3 Classification of BL Lac candidates

3.1 Results from NED and SIMBAD database queries

The 223 candidates were classified in a two-step process. First we searched for known optical counterparts in the NASA/IPAC Extragalactic Database (NED) and in the SIMBAD database. Special care was taken to avoid confusion of BL Bac objects with normal galaxies and other types of AGNs. A database entry of an object as "galaxy'' without spectroscopic information was not accepted as an identification, because nearby BL Lac objects in elliptical galaxies might not have been recognized. For galaxies with redshift information and for objects with "AGN'' or "QSO'' identification but without additional information (e.g. redshift) the original literature was consulted before the object was dismissed as BL Lac.

In total 101 objects could be classified this way. Of the remaining objects a few candidates were classifed as stars on the objective prism plates of the HQS, and another few as obvious clusters of galaxies based on direct plates and on the fact that these sources show extended X-ray emission. For all other candidates follow-up observations were obtained.

   

Table 2: Spectroscopic follow-up observation runs. The last column gives the number of objects observed. Some of the objects were observed several times.

Telescope & Instrument	Date	#nights	Nb
3.5 m Calar Alto (MOSCA)	March 1997	4	30
WHT / La Palma (ISIS)	April 1997	2	19
3.5 m Calar Alto (MOSCA)	Feb. 1998	6	89
3.5 m Calar Alto (MOSCA)	Feb. 1999	${\sim} 1^{a}$	9

^a Morning and evening hours of three nights.
^b Number of objects observed in this observation run.

3.2 Observations of the remaining unclassified BL Lac candidates

Spectroscopic observations to classify the remaining BL Lac candidates and to determine their redshifts were made with the 3.5 m telescope on Calar Alto equipped with the multiobject spectrograph MOSCA and with the 4.2 m WHT on La Palma equipped with ISIS (Table 2). Most of the results from the 1997 observation runs have already been presented in Bade et al. (1998). For the classification with MOSCA, we used the G500 grism, which covers a wavelength range 4250 - 8400 Å  with a pixel-to-pixel resolution of 12 Å. If necessary, additional spectra were taken with the G1000 and R1000 grisms to determine redshifts. These spectra have a resolution of 6 Å  and cover the ranges 4400-6600 Å  and 5900-8000 Å  respectively. The spectra were reduced in a standard way: bias subtraction, flat-field correction using morning and evening skyflats, and response determination of the detector using spectrophotometric standard stars. BL Lac objects by definition have no or very weak emission lines. Integration times of ${\approx} 1000 \dots 2000~ \rm s$ were needed to detect the weak absorption lines of the host galaxy, which is often out-shined by the non-thermal continuum of the point-like central synchrotron source. Most observations were made under non-photometric conditions.

Optical photometry in the Johnson B band has been obtained for many of the optically faint BL Lacs with the Calar Alto 1.23 m telescope (Beckmann 2000a). Especially for several of the very faint objects ( $B > 20 ~{\rm mag}$) no reliable photometry was available before. For these objects we have now optical magnitudes with an error of $\Delta B \le 0.1 ~{\rm mag}$. For the other objects the acquisition frames of the spectroscopic runs have been used to determine a B magnitude, or values from the literature have been taken. For the brighter objects ( $B < 18 ~{\rm mag}$) also the HQS calibrated objective prism plates have been used, which have an error of $\Delta B \le 0.3 ~{\rm mag}$.

3.3 Results from follow-up observations

Follow-up observations were made of 117 objects, including the unidentified candidates from the BSC/NVSS correlation and additionally a number of objects, which were considered to be promising BL Lac candidates, but did not match the selection criteria described before (e.g. too low X-ray count rate). In total we discovered 53 BL Lac objects according to the classification criteria, which are discussed in Sect. 3.4. As a considerable number of the objects was discovered independently by other groups in the meantime, we are left over with 26 new BL Lacs: 11 from the BSC/NVSS correlation and 15 from the additionally observed objects.

3.3.1 The complete HRX BL Lac sample

The 11 new BL Lac objects discovered among the objects from the BSC/NVSS correlation are marked in Table 8 and their spectra are included in Fig. 8. Based on our spectra we could confirm or revise redshifts for several other BL Lac objects. Five objects from this correlation with other identifications than BL Lac and without NED or SIMBAD entry so far, are marked in Table 8 in addition.

Summarizing, the optical identification of the 223 BSC/NVSS objects leads to the following distribution of object classes within the radio/X-ray correlation (Table 3): 35% are BL Lac objects, 36% are other AGNs (QSO, Seyfert 1/2, FSRQs), 11% galaxies (including starburst galaxies and LINERs), 13% cluster of galaxies, and 4% stars (including 2 supernova remnants). Only a fraction of 1% of the 223 candidates is yet not identified.

 

 

Table 3: Summary of identifications of objects from the BSC/NVSS correlation (cf. Table 8).

object type	total number	fraction
BL Lac	77	34.5%
Seyfert 1	65	29.1%
Seyfert 2	8	3.6%
Quasar	6	2.7%
blazar	2	0.9%
LINER	5	2.2%
Galaxy cluster	29	13.0%
Galaxies	19	8.5%
Stars	8	3.6%
SNR	2	0.9%
Unidentified	2	0.9%
Total	223

The 77 BL Lacs from the BSC/NVSS correlation are called the complete HRX BL Lac sample. In comparison to the EMSS BL Lac sample, this sample probes a population of objects with lower $\alpha_{\rm RO}$ and $\alpha _{\rm OX}$ values and contains therefore more radio quiet and stronger X-ray dominated objects. The HRX-BL Lac sample is the largest complete sample of X-ray selected BL Lac objects. Table 4 compares the HRX BL Lac sample with four other X-ray selected BL Lac Surveys: the EMSS based sample (Rector et al. 2000), the sample by Laurent-Muehleisen et al. (1999) based on the correlation of the RASS with the Green Bank radio survey, the REX survey using the NVSS in combination with the sources found in the ROSAT pointed observations (Caccianiga et al. 1999), and the DXRBS (Perlman et al. 1998), which uses the ROSAT data base WGACAT and PMN/NVSS radio data.

   

Table 4: Properties of the Hamburg BL Lac samples in comparison to other recent samples.

sample	Reference	number of	X-ray	radio	optical
		objects	limit	limit	limit
HRX core sample	Bade et al. (1998)	39	$0.075/0.15 ~{\rm s}^{-1}$ a,g	-	-
HRX-BL Lac	this work	77	$0.09 ~{\rm s}^{-1}$ a,g	$2.5 ~{\rm mJy}^{b}$	-
RGB	Laurent-Muehleisen	127	$0.05 ~{\rm s}^{-1}$ a,c	$15 \dots 24 ~{\rm mJy}^{d}$	$18.5 ~{\rm mag}^{e}$
RGB complete	et al. (1999)	33	$0.05 ~{\rm s}^{-1}$ c	$15 \dots 24 ~{\rm mJy}^{d}$	$18.0 ~{\rm mag}^{e}$
EMSS	Rector et al. (2000)	41	$2 \times 10^{-13}$ f	-	-
REX	Caccianiga et al. (2002)	55	$4 \times 10^{-13}$ g	$2.5 ~{\rm mJy}^{b}$	$B \le 20.5 ~{\rm mag}$
DXRBS	Padovani (2001)	30	few $\times 10^{-14}$ c	${\sim} 50 ~{\rm mJy}$	-

^a ROSAT All Sky Survey count rate limit.
^b NVSS radio flux limit at $1.4 ~{\rm GHz}$.
^c Full ( $0.1 {-} 2.4 ~{\rm keV}$) PSPC energy band.
^d GB catalog flux limit at $5 ~{\rm GHz}$.
^e O magnitude determined from POSS-I photographic plates.
^f EINSTEIN IPC ( $0.3{ -} 3.5 ~{\rm keV}$) flux limit in $[{\rm erg} ~ {\rm cm}^{-2} ~ {\rm s}^{-1}]$.
^g Hard ( $0.5 {-} 2 ~{\rm keV}$) PSPC energy band.

3.3.2 The extended HRX BL Lac sample

Among the BL Lac candidates observed additionally we could confirm another 27 BL Lac objects, from which 15 did not appear in the literature so far. Together with the 77 BL Lacs from the complete sample they form the extended HRX BL Lac sample comprising 104 objects. The spectra of the 15 new BL Lacs are presented in Fig. 8 together with the 12 new BL Lacs from the complete sample. The spectra of all other objects including those from objects not identified as BL Lacs will be accessible on the web.

The properties of the 104 BL Lacs of the extended sample are presented in Table 9 (this table is only availlable in electronic form). The BL Lacs discovered additionally are marked by an asterisk and the new BL Lacs are labeled by "new''. This Table lists the object names, the NVSS radio coordinates (J2000.0), redshifts, ROSAT PSPC (0.5-2.0 keV) X-ray fluxes in $10^{-12}~ {\rm erg} ~ {\rm cm}^{-2} ~ {\rm s}^{-1}$, 1.4 GHz radio fluxes in mJy from the NVSS radio catalogue, B magnitudes, K magnitudes, and the calcium break index. The radio positions have an error of less than 5 $^{\prime\prime}$ (for the faintest objects) and are therefore considerably more accurate than the X-ray positions given in Table 8.

The RASS-BSC fluxes have been computed by using the count rate and a single-power law with free fitted absorption $N_{\rm H}$. The spectral slope and $N_{\rm H}$ are determined by the hardness ratios, a method described by Schartel (1994). The hardness ratio is defined as HR = (H-S)/(H+S) with H and S being the number of counts in the hard and soft energy bands; typically two ratios are computed: HR1 with energy ranges $S = 0.1 {-} 0.4 \rm ~ keV$ and $H = 0.5 {-} 2.0 \rm ~ keV$, and HR2 with $S = 0.5 {-} 0.9 \rm ~ keV$ and $H = 1.0 {-} 2.0 \rm ~ keV$ (Voges et al. 1999). The values for the hardness ratios range by definition from +1 for extremely hard to -1 for very soft X-ray spectra. The error estimate for the $N_{\rm H}$ and $\alpha_{\rm X}$ values is based on the hardness ratios only, not on the photon spectrum itself. Therefore this method does not give $\chi^2$ values, but is able to determine  $68 \% ~ (1 \sigma)$ errors. This is done by exploring the hardness-ratio, spectral slope, and $N_{\rm H}$ parameter space, determining the $1 \sigma$region within it for a given set of parameter components.

The near infrared data are taken from the Two-Micron All-Sky Survey (2MASS, Skrutskie et al. 1995; Stiening et al. 1995). In Table 9 only the K-magnitude is listed, but for the analysis we also used J and H from the 2MASS.

The calcium break index (Ca-break) is defined as follows (Dressler & Shectman 1987):

$${\rm Ca{-}break}[\%] = 100 \cdot \frac{f_{\rm upper} - f_{\rm lower}}{f_{\rm upper}}$$ (1)

with $f_{\rm upper}$ and $f_{\rm lower}$ being the mean fluxes measured in the 3750 Å  $< \lambda < 3950$ Å  and 4050 Å  $< \lambda < 4250$ Å  objects rest frame band respectively. The measurement of the break was possible only if the redshift was known and if the break was within the spectral range covered by the observation. Therefore break values are available for 30 of the HRX-BL Lac only. In seven cases the break value is negative. This is in most cases due to a low signal to noise of the spectra. Only for one object (1RXS J111706.3+201410) the negative calcium break index is not consistent with a value of 0% and we assume that here the underlying power law of the jet emission outshines the host galaxy, so that the measured negative "break value'' is in fact based on the synchrotron component.

Figure 1 shows the redshift distribution of the HRX-BL Lac extended and complete samples. The mean redshift for the complete and extended sample are $\bar{z} = 0.31$ and $\bar{z} = 0.34$, respectively. We note that in comparison to the core sample no new BL Lacs with z > 0.7 were found, which contribute to the complete sample.

  
3.4 BL Lac classification criteria

The characterizing feature of BL Lac spectra in the optical is the presence of a non-thermal continuum which is well described by a single power law. A second component is the emission of the host galaxy, which contributes absorption features in addition to continuum emission. If the BL Lac itself shows no emission lines at all, redshift determination is only possible by identifying these absorption features. The host galaxies are in majority giant elliptical galaxies (e.g. Urry et al. 2000), having strong absorption features caused by the stellar content.

Expected absorption features in the optical, which can be used for redshift determination, have already been discussed in detail by Bade et al. (1998). The most prominent feature in the spectra of elliptical galaxies is the so-called "calcium break'' at 4000 Å. Its strength is given by the calcium break index, as defined before.

Most of the AGN with emission lines found in the radio/ X-ray correlation are Seyfert type galaxies or LINER (see Table 3). These AGN do not show a calcium break. For the other objects the strength of the calcium break can be used to distinguish between normal elliptical galaxies and BL Lac objects. For the former, this contrast is $\ge$40% with the higher flux to the red side of the break. Our criteria to classify BL Lac objects were defined by Bade et al. (1998) for the core sample and are spectroscopically similar to those applied to the Einstein Medium-Sensitivity Survey (EMSS; Stocke et al. 1991). However, we relaxed the upper limit for the strength of the calcium break index from 25% to now 40% when other properties of the object were consistent with a BL Lac classification. This follows the findings of previous studies (Marchã et al. 1996; Laurent-Muehleisen et al. 1999; Rector et al. 2000) that there exist galaxies with strengths $25\% <$ Ca-break $< 40\%$, which fulfill all other selection criteria for BL Lac objects. Explicitly the selection criteria are now:

• no emission lines with W $_{\lambda} > 5$ Å;
• the contrast of the Ca II break from the host galaxy must be less than 40%.

With respect to the first criterium no misclassifications are expected as there were no objects found within the BSC/NVSS correlation with weak emission lines and equivalent widths of several 10 Å.

Figure 1: Distribution of redshifts in the extended HRX-BL Lac sample. The hatched part refers to the complete sample.

Borderline cases are more likely with respect to the calcium break index, because the transition between non-active elliptical galaxies and BL Lacs is smooth. This is clearly shown in Fig. 2, in which our measured break strength is plotted vs. the optical luminosity L_B, as derived in Sect. 4.2. Both quantities are correlated and almost evenly distributed up to ${\rm Ca{-}break} \sim40\%$.

The observed correlation might be affected by a varying fraction of host galaxy light included in the spectra. In nearby objects the BL Lac host galaxy might not have been fully covered by the slit and therefore the calcium break strength could have been underestimated. However, as the low-redshift objects are mainly the less luminous ones, this effect cannot explain the decreasing strength of the calcium break with increasing luminosities.

This correlation is not only seen in the optical domain, but is also present if we use radio, near infrared or X-ray luminosity instead. In all wavelength regions from the optical to the X-rays the correlation between emitted luminosity and break strength is significant. Therefore we would like to stress the point that the observed correlations are not due to observational selection effects.

Misclassifications might have been occurred also due to large errors for the measured break strengths in some of our spectra with low signal to noise ratio. However for all objects except three of the HRX-BL Lac complete sample the break strengths are <25%, making a misclassification unlikely. The three objects with a calcium break strength in the range $25\% < {\rm Ca{-}break} < 40\%$ are 1ES 0927+500, 1RXS 114754.9+220548, and 1RXS 151040.8+333515 (cf. Table 9) and they were included in the sample, because they fulfill other BL Lac properties, for example strong polarization ($P >6\%$) in the NVSS.

Figure 2: Strength of the calcium break versus monochromatic luminosity LB in the optical B-band. Circles refer to the complete sample while triangles mark additional objects found within the course of the work.