We will be presenting updated results to our optical survey of the "error boxes" of EGRET unidentified sources at high Galactic latitude. It is our intention to search for potential blazars that may have been missed in the original identification process. We have first searched the error boxes of unidentified sources at |b|>20° for flat spectrum radio sources using NASA Extragalactic Database (NED). For each such radio source found we conducted optical searches for counterparts using the Palomar 60-inch telescope. Many of the radio sources have plausible optical counterparts, and spectroscopy will be conducted at a later date to determine which sources are quasars or active galaxies (AGN's). Though it is plausible that several of these sources are extragalactic, no blazar-like activity has yet been observed

Bloom, Steven D.

Dale, Daniel A.

Caltech Authors - Main

Optical Identification and Monitoring of High
Energy Gamma-Ray Sources
Steven D. Bloom* and Daniel A. Dale11
^Department of Physics and Astronomy, Hampden-Sydney College, Box 821,
Hampden-Sydney, VA, 23901
11nfrared Processing and Analysis Center, JPL/Caltech, Pasadena, CA, 91125
Abstract. We will be presenting updated results to our optical survey of the "error boxes" of
EGRET unidentified sources at high Galactic latitude. It is our intention to search for potential
blazars that may have been missed in the original identification process. We have first searched
the error boxes of unidentified sources at Ibl > 20° for flat spectrum radio sources using NASA
Extragalactic Database (NED). For each such radio source found we conducted optical searches
for counterparts using the Palomar 60-inch telescope. Many of the radio sources have plausible
optical counterparts, and spectroscopy will be conducted at a later date to determine which
sources are quasars or active galaxies (AGN's). Though it is plausible that several of these
sources are extragalactic, no blazar-like activity has yet been observed.
INTRODUCTION
Though the EGRET instrument detected 271 sources, most of these sources remain
unidentified with counterparts at another wavelength [7]. However, the distribution of
sources on the sky does suggest at least two major components: Those near the
Galactic plane, where 90 % of the sources are unidentified, and those above the plane,
where 50% are unidentified [2]. Grenier [5] further suggests that those sources above
the plane may have three subcomponents: extragalactic, Gould Belt, and Galactic halo.
Though nearly all of the firmly identified extragalactic sources are blazars, there have
been recent studies suggesting that some sources may be related to galaxy clusters [3].
The task of investigating all of these sources individually seems daunting. Therefore,
several investigators have gone the route of examining several individual sources in
depth ([6], [11], [13]). The advantage of this technique is that such searches are much
more likely to be fruitful. This has particularly been true for finding new blazar-like
objects at high Galactic latitude. We continue along these lines by searching within the
error contours of high Galactic latitude EGRET unidentified sources for new blazars.
We will then ascertain whether any such objects found are likely to be the source of
the gamma rays.
CP587, GAMMA 2001: Gamma-Ray Astrophysics 2001, edited by S. Ritz et al.
© 2001 American Institute of Physics 0-7354-0027-X/01/$18.00
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SOURCE SELECTION
Using the NASA Extragalactic Database (NED), we have searched for flat spectrum
radio sources (spectral index, -0.5<a<0.5) within the 95% error contours of the
EGRET unidentified sources with lbl>20°. Aside from a handful of pulsars, blazars
with flat radio spectra are the only objects that have been detected by EGRET as a
class. We have chosen this range of latitudes so that there will be minimal overlap with
Galactic objects. We further restricted ourselves to the ranges of right ascension and
declination that would be visible from Mount Palomar during the nights in November
1999. These radio data referenced in NED were usually from the Parkes or Green
Bank surveys, which had positional accuracies of about 1-3 arc minutes (depending on
observing frequency and brightness of the source), which is far too coarse for optical
searches. We have therefore additionally searched for these radio sources in the
NRAO-VLA Sky Survey (NVSS) in order to obtain positions accurate to
approximately 1" [4]. In addition to the flat spectrum radio sources, we have compiled
a secondary observing list consisting of radio sources of unknown spectral index,
and/or X-ray and optical sources with accurate positions.
An extensive study of the identification of radio sources with EGRET sources was
conducted by Mattox et al. [10]. Essentially they conclude that it is difficult to
positively identify a gamma-ray source with any radio source with flux density less
than about 500 mJy at 5 GHz mostly because the sky density of such sources is so
high, and thus the probability for chance correlations is correspondingly high. They
assume that the radio spectra would be near their peak fluxes at these frequencies, and
thus 5 GHz flux densities would be representative of the synchrotron output of the
source. This assumption was made out of necessity, since there are no complete high
frequency surveys with sensitivity less than one Jansky. However, other investigators
have shown that several sources might be fairly dim at these low frequencies, and have
a brighter (and flatter) spectrum extending to at least 200 GHz (eg., [1]). Thus, some
dim 5 GHz radio sources, which by the method of [10] would not be considered to be
counterparts to EGRET sources, may indeed be counterparts to gamma-ray blazars.
We intend to make up for this possible deficiency by searching for the optical
counterparts to sources that may be fairly dim (< 100 mJy) at 5 GHz. If a counterpart
is found we will search for blazar-like activity, and go on to investigate the optical and
radio spectra of these objects.
OBSERVATIONS
We have conducted the search for optical counterparts using the 60-inch telescope
at Mount Palomar with a Tek 2048x2048 CCD. The pixel size is 0.37 arc seconds,
however we have 2x2 binned the pixels to a size of 0.74 arc seconds. Since the seeing
for those nights (and most at Palomar) was worse than 1 arc second, no crucial
information on source structure is lost. In addition, we would expect most of our
sources to be point-like. We have used the Johnson B, V and Kron R filters for
determining magnitudes in those bandpasses. The procedure was to first observe a
candidate source in R for 10 minutes (and then adjust to a shorter or longer exposure
depending on results). If no obvious counterpart was seen after 20 minutes, we did not
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go to the other filters. If a potential source was seen, we observed in V and B as well.
In addition to the program sources, we have observed Landolt [8] standard stars
throughout the evening to determine the photometric scale and atmospheric absorption
in the usual manner (more details on procedure and reductions will appear in a
forthcoming journal article by the authors). Galactic extinction corrections have been
applied using the values from NED. We have repeated our observations on a second
night to test for variability. We have used the Guide Star Catalogue to determine the
astrometric plate solutions for our CCD fields. Typical rms uncertainties in our plate
solutions were less than that of the individual stellar positions from the catalogue
(about 0.3 arc seconds).
RESULTS
We have observed 10 radio sources (within four different EGRET error boxes)
and found optical counterparts for seven, breaking into magnitude in the following
manner. Of the seven sources seen visually, five are in the range of V=16-19, one is in
the range of V= 19-21 and one is dimmer than 21 (near the magnitude limit). A
summary of the results for the visually detected sources is in Table 1. A more
complete summary with multicolor photometry will be presented in a forthcoming
paper. Column (1) gives the name of the EGRET source, Column (2) gives the name
of the radio source that is a candidate counterpart, Column (3) gives the V magnitude
attained by us as described in the previous section, Column (4) give the radio flux
density at 5 GHz (in milliJanskys) as found in NED and Column (5) gives the spectral
index between 1.4 and 5 GHz and is provided by NED if the information is known.
We note that no source has shown significant variability across the two nights. Of
particular interest is that two candidate counterparts (B0214+108 and NVSS
J032850+21282) are in the catalogue of ROSAT/Green Bank ("RGB") sources [9].
Punsly [12] suggested that "RGB" sources would be likely gamma-ray blazars and that
the counterparts are BL Lac objects with their spectral energy distributions (SED)
peaked at intermediate to high frequencies (that is, optical to X-ray). These particular
objects in our study, however, do not have well determined SED's. The brighter
objects have been slated for spectroscopic and high frequency radio observations
within the next several months (to be conducted by other groups, in collaboration with
us in this study).
TABLE 1. Possible Counterparts to Gamma-Ray Sources
EGRET Source Radio Source
(1)
3EG J0038-0949
3EGJ0215+1123
3EGJ0215+1123
3EG J0245+1758
3EG J0245+1758
3EGJ0245+1758
3EGJ0329+2149
(2)
NVSS J003906-094247
NVSS J021527+1 12318
B0214+108
NVSS J024437+172221
NVSSJ024640+180144
NVSS J02461 1+182330
NVSS J032850+2 12825
V
(3)
21.5
17.9
16.3
19.8
18.1
18.8
18.1
Radio
(4)
214
56
440
33
53
214
57
S.I.
(5)
-0.1
-0.8
-0.8
-0.1
0.1
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CONCLUSIONS
After examining the error boxes of four EGRET sources for coincident radio/optical
objects, we have found four plausible counterparts to the gamma-ray sources. A
summary of the results is shown in Table 1. For 3EG J0038-0949 there is only one
plausible counterpart. For 3EG J0215+1123, we consider B0214+108 to be the most
plausible counterpart, though even this source has difficulties. The extended radio
source, 4C +10.06 is ambiguously identified with this catalogued QSO. The two
nearest compact NVSS radios source are over 40 arc seconds from the optical object.
Search results from NED indicate that there are many galaxies in this field, at least one
of which has a red shift similar to the QSO. For 3EG J0245+1758, the second NVSS
source listed in the table is the more likely counterpart (brighter, flat spectrum). This
source is also a known QSO. At first, it may not seem that the suggested counterpart
for 3EG J0329+2149 is plausible, due to its relatively low radio flux density; however,
we note that this source is a coincident optical, IR, radio, and X-ray source. The
compact core is known to have a flat spectrum [9]. We emphasize that we do not yet
consider any of these suggestions to be firm identifications. We suggest that optical
monitoring of these objects continue, and that spectroscopy conducted for any of the
objects for which the optical counterpart has not yet been classified spectrally. The
Hampden-Sydney College 16 inch telescope could be used to monitor the brightest 5
or so of these sources when they next become visible in northern Autumn 2001.
ACKNOWLEDGMENTS
This work has made use of the NASA Extragalactic Database and the Astrophysical
Data System web abstract server.
REFERENCES
1. Bloom, S. D. et al., Ap. J., 488, L23 (1997)
2. Caraveo, P., these proceedings (2001)
3. Colafrancesco, S., these proceedings (2001)
4. Condon, J. J. et al., A. J., 115, 1693 (1998)
5. Grenier, I., these proceedings (2001)
6. Halpern, J. et al., Ap. J, 551, 1016 (2001)
7. Hartman, R. C. et al., Ap. J. Suppl. 123,79 (1999)
8. Landolt, A. U., A. J., 104, 340 (1992)
9. Laurent-Muelheisen, S. A. et al., A&A Suppl., 122, 235 (1997)
10. Mattox, J. R. et al., Ap. J., 481, 95 (1997)
11. Mukherjee, R. et al., Ap J, 542, 740 (2000)
12. Punsly, B., Ap. J., 516, 141(1999)
13. Wallace, P. M., these proceedings (2001)
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Optical identification and monitoring of high energy gamma-ray sources

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Optical identification and monitoring of high energy gamma-ray sources

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