The discovery of 13 serendipitous BL Lac objects in the Einstein IPC Slew Survey by means of x ray/radio vs. x ray/optical color-color diagrams and confirmation by optical spectroscopy are reported. These 13 BL Lacs were discovered using a technique which exploits the characteristic broad band spectra of BL Lacs. New VLA detections provide accurate fluxes (f(6 cm) is approximately 0.5 mJy) and 2 in. positions, facilitating the determination of an optical counterpart. All 13 new BL Lacs show essentially featureless optical spectra. Nine of these lie within the range of colors of known x ray selected BL Lacs. Of the remaining four, one is apparently x ray louder (by a factor of 1.5) or optically quieter (by 0.8 mags); and three are optically louder (by 1-1.3 mags) than x ray selected BL Lacs. Approximately 50 new BL Lacs in total are expected from VLA work and upcoming Australia Telescope observations, yielding a complete Slew Survey sample of approximately 90 BL Lacs

Elvis, Martin S.

Huchra, John P.

Humphreys, Roberta

Luu, Jane

Perlman, Eric

Remillard, Ron

Schachter, Jonathan F.

Stocke, John T.

Wallin, John

English

NASA Technical Reports Server

N ... .... • _j
X-RAY/RADIO/OPTICAL TECHNIQUE 1 If / _/
Jonathan F. Schachter _, John T. Stocke ='3, Eric Perlman 2'3, Martin Elvis =, Jane Luu 4,
John P. Huchra, Roberta Humphreys 5, Ron Remillard 6, and John Wallin 7
Harvard-Smithsonian Center for Astrophysics, MS #4,
60 Garden St., Cambridge, MA, 02138.
Received: ; Accepted:
ABSTRACT
We report the recent discovery of 13 serendipitous BL Lac objects in the Einstein IPC Slew
Survey by means of X-ray/radio vs. X-ray/optical color-color diagrams and confirming
optical spectroscopy. These 13 BL Lacs were discovered using a technique proposed by
Stocke et al. (1989) which exploits the characteristic broad-band spectra of BL Lacs. New
VLA detections provide accurate fluxes If(6 cm) _ 0.5 mJy] and 2" positions, facilitating
the determination of an optical counterpart. All 13 new BL Lacs show essentially featureless
optical spectra. Nine of these lie within the range of colors of known X-ray selected BL
Lacs. Of the remaining four, one is apparently X-ray louder (by a factor of 1.5) or optically
quieter (by 0.8 mags); and 3 are optically louder (by 1-1.3 mags) than X-ray selected BL
Lacs. We expect ,-,50 new BL Lacs in total, from our VLA work and upcoming Australia
Telescope observations, yielding a complete.Slew Survey sample of ,--90 BL Lacs.
Subject Headings: BL Lacertae objects: general -- galaxies: nuclei -- galaxies: X-
rays -- X-rays: sources -- surveys
1 Research reported here used the Multiple Mirror Telescope Observatory, which is a joint
facility operated by the Smithsonian |nstitution and the University of Arizona.
2 Visiting Astronomer at the National Radio Astronomy Observatory (NRAO), which
is operated by Associated Universities, Inc., under contract with the National Science
Foundation (NSF).
3 Postal Address: University of Colorado, Dept. APAS, Campus Box 391, Boulder, CO
80309.
4 Hubble Fellow.
5 Postal Address: University of Minnesota, School of Physics & Astronomy, Minneapolis,
MN 55455.
6 Postal Address: Massachusetts Institute of Technology, Center for Space Research, Room
37-595, Cambridge, MA 02139.
r Postal Address: Naval Research Laboratory, Code 4129 6, 4555 Overlook Avenue SW,
Washington, DC 20375.
https://ntrs.nasa.gov/search.jsp?R=19930011607 2020-03-17T08:08:06+00:00Z
I. INTRODUCTION
BL Lac objects have been hard to find: in the 20 years following the discovery of the first
example (Schmidt 1968), their numbers grew by only a factor ,-_10 (Veron-Cetty and Veron
1991). By comparison, in the 20 years following the discovery of quasars (Schmidt 1963)
their numbers grew 500-fold (Veron-Cetty and Veron 1991). Their elusiveness is due to the
very lack of strong emission lines that make BL Lacs interesting. Even now less than 200
BL Lacs are known (Veron-Cetty and Veron 1991), ,,,2% of known active objects. BL Lac
objects account for ,-_ 2% (162 out of 7765) of sources in the Veron-Cetty and Veron (1989)
AGN catalog.
In the last few years, techniques using their distinguishing properties (Stocke et
al. 1989) have made it possible to discover many new BL Lacs in the Einstein Extended
Medium Survey (EMSS; Gioia et al. 1991; Stocke et al. 1991). This letter reports the
extension of these efforts to the Einstein Slew Survey: we find 13 new BL Lac objects, which
are the first results of a new, uniformly selected, sample of 85-90 bright BL Lacs.
II. AN X-RAY/RADIO/OPTICAL TECHNIQUE FOR IDENTIFYING
BL LACS
The key to finding BL Lacs is to exploit their salient characteristics:
First, all BL Lacs are radio loud [aRo > 0.3, i.e. f(6 cm) >1 mJy for V _< 20; Stocke
et al. 1991]. For this reason, radio suryeys .with follow-up optical spectroscopy (e.g., Kfihr
et al. 1981), have been considerably more efficient than optical surveys (6-8%).
Second, 98% of BL Lacs have distinctive X-ray/radio/optical colors (Stocke et
al. 1991). Thus, X-ray surveys, combined with radio and optical data, have been shown to
be the best approach to date (,--12% efficiency; Stocke et al. 1991). The X-ray/radio/optical
approach was highly successful in identifying 36 BL Lacs in the EMSS.
Third, the surface density of X-ray--selected BL Lacs flattens at fluxes below
10 -_2 ergs s -_ cm -2 (Wolter et al. 1991). Therefore, wide-angle (all-sky) surveys are better
for detecting BL Lacs than narrow-beam deep surveys. Because the EMSS covers only 2% of
sky (up to --, 5 × 10 -12 ergs s -1 cm-2), we need to use a shallow X-ray survey: the Einstein
Slew Survey. The Slew Survey, constructed from 2799 individual slews of the IPC, covers
50% of sky at an exposure of 6 s (or, equivalently, to ,-, 1 × 10 -9 ergs s -1 cm-2; Elvis et
al. 1992). We estimate from the EMSS that a large number (--, 50) of new Slew BL Lacs
will be identified, increasing the currently known number of BL Lacs by more than 30%.
Together these properties allow us to define a multi-step approach to identify BL
Lacs, starting from the 193 unidentified Slew Survey sources:
1. Choose sources at high Galactic latitudes ([b[ > 15°), with previous radio detections
or a suggestive X-ray/radio flux upper limit;
..
4.
.
6.
Observe them with the VLA (hybrid CnB configuration, 2" resolution, cf. 2' X-ray
position), giving an accurate flux (,--, 0.5 mJy);
Select point sources from the VLA maps as likely BL Lacs;
Use arcsecond VLA positions to find optical counterparts and magnitudes from
digitized sky survey plates;
Place objects in the X-ray/radio/optical color-color diagram;
Obtain optical spectra of candidates with correct colors.
III. CURRENT APPLICATION OF THE TECHNIQUE
The EMSS (Gioia et al. 1990) showed that the presence of a centimeter radio source of the
proper flux within an X-ray error circle will yield a BL Lac object at >_ 80% efficiency (Stocke
et al. 1991). In Figure 1 (after Stocke et al. 1991), identified BL Lacs occupy a unique area in
the radio/optical/X-ray (or c_ox-eRo) color-color diagram, compared to other extragalactic
classes. The few (< 20%) non-BL Lacs in the region of Figure 1 bounded by aRo = (0.6, 0.3)
and _ox = (1.2,0.55) are either highly variable AGN or dominant galaxies in cooling-flow
clusters, which are easily distinguished from BL Lacs with optical spectra.
Sources likely to be BL Lacs were selected primarily from the 193 unidentified Slew
Survey sources, i.e. sources currently lacking proposed optical counterparts. As most of the
unidentified sources are accessible from the North (76% with 6 > -40°), we obtained VLA
observing time, as described below. Or_fy 10"% (19) of the targets had previous radio survey
detections [f(6 cm) > 25 mJy in Condon & Broderick 1985, 1987; or f(6 cm) > 60 mJy in
Condon and Broderick 1992]. We therefore prioritized sources as follows:
.
.
All high-latitude (]b[ > 15 °) Unidentified sources (both radio detections and nondetec-
tions; see Table 1).
Slew sources with proposed optical counterparts of questionable validity, due to the
lack of previous X-ray detections or confirming spectroscopy. These were all (a)
stellar identifications, which are possible superpositions of foreground objects, or;
(b) "normal" galaxy identifications, which may have active nuclei, or; (c) cluster
identifications.
.
.
Unidentified sources at low latitudes, where a radio detection may indicate the presence
of a pulsar.
Slew sources rejected from the source list because of a 10%-15% false-source rate and
the requirement of high source reliability > 98% (Elvis et al. 1992). In the 203 rejected
unidentified sources, the false--source rate is slightly smaller (9%; from Figure 12 of
Elvis et al.), but VLA detections would be highly significant.
In January, 1992, we observeda prioritized list of 152 Slew Survey sourcesat 6 cm
with the VLA CnB hybrid configuration. The list contained 100 objects of Priority 1 (see
above), 20 of Priority 2, 13 of Priority 3, and 19 of Priority 4. Using snapshot exposures
of 3 minutes each, we achieved a 5o"flux sensitivity of 0.5 mJy, and a positional accuracy
of 2-3 ". The angular resolution clearly shows double-lobed, wide-angle-tail, or head-tail
structure in many cases. Some 52 of the VLA detections identified with Slew sources (within
the 80" 91% confidence radius of the Slew Survey) have the point-source radio morphology
typical of BL Lac objects. Five of these have a second VLA point source within the Slew
error box, where the ambiguity will be resolved using our color-color criteria once we have
magnitudes for all sources. The 47 remaining BL Lac candidates are divided into 21 strong
sources [f(6cm) > 10 mJy], and 26 weaker sources [0.5 mJy < f(6 cm) _< 10 mJy].
To find optical magnitudes for the VLA sources, we are primarily using two B --, 22
digitized catalogs: in the South, the NRL catalog of objects derived from ROE scanning of
the UK Schmidt plates; in the North, the Minnesota catalog derived from scanning the POSS
plates. At this writing, the UK Schmidt plates (in the Bj band) are easier to work with,
as NRL has all of the scanned UK Schmidt plates on EXABYTE tape. The U. Minnesota
analysis is more time consuming, but provides both O and E band plate data, which is
important for identifying optical counterparts to Slew Survey sources. In the case of either
the POSS or the UK Schmidt digitized data, magnitudes are typically accurate to better
than 0.5 mag, and positions to 0.5". These values are completely adequate for our arialysis..
We obtained the magnitudes of optical counterparts in the VLA error circle. In no
case has there been more than one possible counterpart, and we have found only one case
without any possible counterparts, suggesting extreme optical variability (the Slew source
1ES1218+237). This is consistent with our expectation that the faintest expected Slew
Survey objects will have B -,_ 19 (Schac'hter et al. 1992). Using star counts from the Galactic
models of Bahcall & Soneira 1980, it is easy to show that even at the POSS or UK Schmidt
plate flux limit of B ,--, 22, and b = 20 °, the probability of a foreground source in the VLA
error circle is small (<5%).
Using optical magnitudes derived from digitized plate data, we can place 13 of
the VLA sources in Figure 1. We scale POSS O-magnitudes and UK Schmidt B j-
magnitudes by +0.5 to obtain approximate V magnitudes, using the median (B - V) of
0.5 in Veron-Cetty & Veron 1989. Bearing in mind the uncertainties in the IPC flux and V
magnitude (shown graphically in Figure 1), 9 sources have the correct colors to be BL Lacs.
These only require confirming spectroscopy for an absolute identification (see below). The
remaining four sources have systematically smaller values of either aox (1ES0347-121) or
aRo (1ES0303+067, iES1440+122, 1ES1544+820) relative to EMSS BL Lacs.
The simplest explanation of the 4 outlier points is X-ray or optical flaring, which is
plausible given the non-simultaneity of the X-ray, optical photometric, and radio observa-
tions, and the variability of BL Lacs at all known wavelengths (e.g., Schwartz et al. 19??).
The low-c_Ro sources 1ES0303+067, 1ES1440+122, and 1ES1544+820 could have flared
optically while observed by the POSS by 1.3, 1.1, and 1.0 magnitudes and could still lie in
the proper region of the diagram. Similarly, the low-aox source 1ES0347-121 could have
been caught in a flaring X-ray state by the Slew Survey. A reduction in the observed Slew
flux by a factor of 1.5 would give this sources the correct BL Lac colors. This value, which
. . . ,
cannot be attributed to uncertainty in determination of the Slew Survey flux (18%), is within
the range of known BL Lac variability. Equivalently, 1ES0347-121 could have been fainter
optically when observed in the sky survey plates by 0.8 mags and still have the correct colors.
Finally, the third and most speculative explanation is that 1ES0347-121 is a prototype of
a new class of X-ray loud BL Lac objects. In the mean, these would be a factor of ,-,4 more
X-ray loud than EMSS BL Lacs.
Optical spectra, beginning in June, 1991, were obtained by us at the Michi-
gan/Dartmouth/MIT 1.3 m (R. R.), and the MMT (Blue Channel: J. P. H; Red Channel:
J. L., J. T. S). No attempt was made at this stage to identify a complete sample; we simply
aimed to identify as many sources favorable in each observing season as possible. For the 13
objects with correct colors (or discrepancies explainable by flaring), we obtained spectra, all
of which turned out to be featureless.
Figure 2 shows spectra of the 13 confirmed BL Lac objects. The positions of
these objects are also marked in the X-ray/radio/optical color-color diagram (Figure 1).
Differences in signal-to--noise ratios result from different detectors and telescopes used in the
observations. Despite these differences, it is clear that the spectra are uniformly featureless.
Table 2 provides VLA positions, 6 cm fluxes, optical magnitudes and colors (if available),
for the 13 confirmed BL Lacs. In addition, there are 4 more featureless spectra (n0t shown),
which are VLA nondetections, but which have the correct colors to be BL Lacs in Figure 1
(based on Condon nondetections; §3b).
IV. DISCUSSION
.f_J L"
The combined X-ray/radio/optical technique for identifying BL Lacs is evidently very
efficient. There have been no cases in which a VLA point source in the BL Lac part of-
the ctRo/o_ox diagram has not turned to be in fact a BL Lac object, as defined by the
featurelessness of its optical spectrum. The payoff is large, considering the small amount
of observing time required per source (5 rain snapshot at the VLA, then 10-20 min at an
optical telescope).
We can compare the new 13 Slew Survey BL Lacs identified to date with the complete
BL Lac sample of the EMSS, and the optically selected BL Lacs in Veron-Cetty and Veron
(hereafter, VV; VV 19??). The median V magnitudes are 17.0 for VV, 19.1 for the EMSS, and
18.2 for the Slew Survey. This illustrates that we are filling in an important gap in presently
known optically and X-ray_selected BL Lac samples. In units of 10 -12 ergs s -1 cm -2, the
mean Slew Survey BL Lac X-ray flux is 2.6, compared to 1.5 for the EMSS. The Slew Survey
BL Lacs are apparently ,-.,2-2.5 times brighter than the EMSS BL Lacs, although (except
for the two objects noted above) the X-raymto-optical flux ratios between the two groups
are consistent.
There are 38 more VLA point sources to be identified; using the EMSS as a guide, we
eventually expect a yield of ,-, 40 BL Lacs from the 52 point sources detected at the VLA.
Our observational program is accelerating. We have Australia Telescope time scheduled to
pursue the deep southern (6 < -40 °) radio sources. Some 33 of these lie at high latitude.
Assuming a detection efficiency similar to our VLA work, we expect -,_ 15 AT detections,
from which we will identify a dozen new BL Lacs. Hence, together with the 35 Slew Survey
sources known from catalogs to be BL Lacs, and the complete VLA sample, we expect about
90 BL Lacs in the whole Slew Survey. Since they will be relatively bright and uniformly
selected, they should prove valuable for both detailed follow-up observations of individual
objects, and statistical population studies.
This work was supported by NASA grants NAGS-1746 (Slew Survey ADP), NAGW-
2201 (Long Term Space Astrophysics), and NAS8-3751 (Einstein Data Center) to SAO; and
by NASA grant NAGW-2645 and NSF grant AST-9020008 to the University of Colorado.
Source
Group
Table 1: VLA Observations of BL Lac Candid.a.tes
Number Number Observed
Unidentified at VLA (Pct.)
Slew Survey ° 193 113 (59%)
Northern (_ > -40 ° ) 147 113 (77%)
High Latitude (Ibl > 15 °) 111 100 (90%)
Low Latitude 36 13 (36%)
Southern 46 0 b
a. From source list of 809 objects.
b. Southern sources will be observed at the Australia Telescope in July, 1992.
Table 2: New BL La¢ Identifications
Index Name Posn. RA DEC f(6 cm) mo
J2000 Off.(") Off.(") (mJy) (mags)
mO -- mE
(mags)
Spec.
Note*
1 1ES0229+200 02 32 48.6 +20 17 17 +25 -12 41.5 18.0 4.7
2 1ES0303+067 03 06 09.7 +05 58 28 +71 -116 3.9 17.6: ...
3 1ES0347-121 03 49 23.2 -11 59 27 +3 -1 7.6 19.1(J) ...
4 1ES0502+675 b 05 07 56.2 +67 37 24 -4 -1 31.3 16.5: ...
5 1ES0525+713 ' 05 31 41.7 +71 22 17 +3 +68 8.6 19.9: ...
6 1ES0647+250 06 50 46.5 +25 03 00 -11 +11 61.0 15.3: ...
7 1ES0806+524 b 08 09 49.2 +52 18 58 0 +3 169 15.3 1.8
8 1ES1028+511 10 31 18.5 +50 53 36 -1 +1 42.5 17.0 0.7
9 1ES1440+122 14 42 48.3 +120040 -9 -1 40.2 15.3: ...
10 1ES1544+820 15 40 15.7 +81 55 06 +2 +3 45.1 15.3: ...
11 1ES1927+654 19 28 19.5 +65 33 55 +13 -3 16.0 18.7: ...
12 1ES1959+650 19 59 59.9 +65 08 55 -5 +14 241.0 14.6: ...
13 1ES2343-151 23 45 38.4 -14 49 29 +16 -11 7.7 19.2(3) ...
Luu/Rem.
Rem.
Rem.
Sto./Rem.
Rem.
Ram.
Sto.
Sto.
Sto.
Sto.
Rem.
Sto./Rem.
ttuch.
a. Notes on spectra: Huch.--J. Huchra, MMT Blue Channel, Luu--J. Luu, MMT Red Channel, Rem.--
R. Remillard, MDM 1.3 m, Sto.--J. Stocke, MMT Red Channel.
b. Priority 4 source; see §3.
c. Possible wide--angle-tail source.
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Figure 1: Color-Color diagram for the 13 new BL Lac objects reported in this paper (open
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Thirteen new BL Lacertae objects discovered by an efficient x ray/radio/optical technique

Thirteen new BL Lacertae objects discovered by an efficient x ray/radio/optical technique

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