.5 could not be revealed with the quality of the present data which are limited by poor seeing conditions (about 1.8 arcsec). Although the present findings indicate that a mildly obscured AGN could be hosted by P3, we note that hard X-ray selected objects with L X > 10 42 erg s −1 are generally characterized (with some exceptions, e.g. Brandt et al. 1997) by a dust to gas ratio much lower than the Galactic standard . Taking these results at the face value the putative X-ray mildly obscured nucleus would have probably been detected in the optical band, though this conclusion is subject to some uncertainties on the relation between X-ray and optical absorption.
FIG. 5.-The observed SED is compared with that of the highly obscured Seyfert 2 galaxy NGC 6240.
As a final possibility we have considered the hypothesis of a Compton thick (N H > ∼ 1.5 × 10 24 cm −2 ) AGN. The P3 SED is compared (Fig. 5) with that of NGC 6240, a highly obscured AGN considered to be the prototype of this class of objects (Vignati et al. 1999). The NGC 6240 SED is normalized to match the P3 optical flux. Besides the disagreement in the radio band and at 60 µ, which can be due to a more intense starformation in NGC 6240, the P3 multiwavelength data are in relatively good agreement with the obscured AGN template. If P3 hosts a Compton thick AGN with a SED similar to that of NGC 6240 then a luminous (L X ≃ 10 44 erg s −1 ) X-ray source should be present at higher energies. In this case the observed X-ray emission would be due to a scattered nuclear component known to be common among highly obscured AGN (Vignali et al. 2001 and references therein). The lack of strong X-ray variability would support this hypothesis.
Whatever is the nature of the AGN powering P3, it is interesting to see how a source with similar broad-band properties would appear at high redshift and whether P3-like objects constitute a significant fraction of the X-ray faint sources which are being discovered in deep surveys ). The 2-10 keV P3 luminosity can be detected, at the limit of the present Chandra surveys, up to z≃1.5. At this redshift the optical and infrared magnitudes of a passively evolving elliptical galaxy (Pozzetti et al. 1998) would be R ≃ 24.2 and K ′ ≃ 18.5. Such an object would not belong to the optically faint X-ray source population investigated by Alexander et al. (2001), but rather to the class of Extremely Red Objects (EROs; R-K > 5). This is not surprising since passively evolving elliptical galaxies have extremely red colors at z >1. The EROs Xray properties have been recently investigated using the 1 Ms exposure in the Chandra Deep Field North (Alexander et al. 2002). The objects detected in the hard (2-8 keV) band have extremely flat spectral slopes and X-ray properties consistent with those expected from obscured AGN. It is also worth to remark that a high-z P3-like object would be bright enough in the optical band to allow spectroscopic observations at 8m class telescopes. Optical and infrared observations of faint Xray sources might thus uncover several examples of completely hidden high-redshifts AGN.

CONCLUSIONS
The most important results obtained from our analysis of multiwavelength data can be summarized as follows: • The high X-ray luminosity and hard X-ray spectrum clearly indicate the presence of nuclear activity in P3.
• There is no evidence of AGN emission lines both in the optical and infrared spectra.
• The upper limit on the 5 GHz radio emission qualifies the AGN hosted by P3 (regardless of its origin) as a radio-quiet object.
• The multiwavelength properties allows us to rule out ADAF emission and the presence of a low-luminosity AGN with a SED similar to those of the few nearby objects in the Ho (1999) sample.
• An extreme BL Lac object, where the level of the nonthermal continuum in the radio and optical bands is very low, could provide an acceptable explanation of the P3 SED. The lack of X-ray variability, however, seems to be at odd with this interpretation.
• The AGN responsible for the high X-ray luminosity is likely to be hidden by a substantial (possibly Compton thick) column of cold absorbing gas. If this is the case most of the observed X-ray emission would be due to a scattered/reprocessed nuclear component.
Sensitive broad band observations of a sample of X-ray bright, optically quiet galaxies are required to better understand their nature. Deep optical and infrared spectroscopy of X-ray faint sources would allow to assess their fraction in deep Xray surveys with important consequences for our understanding of the nature and the evolution of the X-ray background constituents.