On the redshift of TeV BL Lac objects
Simona Paiano, Marco Landoni, Renato Falomo, Aldo Treves, Riccardo Scarpa, Chiara Righi
DDraft version October 12, 2018
Preprint typeset using L A TEX style AASTeX6 v. 1.0
ON THE REDSHIFT OF TEV BL LAC OBJECTS
Simona Paiano , Marco Landoni , Renato Falomo , Aldo Treves , Riccardo Scarpa , Chiara Righi INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5 I-35122 Padova (PD) - ITALY INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46 I-23807 Merate (LC) - ITALY Universit`a degli Studi dell’Insubria, Via Valleggio 11 I-22100 Como - ITALY Instituto de Astrofisica de Canarias, C/O Via Lactea, s/n E38205 - La Laguna (Tenerife) - ESPANA
ABSTRACTWe report results of a spectroscopic campaign carried out at the 10 m Gran Telescopio Canariasfor a sample of 22 BL Lac objects detected (or candidates) at TeV energies, aimed to determine orconstrain their redshift. This is of fundamental importance for the interpretation of their emissionmodels, for population studies and also mandatory to study the interaction of high energy photonswith the extragalactic background light using TeV sources. High signal-to-noise optical spectra inthe range 4250 - 10000 ˚A were obtained to search for faint emission and/or absorption lines fromboth the host galaxy or the nucleus. We determine a new redshift for PKS 1424+240 (z = 0.604)and a tentative one for 1ES 0033+595 (z = 0.467). We are able to set new spectroscopic redshiftlower limits for other three sources on the basis of Mg II and Ca II intervening absorption features:BZB J1243+3627 (z > > > Keywords:
BL Lac object spectroscopy — Redshift — TeV astronomy — Extragalactic BackgroundLight INTRODUCTIONBlazars are luminous emitters over the whole elec-tromagnetic spectrum up to TeV energies. They arehighly variable and polarized and are often dominated,especially during outbursts, by the gamma-ray emission.The standard paradigm for these sources is that theyowe their extreme physical behavior to the presence of arelativistic jet closely aligned with the observers direc-tion, a model that explains most of the peculiar prop-erties of these sources: superluminal motion, rapid vari-ability, huge radio brightness temperature, etc. Fromthe optical point of view, blazars showing very weaklines or a completely featureless spectra are named BLLac objects (BLLs) (see e.g. the review of Falomo et al.2014).Compared to other AGN, the featureless spectrumof BLLs is due to the extreme dominance of the non-thermal emission over the stellar emission of the host [email protected] galaxy, which make the assessment of their redshift verydifficult. (Sbarufatti et al. 2005b, 2006b,a, 2008; Lan-doni et al. 2012; Shaw et al. 2013; Landoni et al. 2013,2014; Massaro et al. 2014, 2015; Landoni et al. 2015;´Alvarez Crespo et al. 2016).The knowledge of the distance is, however, crucialto understand the nature of these sources, the physi-cal mechanism responsible for their extremely energeticemission, their intrinsic luminosity, and cosmic evolu-tion. Furthermore, in the case of TeV BLLs, the sim-ple knowledge of the redshift converts these sources intopowerfull probe of the Extragalactic Background Light(EBL) through γ − γ absorption, also improving our un-derstanding of supersymmetric particles thought to beproduced in their ultra-relativistic jet (see e.g. Tavec-chio et al. 2015). Thus, we undertook a spectroscopicobservational campaign of a sample of TeV (or TeV can-didate) BLLs with unknown or uncertain redshift tobe observed at the 10.4m Gran Telescopio CANARIAS(GTC), in order to improve our knowledge of the redshiftof TeV BLLs, a possibly unique test-bench for ultra-high a r X i v : . [ a s t r o - ph . GA ] J a n energy fundamental physics.The first results of this program were presented inLandoni et al. (2015) for S4 0954+65 and Paiano et al.(2016) for S2 0109+22. In this paper we report resultsfor 22 additional BLLs: 15 of them detected at TeVenergies, while 7 being good TeV candidates (Massaroet al. 2013).In Section 2 we outline the selection criteria of oursample and discuss their main properties. In Section 3we present the data collection and the reduction proce-dure. In Section 4 we show the optical spectra of eachobject, underlying their main features, and discuss theirredshift. In Section 5 we give detailed notes on individ-ual objects and finally in Section 6 we summarize anddiscuss the results.In this work we assume the following cosmologicalparameters: H = 70 km s − Mpc − , Ω Λ =0.7, andΩ m =0.3. THE SAMPLEWe selected all BLLs that are detected at Very HighEnergy band (VHE; >
100 GeV) from the online refer-ence catalog of TeV sources (TeVCAT ) with unknownor uncertain redshift and that are observable from LaPalma ( δ > -20 ◦ ). For objects with uncertain redshiftwe choose sources with contrasting redshift values re-ported in literature or with measurements from opticalspectra of low signal-to-noise ratio. This selection yields18 targets and we obtain observations for 15 of them (seeTab.1) that represent about 70% of the whole sample ofTeV blazars with uncertain or unknown redshift.In addition we selected BLLs from a sample of 41objects proposed as TeV emitters by Massaro et al.(2013) on the basis of the combined IR and X-ray prop-erties of BLLs reported in the ROMA-BZCAT cata-log (Massaro et al. 2009), satisfying the criteria of un-certain redshift and observability. This selection pro-duced 12 TeV candidates and we obtained spectra for7 of them (see Tab.1) that represent ∼
60% of the un-known/uncertain TeV candidate emitters proposed byMassaro et al. (2013). 1424+240 OBSERVATIONS AND DATA REDUCTIONObservations were obtained between February 2015and August 2016 in Service Mode at the GTC usingthe low resolution spectrograph OSIRIS (Cepa et al.2003). The instrument was configured with the twogrisms R1000B and R1000R , in order to cover the spec- http://tevcat2.uchicago.edu/ half of them have unknown or uncertain redshift tral range 4000-10000 ˚A, and with a slit width = 1”yielding a spectral resolution λ /∆ λ = 800.For each grism three individual exposures were ob-tained (with exposure time ranging from 300 to 1200seconds each, depending on the source magnitude), thatwere then combined into a single average image, in or-der to perform optimal cleaning of cosmic rays and CCDcosmetic defects. Detailed information on the observa-tions are given in Tab.2.Data reduction was carried out using IRAF andadopting the standard procedures for long slit spec-troscopy with bias subtraction, flat fielding, and badpixel correction. Individual spectra were cleaned ofcosmic-ray contamination using the L.A.Cosmic algo-rithm (van Dokkum 2001).Wavelength calibration was performed using the spec-tra of Hg, Ar, Ne, and Xe lamps and providing an ac-curacy of 0.1 ˚A over the whole spectral range. Spec-tra were corrected for atmospheric extinction using themean La Palma site extinction table . Relative flux cal-ibration was obtained from the observations of spectro-photometric standard stars secured during the samenights of the target observation. For each object thespectra obtained with the two grisms were merged into afinal spectrum covering the whole desired spectral range.Thanks to the availability of a direct image of thetarget, which is obtained at GTC as part of target ac-quisition, the spectra could be flux calibrated. The cal-ibration was assessed using the zero point provided bythe GTC-OSIRIS webpage . For half of our sample itwas also possible to use stars with known flux from theSDSS survey to double check the flux calibration. Wefound no significant difference on average between thetwo methods within ∼ . RESULTSThe optical spectra of the targets are presented inFig. 4. In order to emphasize weak emission and/orabsorption features, we show also the normalized spec- IRAF (Image Reduction and Analysis Facility) is distributedby the National Optical Astronomy Observatories, which are op-erated by the Association of Universities for Research in Astron-omy, Inc., under cooperative agreement with the National ScienceFoundation. https://irsa.ipac.caltech.edu/applications/DUST/ Search for emission/absorption features
All spectra were carefully inspected to find emissionand absorption features. When a possible feature wasidentified, we determined its reliability checking that itwas present on the three individual exposures (see Sec.3 for details). We were able to detect spectral linesfor 9 targets. In particular we observe [O III] 5007 ˚Aweak emission in the spectra of 1ES 1215+303, W Co-mae, MS 1221.8+2452 and PKS 1424+240, [O II] 3727 ˚Ain 1ES 0033+595, 1ES 1215+303 and PKS 1424+240,the Ca II 3934,3968 ˚A doublet absorption system andthe G-band 4305 ˚A absorption line in MS 1221.8+2452,a strong emission of Mg II 2800 ˚A in S3 0218+357and intervening absorption systems due to Mg II 2800˚A in BZB J1243+3627 and BZB J1540+8155 and, fi-nally the Ca II 3934,3968 ˚A doublet in the spectrum ofBZB J2323+4210. Details in Fig. 5 and Tab. 5. Thespectrum of 7 additional targets is found completely fea-tureless even though a redshift is reported in literature.Details about the optical spectra and redshift estimatesfor each objects of our sample are given in Sec. 5.4.2.
Redshift lower limits
Based on the assumption that all BLLs are hosted bya massive elliptical galaxy (e.g. Falomo et al. 2014) oneshould be able to detect faint absorption features fromthe starlight provided that the SNR and the spectralresolution are sufficiently high. In the case of no detec-tion of spectral features it is possible to set a lower limitto the redshift based on the minimum Equivalent Width(EW) that can be measured in the spectrum.The minimum measurable equivalent eidth (EW min )was set according to the scheme outlined by Sbarufattiet al. (2006a,b), though in a more elaborated procedure(see Appendix A). In brief from the normalized spec-trum (see Fig. 4) we computed the nominal EW adopt-ing a running window of 15 ˚A for five intervals of thespectra that avoid the prominent telluric absorption fea-tures (see Tab. 3). The procedure yields for each giveninterval a distribution of EW and we took as minimummeasurable EW three times the standard deviation ofthe distribution (see details in Appendix A).Five different intervals were considered because theSNR changes with wavelength. The range of EW min is reported in Tab. 3 and we give a lower limit on z assum-ing a standard average luminosity for the host galaxyM R = -22.9 (or M R = -21.9 in parenthesis). NOTES FOR INDIVIDUAL SOURCES
BZB J0035+1515 : The source was first discov-ered by Fischer et al. (1998) and catalogued asBLL on the basis of its featureless optical spec-trum. A more recent optical spectrum, obtainedas part of the SDSS survey, exhibits no features(although the automatic procedure suggests sometentative values, also included in NED). Also Shawet al. (2013) found a featureless spectrum.We confirm the featureless nature of the spectrumfrom 4200 to 9000 ˚A and from our high SNR weobtain an EW min of 0.09 - 0.18 ˚A, which corre-spond to a redshift lower limit of z > : Perlman et al. (1996) identi-fied this Einstein Slew Survey source as a BLLfinding a featurless optical spectrum (although atentative redshift z = 0.086 was derived by Perl-man et al. as mentioned in Falomo & Kotilainen(1999)). In Scarpa et al. (1999) the HST imagesof this object shows two unresolved sources, “A”and “B”, separated by 1”.58 and with magnitude m R =17.95 ± ± ∼
100 (see Fig. 4) andalthough there is some contamination of the spec-trum by the companion, the non detection of H α indicates that the “B” object has an extragalacticnature and it is the blazar counterpart as proposedby Scarpa et al. (1999). We found an emission fea-ture at 5468 ˚A of EW = 0.4 ˚A (see Fig. 5). Thisfeature is detected in all three individual spectraand therefore we consider it a secure detection. Ifidentified as [OII] 3727˚A emission, a tentative red-shift of z = 0.467 can be provided.Finally, comparing our photometry with Scarpaet al. (1999), we obtain the same value for the Aobject, while for the object B we obtain a magni-tude difference of 1.2 with respect to the previousone reinforcing the classification of this source asa BLL. Figure 1 . r-band optical image of the sky region around theBL Lac object 1ES 0033+595 obtained at the GTC. Thesource flagged as “A” is a foreground star and the BLL isthe source labelled as “B”.
Figure 2 . GTC spectrum of the companion source, labelledas “A” (see Fig.1, of the BL Lac object 1ES 0033+595. Ab-sorption lines due to CH (4299 ˚A), hydrogen (4342 ˚A, 4863˚A, 6565 ˚A), and Mg I (5176 ˚A) are clearly detected. Telluricbands are indicated by ⊕ . This object can be classified as aG-type star. RGB J0136+391 : The first identification ofthis source as BLL was proposed by Laurent-Muehleisen et al. (1998) showing a featureless op-tical spectrum. The same result was found in Weiet al. (1999); Piranomonte et al. (2007) and Shawet al. (2013). A lower limit on the redshift ofz > ∼ > S3 0218+357 : This source was discovered to be agravitational lens by Patnaik et al. (1993) who de-tected two similar radio sources with ∼ β and[OIII] at z = 0.684 attributed to the lens galaxy.Moreover they claimed the detection of weak H β and [OIII] emission in the red noisy spectrum, alsoattributed to the blazar at z = 0.944We obtain an optical spectrum ranging from 4500to 10000 ˚A with a SNR in the range 25-50. Weconfirm the detection of Mg II and Ca II absorp-tion lines at z = 0.684, and in addition we clearlydetect an absorption line at 9920 ˚A identified asNa I 5892 ˚A at the redshift of the lens. We donot detect the emissions line [OII], H β and [OIII](claimed by Cohen et al. 2003). We note that someof these latter features occur inside the telluric ab-sorptions of the O and H O. We clearly detect thestrong broad emission line at 5480 ˚A (EW=35 ˚A,FWHM=4700 km/s) that if attributed to Mg II2800˚A, yields the redshift of z = 0.954. We stressthat in our spectrum we do not detect the claimedemissions H β and [OIII] attributed to the blazar byCohen et al. (2003). We note again that these fea-tures are placed in a spectral region that is heavilycontaminated by strong H O atmospheric absorp-tion. Therefore we conclude that the redshift ofthe S3 0218+357 is still tentative since it is basedon only one line. If confirmed, this source is themost distant blazar detected at frequencies >
3C 66A : Wills & Wills (1974) identified thisstrong radio source as a BLL because of its fea-tureless optical spectrum. Miller et al. (1978) pro-posed a redshift of z = 0.444, on the basis on oneemission line attributed to Mg II 2800 ˚A. A valueconsidered by the authors as tentative and highlyuncertain. No other optical spectroscopy was donefor thirty years. More recently Finke et al. (2008)showed an optical spectrum in the range from 4200to 8500 ˚A with no detectable optical features. Thefeatureless spectrum was also confirmed by Shawet al. (2013).Our high SNR ( ∼ EW min , due to the relativelybright source we can set only a modest lower limitof z > β emissionline at 7020 ˚A, where we do not detect any linewith EW > VER J0521+211 : On the basis on a weak emis-sion line at 5940˚A attributed to [N II] 6583˚A,Shaw et al. (2013) proposed this source to be atz = 0.108. This feature was not confirmed by Ar-chambault et al. (2013) that reports a featurelessspectrum.We do not confirm the redshift of the source, whichtherefore is still unknown, setting a lower limit ofz > : An optical spectrum of thissource, with modest SNR was found featurelessby Schachter et al. (1993), a result later confirmedby a better spectrum obtained with the Keck tele-scope by Shaw et al. (2013). A relatively highredshift can be supported by the absence of detec-tion of the host galaxy from high quality image byKotilainen et al. (2011).Our GTC higher SNR ( ∼ ∼ > S5 0716+714 : This is a bright (V ∼
15) andhighly variable (Bach et al. 2007) source forwhich several attempts to detect the redshift failed((Stickel & Kuhr 1993; Rector & Stocke 2001;Finke et al. 2008; Shaw et al. 2013)). From opticalimages Sbarufatti et al. (2005a) set a lower limitof z > ∼ (cid:46) > BZB J0915+2933 : Wills & Wills (1976) showeda continuous optical spectrum for the source andclassified it as a BLL. The featureless behaviourwas also found by White et al. (2000) and by Shawet al. (2013).Through our high SNR optical spectrum, we con-firm the featureless spectrum and set a lower limitto the redshift of z > BZB J1120+4212 : This object (also known asRBS 0970) is a point-like radio source detectedby various X-ray surveys (see e.g. Giommi et al.2005). Optical spectral classification of the sourceas BLL was proposed by Perlman et al. (1996) onthe basis of the quasi-featureless spectrum. Theyclaim the detection of starlight absorption featuresat z = 0.124. However, based on the spectrum re-produced in their Fig. 4, the reliability of this fea-tures is quite uncertain. This redshift is not con-firmed in other spectra obtained by White et al.(2000) and Massaro et al. (2014). Also the spec-trum obtained by SDSS (J112048.06+421212.4)appears to us featureless.Our spectrum with SNR ∼ > : Bade et al. (1998) reported aredshift z = 0.130 for this target, but no infor-mation about the detected lines are given. Onthe contrary White et al. (2000) showed an opti-cal spectrum claiming a redshift of 0.237, althoughit appears featurless from their figure.A more recent spectrum (SNR = 60) obtainedby Ricci et al. (2015) was also found featurless.The target was clearly resolved in HST exposures((Scarpa et al. 2000)) revealing a massive ellip-tical host galaxy, suggesting the source is at lowredshift. Given these different redshift values, we secureda high quality optical spectrum (SNR ∼ W Comae : Weistrop et al. (1985) providedan optical spectrum and estimated a redshift ofz = 0.102 based on the detection of [OIII] 5007˚Aand H α emission lines. This redshift was notconfirmed by Finke et al. (2008), though theirspectra cover only the range from 3800 to 5000˚A. In addition the spectrum obtained by theSDSS (J122131.69+281358.4) proposes a redshiftof z = 1.26. In 2003 the host galaxy of W Comaewas resolved by Nilsson et al. (2003).From our (SNR ∼ α emissionlines at z = 0.102. In addition we detect at thesame redshift the absorption lines due to Ca II(3934, 3968 ˚A) doublet, G-band 4305 ˚A, and MgI 5175 ˚A from the host galaxy. MS 1221.8+2452 : A tentative redshift ofz = 0.218 was proposed by Morris et al. (1991)and Rector et al. (2000). Imaging studies of thissource were able to resolve the host galaxy andare consistent with the low redshift of the target(Falomo & Kotilainen 1999; Scarpa et al. 2000).We detect the Ca II doublet and G-band 4305 ˚Aabsorption lines at z= 0.218 and we find emissionlines at ∼ ∼ AA that if confirmedcould be attribuited to H α and N II 6583 ˚A. S3 1227+255 : Nass et al. (1996) reportedz = 0.135 but no information on the detected spec-tral lines were provided. In spite of the allegedlow redshift, high quality images failed to detectthe host galaxy Nilsson et al. (2003). Shaw et al.(2013) did not confirm this redshift and no spec-tral features were found.Our optical spectrum (SNR ∼ > BZB J1243+3627 : White et al. (2000) reporteda featureless spectrum for this source. An absorp-tion feature of Mg II 2800˚A at λ ∼ ≥ ∼ ∼ > BZB J1248+5820 : The source was classified asa BLL by Fleming et al. (1993) and no redshiftwas available. The featureless nature of the spec-trum is reported in (Henstock et al. 1997; Plotkinet al. 2008) and Shaw et al. (2013). Note thatNED report z = 0.847 based on the SDSS DR3spectrum, although this is not confirmed by SDSSDR13 analysis. Scarpa et al. (2000) failed to de-tect the host galaxy from HST images.Our high SNR spectrum is featureless and we candetermine a lower limit to the redshift of z > PKS 1424+240 : The source was classified asBLL by Fleming et al. (1993) and a featurelessspectra was reported by Marcha et al. (1996);White et al. (2000) and Shaw et al. (2013). Furnisset al. (2013), from the Ly β and Ly γ absorptionsobserved in the far-UV spectra from HST/COS( Hubble Space Telescope /Cosmic Origins Spectro-graph) spectra, reported a lower limit z > ∼
350 optical spectrum, we de-tect two faint emission lines at 5981 and 8034 ˚A(see Fig.5) due to [O II] 3727˚A and [O III] 5007˚A.The redshift corresponding to this identification is0.6047, suggesting that the absorber at that red-shift limit is associated to the BLL. Note also thatin the environment of the target there is a groupof galaxies at z ∼ BZB J1540+8155 : The source was identified asBLL by Schachter et al. (1993). The optical spec-tra obtained by Perlman et al. (1996) failed todetect emission or absorption features. The hostgalaxy was not detected by HST images (Scarpaet al. 2000) posing the source at relatively highredshift.In our GTC spectrum we detect an interveningabsorption doublet at ∼ > RGB J2243+203 : Laurent-Muehleisen et al.(1998) presents the first optical spectrum of thissource, found featureless. Similarly the spectrumobtained by Shaw et al. (2013) is featureless butthe authors claimed the detection of an absorptionline at ∼ > min < > BZB J2323+4210 : From a poor optical spec-trum Perlman et al. (1996) claimed the detectionof two starlight absorption features identified asMg I (5175˚A)and Na I (5892 ˚A) and they proposedat redshift z = 0.059. We disprove this redshift be-cause the Na I absorption coincides with the tel-luric absorption at 6280 ˚A. Shaw et al. (2013) doesnot confirm the above redshift either.Our high SNR ( ∼ F λ ∝ λ α ; α = -1.2). We clearly detect an absorption doublet at ∼ ∼ lim > ∼
10 ˚A) compared to typ-ical Ca II line width from galaxies and are indica-tive of interstellar absorptions. Indeed at ∼ ∼
40 kpc.We conclude that the redshift of BZB J2323+4210is still unknown and we set a spectroscopic lowerlimit of z > > Figure 3 . Optical R-image of the BL Lac objectBZB J2323+4210 taken at the NOT telescope (Falomo &Kotilainen 1999). Two spiral galaxies, labelled as G1 andG2, are present in the field of view of the BL Lac object ata distance of ∼ ∼ DISCUSSIONWe secured high SNR spectra in the range 4200-9500 ˚A for a sample of 22 BLLs, selected for beingTeV emitters or good candidates based on the theirIR properties. Most of these sources either had un-known redshift or the value was rather uncertain. Fromthe new spectroscopy we are able to determine theredshift for 5 objects (S3 0218+357, 1ES 1215+303,W Comae, MS 1221.8+2452, and PKS 1424+240). ForPKS 1424+240, one of the farthest BLL detected inthe TeV regime, no previous estimate of the redshiftwas available. For three objects, BZB J1243+3627with an uncertain redshift and BZB J1540+8155, andBZB J2323+4210 with previously unknown redshift, wefound intervening absorptions that allow us to set spec-troscopic lower limits. For the remaining 13 sources wefound that in spite of the high SNR their spectrum isfeatureless. We can set limits to any emission or absorp-tion features to 0.05-0.50 ˚A depending on the SNR of thetargets and the wavelength region. For seven of thesetargets there was a previous tentative redshift that we donot confirm from our observations. The main reasons forthis difference are: old spectra have poor SNR, wrongsource identification, very tentative line identificationsand redshift given without information on the detectedspectral features (no spectrum shown). It is worth tonote that unfortunately these unconfirmed, likely wrongvalues (also appearing in NED) are often used to derivephysical properties of the sources.On the basis of the assumption that all the objectswith pure featureless spectra are hosted by a massiveelliptical galaxy as is the case of most (virtually all)BLLs, we have then determined a lower limit of theirredshift from the minimum detectable EW of some ab-sorption lines of the host galaxy (see Appendix for de-tails). Depending on the brightness of the observed nu-clei (r = 13.6 to 19.9 ) we can set redshift lower limitsfor these objects from z = 0.1 to z = 0.55 (see Tab. 3).In addition to the lower limits of the redshift for ob-jects with featureless spectra we can also estimate anaverage upper limit to the redshift of the sample of ob-served targets based on the number of Mg II 2800 ˚Aintervening absorption systems observed in our spectra.Given our observed spectral range we are potentiallyable to detect Mg II 2800 ˚A intervening absorption lines(of EW (cid:38) > < ∼ ∼
10 Mg II absorption systemsin the spectra of 18 targets.The relatively low upper limit of the redshift derivedabove together with the lack of detection of absorptionlines from the host galaxies suggests that these targetshave a high Nucleus-to-Host galaxy ratio (N/H). Foreach object with a featureless continuum we have de-rived a lower limit to the redshift on the basis of theassumption that the source is hosted by a massive earlytype galaxy (see Appendix for details) and at a givenlimit of detectable EW of an absorption feature. We cannow associate a minimum N/H to these redshift lowerlimits (see Tab. 3). It turns out that some objects inour sample have a N/H >
10 (assuming the targets arehosted by a standard galaxy (e.g. Falomo et al. 2014).This is significantly higher than the typical value (N/H ∼ δ extremely high, we could have N/H ∼ Table 1 . THE SAMPLE OF TEV BLLAC AND TEV-CANDIDATESObject name RA δ CLASS V E ( B − V ) z literature Reference(J2000) (J2000)BZB J0035+1515 00:35:14.70 15:15:04.0 TeVc 16.9 0.062 ?1ES 0033+595 00:35:52.60 59:50:05.0 HBL 19.5 1.386 ?S2 0109+22* 01:12:05.08 22:44:39.0 IBL 15.7 0.034 0.265 ? Healey et al. (2008)RGB J0136+391 01:36:32.50 39:06:00.0 HBL 15.8 0.068 ?S3 0218+357 02:21:05.50 35:56:14.0 HBL** 20.0 0.061 0.944 Cohen et al. (2003)3C 66A 02:22:39.60 43:02:08.0 IBL 15.0 0.075 0.444 ? Miller et al. (1978)VER J0521+211 05:21:45.90 21:12:51.0 IBL 17.5 0.604 0.108 ? Shaw et al. (2013)1ES 0647+250 06:50:46.50 25:03:00.0 HBL 15.7 0.087 ?S5 0716+714 07:21:53.40 71:20:36.0 IBL 15.5 0.027 ?BZB J0915+2933 09:15:52.40 29:33:24.0 TeVc 15.8 0.021 ?S4 0954+65*** 09:58:47.20 65:33:55.0 LBL 17.0 0.106 0.367 ? Lawrence et al. (1986)BZB J1120+4212 11:20:48.00 42:12:12.0 TeVc 17.3 0.001 0.124 ? Perlman et al. (1996)1ES 1215+303 12:17:52.10 30:07:01.0 HBL 15.8 0.020 0.13 ? Bade et al. (1998)W Comae 12:21:31.70 28:13:59.0 IBL 15.4 0.021 0.102 ? Weistrop et al. (1985)MS 1221.8+2452 12:24:24.20 24:36:24.0 HBL 16.7 0.019 0.218 ? Morris et al. (1991)S3 1227+255 12:30:14.10 25:18:07.0 IBL 14.7 0.017 0.135 ? Nass et al. (1996)BZB J1243+3627 12:43:12.70 36:27:44.0 TeVc 16.2 0.010 ?BZB J1248+5820 12:48:18.80 58:20:29.0 TeVc 15.4 0.011 ?PKS 1424+240 14:27:00.40 23:48:00.0 HBL 14.6 0.050 ?BZB J1540+8155 15:40:15.80 81:55:06.0 TeVc 17.6 0.044 ?RGB J2243+203 22:43:54.70 20:21:04.0 HBL 16.0 0.042 ?BZB J2323+4210 23:23:52.10 42:10:59.0 TeVc 17.0 0.134 0.059 ? Perlman et al. (1996)
Col.1 : Name of the target;
Col.2 : Right Ascension;
Col.3 : Declination;
Col.4 : Class of the source: High-synchrotron peaked BL Lac(HBL), Intermediate-synchrotron peaked BL Lac (IBL), Low-synchrotron peaked BL Lac (LBL), TeV Candidate BL Lac (TeVc);
Col.5 :V-band magnitudes taken from NED;
Col.6 : E ( B − V ) taken from the NASA/IPAC Infrared Science Archive(https://irsa.ipac.caltech.edu/applications/DUST/); Col.7 : Redshift;
Col.8 : Reference to the redshift.* Details for S2 0109+22 are reported in Paiano et al. (2016), ** Gravitationally lensed system , *** = Details for S4 0954+65 arereported in Landoni et al. (2015) Table 2 . LOG OBSERVATIONS OF TEV SOURCES AND TEV-CANDIDATES OBTAINED AT GTCGrism B Grism RObejct t
Exp (s) Date Seeing t
Exp (s) Date Seeing rBZB J0035+1515 2100 2015 Sept 30 0.6 1800 2015 Oct 01 0.6 17.001ES 0033+595 3600 2015 Sept 18 1.3 2700 2015 Sept 25 0.9 17.80S2 0109+22 750 2015 Sept 19 1.8 750 2015 Sept 19 1.8 15.20RGB J0136+391 900 2015 Sept 28 0.9 600 2015 Sept 28 0.9 15.80S3 0218+357 3600 2015 Feb 05 0.9 8700 2015 Feb 05 1.2 19.903C 66A 750 2015 Sept 09 0.8 210 2015 Sept 06 0.8 14.70VER J0521+211 900 2015 Sept 21 0.8 1050 2015 Sept 21 0.8 16.401ES 0647+250 1500 2015 Sept 22 1.4 1200 2015 Sept 22 1.4 15.80S5 0716+714 210 2015 Nov 30 1.6 210 2015 Nov 30 1.6 13.60BZB J0915+2933 750 2015 Dec 24 2.0 450 2015 Jun 06 2.0 15.90S4 J0954+65 300 2015 Feb 28 1.0 450 2015 Feb 28 1.0 15.5BZB J1120+4212 3000 2016 Jun 24 1.5 3600 2015 Jul 01 0.7 16.101ES 1215+303 900 2015 May 20 1.5 900 2015 May 20 1.5 14.50W Comae 1800 2015 Jun 30 1.4 1800 2015 Jun 30 1.4 15.50MS 1221.8+2452 3000 2015 May 31 1.3 3000 2015 May 31 1.2 16.70S3 1227+255 450 2015 Dec 25 1.5 500 2015 Dec 25 1.5 14.90BZB J1243+3627 1350 2015 May 21 1.2 1350 2015 May 21 1.2 15.60BZB J1248+5820 600 2015 Dec 25 2.2 900 2015 Dec 25 2.2 15.70PKS 1424+240 450 2015 Jun 30 1.0 450 2015 Jun 30 1.0 14.20BZB J1540+8155 900 2015 Jun 23 1.0 900 2015 Jun 23 1.0 17.30RGB J2243+203 600 2015 Sept 19 2.0 750 2015 Sept 19 2.0 16.20BZB J2323+4210 3000 2016 Aug 07 1.3 3600 2015 Feb 28 0.7 17.50
Col.1 : Name of the target;
Col.2 : Total integration time with the Grism B;
Col.3 : Date of Observation with Grism B;
Col.4 : Seeingduring the observation with the Grism B;
Col.5 : Total integration time with the Grism R;
Col.6 : Date of Observation with Grism R;
Col.7 : Seeing during the observation with the Grism R;
Col.8 : r’ mag measured on the acquisition images. Table 3 . PROPERTIES OF THE OPTICAL SPECTRA OF OBSERVED SOURCESOBJECT α SNR EW min z lim z N/H lim BZB J0035+1515 -1.3 183-275 0.09-0.18 0.55 (0.32) 111ES 0033+595 * 40-135 0.27-0.52 0.53 (0.10) 0.467 e e
33C 66A -1.1 118-314 0.10-0.22 0.10 (*) 2VER J0521+211 -0.9 82-221 0.15-0.37 0.18 (0.10) 11ES 0647+250 -1.3 115-294 0.09-0.21 0.29 (0.12) 7S5 0716+714 -0.8 180-346 0.04-0.14 0.10 (* ) 4BZB J0915+2933 -1.1 89-241 0.14-0.34 0.13 (* ) 1S4 J0954+65 -0.9 50-120 0.15-0.20 0.45 (0.27) 25BZB J1120+4212 -1.6 100-190 0.12-0.23 0.28 (0.12) 51ES 1215+303 -1.0 205-375 0.09-0.14 0.14 (*) 0.129 e
4W Comae -0.6 180-260 0.09-0.17 0.19 (0.10) 0.102 e,g e,g > a e > a > a Col.1 : Name of the target;
Col.2 : Optical spectral index derived from a Power Law fit in the range 4250-10000;
Col.3 : range of SNRof the spectrum;
Col.4 : Range of the minimum equivalenth width (EW min ) derived from different regions of the spectrum (see text),
Col.5 : Lower limit (3 σ level) of the redshift by assuming BL Lac host galaxy with M R = -22.9 (-21.9), in parenthesis we give the redshiftlower limit assuming a host galaxy one magnitude fainter. An asterisk indicates that the redshift limit is out of observed range for thecase of fainter host galaxy (see Appendix), Col.6 : Spectroscopic redshift; the superscript letters are: e = emission line, g = host galaxyabsorption, a = intervening absorption ; Col.7 : Lower limit of the Nucleus-Host galaxy Ratio (N/H) in r-band considering the whole fluxof the host galaxy.
Table 4 . CORRESPONDENCE BETWEEN THE WAVELENGTH RANGE, ABSORPTION LINES AND REDSHIFTRANGE Wavelength Range Absorption Line Redshift Range4250 - 5000 CaII 0.08 - 0.275000 - 6200 CaII 0.27 - 0.586400 - 6800 CaII 0.63 - 0.737800 - 8100 MgI 0.51 - 0.578400 - 8900 MgI 0.63 - 0.72
Col.1 : Wavelength range of the optical spectrum;
Col.2 : Host galaxy absorption line used;
Col.3 : Redshift range corrisponding to thewavelength range Table 5 . MEASUREMENTS OF SPECTRAL LINESOBJECT λ obs EW (observed) Line ID z line ˚ A ˚ A α > > > > > Col.1 : Name of the target;
Col.2 : Barycenter of the detected line;
Col.3 : Measured equivalent width;
Col.4 : Line identification;
Col.5 : Spectroscopic redshift. | ||| ||| || || | Figure 4 . Spectra of the TeV sources and TeV-candidates obtained at GTC.
Top panel : Flux calibrated and dereddered spectra.
Bottom panel : Normalized spectra. The main telluric bands are indicated by ⊕ , the absorption features from interstellar mediumof our galaxies are labelled as IS (Inter-Stellar) | |||| | ||| | | | Figure 4 . Continued from Fig. 4. ||| | || |||| | | | || Figure 4 . Continued. | ||| Figure 4 . Continued. Figure 5 . Close-up of the normalized spectra around the detected spectral features of the TeV sources and TeV-candidatesobtained at GTC. Main telluric bands are indicated as ⊕ , spectral lines are marked by line identification. Figure 5 . Continued.
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