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On the redshift of the bright BL Lac object PKS 0048-097

Aims: The determination of elusive redshifts of bright BL Lac objects Methods: We use the capabilities of newly available spectrograph X-Shooter at European Southern Observatory (ESO) Very Large Telescope, that combines high resolution and a large wavelength range, to obtain UVB to near-IR spectra of BL Lacs. Results: Our observations of PKS 0048-097 detect three emission lines that permit to derive a redshift z = 0.635. Moreover, a Mg II absorption system at z = 0.154 that is associated with a foreground spiral galaxy at 50 Kpc projected distance is found. Conclusions: The obtained redshift allows us to comment about the optical beaming factor and the absorption of the high energy spectrum by the Extragalactic Background Light.


Introduction
PKS 0048-097 (m V ∼ 15.5) is a bright, strongly variable (Pica & Smith 1983) and well studied BL Lac object (Wills et al. 1992;Falomo et al. 1993Falomo et al. , 1994Stickel et al. 1993). Despite many spectroscopical studies (Stickel et al. 1993;Sbarufatti et al. 2006) it remains one of the few bright sources with unknown redshift. Imaging studies of the source were unable to detect its host galaxy (Falomo 1996). Moreover, the upper limits to the brightness of the host galaxy derived from optical and NIR imaging suggest lower limits to the redshift of z > 0.3, even z > 0.5 (Falomo 1996;Kotilainen et al. 1998).
Stimulated by the availability of new spectroscopical instrumentation (ESO X-Shooter, Vernet et al. (2011)) we collected optical to near-IR spectra of the object aimed at detecting very faint features over a large spectral range.
The paper is organized as follow. Observations and data reduction are described in Section 2. Main spectroscopical results are reported in Section 3 where we also comment on Mg II intervening system that required a reconsideration of the close enviroment of the BL Lac. Discussion and conclusions are given in Section 4. Throughout the paper, we consider the following cosmological parameters: H 0 = 70 km s −1 Mpc −1 , Ω m = 0.27 and Ω Λ = 0.73.

Observations and data reduction
We obtained UVB (3100-5500 Å), Optical (5600-10.000 Å) and near-IR (10.100-15.000 Å) spectra of the target using the ESO Very Large Telescope (UT2 -Kueyen) equipped with X-Shooter in Service Mode. Two independent spectra were secured using the Nodding configuration for the instrument (see Table 1 for detailed information).
Data reduction was carried out using the X-Shooter Data Reduction Pipeline (version 1.3.7, see Goldoni (2011)) in Polynomial Mode adopting the configuration recommended by ESO. In particular, for each observation, we calculated the Master Bias Frames and Master Flat Frames for each channel selecting carefully the proper slit values (see Table 1). We also obtained the bidimensional mapping required by the pipeline stack to resample the Echelle Orders. Finally, for each channel, we computed the sensitivity functions required by the pipeline to calibrate in flux the spectra. We normalized in relative units the obtained spectra in order to proper juxtapose the three arms. The reduction of the data and the next extraction and calibration of the spectrum was performed for the two individual exposures (see above). Since no significant spectral differences were apparent between the two spectra we averaged them in order to improve the signal-to-noise ratio.
In Figure 1 we show the mean spectrum of PKS0048-097. The continuum is clearly dominated by the contribution of nonthermal emission which can be described by a power-law of the form F(λ) = λ −α (α ∼ 0.9). In addition to many telluric absorption lines we clearly detect intervening Galactic absorption of Ca II (EW: 0.16 Å -0.05 Å) and Na I (EW: 0.10 Å -0.07 Å). The flux calibration has been assessed throught the relative photometric calibration of the acquisition image. The R band magnitude of PKS 0048-097 is found to be 17.3 ± 0.3 which corresponds to a low state of the source. In fact, in the literature the R magnitude of the source is reported between 14.74 and 16.22 (Fan & Lin (2000) see also REM archives, Angela Sandrinelli private communication). The combination of the high quality instrumentation, in terms of throughput and spectral resolution, and the low state of the source clearly favored the detection of the faint emission lines.  Notes

Results
We searched for intrinsic emission or absorption features above ∼ 3σ signal-to-noise ratio of the spectrum (see Table 1) with the additional condition that the features are clearly detectable in both individual spectra and avoiding obvious defects in the 2D image. This procedure brings three emission lines (see Figure 1, 2 and Table 2) at λ = 6096Å , λ = 8189Å and λ = 10751Å. The line at λ = 6096Å was tentatively detected by Rector & Stocke (2001) (EW: 0.3 Å). We note that no previous near-IR observation of comparable spectral resolution is present in the literature. We identify the first two lines as [O II] (λ 0 = 3727Å) and [O III] (λ 0 = 5007Å) from which a consistent redshift of z = 0.635 is derived. The third emission line (λ = 10751Å) is identified as Hα that corresponds to z = 0.638. The small velocity difference (∼ 600 km s −1 ) between the redshift derived from the narrow emission lines and the broad ones is not uncommon in quasars (see e.g. Boroson (2005)). We therefore assume that the redshift of PKS 0048-097 is consistently z = 0.635.
We detect two narrow absorption features at λλ 3228.20 Å 3236.60 Å (see Figure 3) that we identify as an intervening Mg II absorption system at redshift z = 0.154. In order to search for objects associated with this absorption we investigated the enviroment around the target using high resolution deep image of the field, in the R band, obtained with ESO NTT and SUSI (Falomo 1996). The close companion galaxy (m R = 22.5) at 2.5 ′′ East of PKS 0048-097 (see panel (a) of Figure 4) already noted by Falomo et al. (1990) is too faint to be associated with the absorption system at z = 0.154 and it is likely a background galaxy. On the other hand, the spiral galaxy G1 (m r = 19.2) at ∼ 23 ′′ North-West from the BL Lac corresponding to 50 Kpc projected distance at redshift z = 0.154 (Stickel et al. 1993) is the obvious galaxy associated to the observed Mg II absorption. This  (Melnick J. 1994) by Falomo (1996) that shows its close enviroment. The spiral galaxy G1 (panel b) is the Mg II intervening system detected in the very blue part of the spectrum while the Galaxy G2 is a foreground object respect to the absorption. The panel (a) in the figure shows a deconvolution of the image. The small object nearby PKS 0048-097 is most probably a background galaxy. Panel (c) shows the image of galaxy G1 after the model subtraction while the panel (d) reports the average radial brightness profile of galaxy G1. galaxy ( M r = −20.57) has a disturbed morphology (see panel (b), (c) and (d) of Figure 4) after model subtraction and a scale length of ∼ 5 Kpc. This is a rare case-study of absorption system at low redshift (see e.g. Kacprzak et al. (2011)) in which the counterpart can be well investigated through imaging analysis. The column density correlated to the Mg II features, calculated adopting the Voigt profile fitting method, is log N a MgII = 13.20.

Beaming and Spectral Energy Distribution of PKS 0048-097
The luminosity of Hα (log Hα ∼ 41.93 erg s −1 ) of PKS 0048-097 is comparable with that of other BL Lac objects (Decarli et al. 2011). In order to derive the Optical Beaming Factor, defined as the ratio between L c and L th , we calculated the expected thermal luminosity of the continuum at 5100Å (log L th ∼ 43.16 erg s −1 , see Decarli et al. (2011)) and the continuum luminosity at the same wavelength (log L c ∼ 45.53 erg s −1 ). In the case of PKS 0048-097 this ratio is ∼ 234. The extreme value of this beaming (second only to OJ 287, see Figure  5) is consistent with no detection of the host galaxy (Falomo 1996). PKS 0048-097 is a γ-ray emitter observed with FERMI in the band 100 MeV -10 GeV and it is well fitted by a single power law (α 0 = 2.38±0.24). Assuming the extrapolation of this power law to the TeV band we calculated the expected emission cor-rected for the EBL absorption (Domínguez et al. 2011) adopting the derived redshift. The EBL interaction reduces the intrinsic flux above 50 GeV by a factor 1.7 and above 300 GeV by a factor 60. In Figure 6 we reported the overall SED of PKS0048-097 collected for multifrequency data by Tavecchio et al. (2010). We fitted the data using a standard leptonic one-zone model considering synchrotron and synchrotron self-Compton (SSC) radiation mechanism (see Maraschi & Tavecchio (2003)). We report two curves: the dashed line shows the intrinsic emission, while the solid one takes into account absorption by EBL photons, assuming the estimate of Domínguez et al. (2011). We recall that, despite different approaches and methods, models for the EBL substantially agree in the spectral region interesting for absorption of GeV-TeV photons (see discussion and comparison in Domínguez et al. (2011)). The one-zone models satisfactorily reproduce the SED above the IR band, below which the emission is self-absorbed in the region we consider. The emission at lower frequencies is therefore produced in more distant, transparent, region of the jets not relevant for the modeling of the high-frequency SED. These consideration may be relevant in discussing the observability of the source with Cherenkov telescopes.  FERMI spectra FGL1 and FGL2 are indicated respectively with blue and green points. The parameters of the model are γ min = 500, γ b = 8.5 × 10 3 and γ max = 5 × 10 5 , n 1 = 2, n 2 = 4.1, B = 0.46 G, K = 3.7 × 10 4 , R = 8.2 × 10 15 cm and δ = 25. For the references to the observations and details on the model see Tavecchio et al. (2010).