ATCA observations of Fermi unassociated sources
Introduction
The Fermi two-year
catalogue 2FGL
lists 1872 γ-ray objects. Among those sources that have associations,
for 751 objects radio emission at 8 GHz was detected from compact
parsec-scale regions. The catalogue of VLBI associations in
html or in
text formats provides positions
of sources with sub-milliarcsecond accuracies, estimates of their
correlated flux densities and provides radio images of the bases
of their relativistic jets.
However, 573 sources still do not have associations. We set a goal
to observe at 5 and 9 GHz the areas of 95% probabilities of 2FGL
localization and find there all radio sources with flux densities down
to 2 mJy for a search of possible associations. The broad goal of
our project is to increase the number of radio–γ associations and
to find a population of radio silent γ-ray sources or
to demonstrate that such a population does not exist.
The program has several steps:
- As a pilot project, we first observed with connected
interferometers,
ATCA at the southern hemisphere and
VLA at the northern
hemisphere in the direction of 2FGL pointings.
- Then, we will re-observe in the so-called mosaic mode those
Fermi sources that have 95% probabilities of
2FGL localization greater than FWHM of interferometer beams
and were not detected in the previous runs.
- Finally, we will observe with LBA
in the southern hemisphere and VLBA
in the northern hemisphere detected sources in order to detect emission
at parsec-scales.
Those sources within 95% probabilities of 2FGL localization that show
detectable emission at 8 GHz at parsec scales (0.1–10 mas) are
considered as likely associations.
Observations
We observed areas within positions of all Fermi sources with
< +10° with ATCA at 5.5 and 9 GHz simultaneously.
Each source was observed in three scans of 30 seconds each
separated by four hours.
This will allow us
- to expand the list of associated sources.
Association through proposed radio observations will automatically
improve their position by two order of magnitude and will
make possible to associate γ-ray sources with objects
in other frequency ranges, for instance, optical.
Availability of information at other frequency bands will
significantly boost the value of observations from the
multi-million dollar Fermi mission. In particular,
this information will help to confirm whether
a given source is a blazar of a certain class, and if yes,
follow-up observations have a potential to determine
its redshift. Achieved position accuracy will make possible
optical and VLBI follow-up observations.
- to assign a preliminary class of
a given source as a blazar.
Although as Kovalev et al. (2009) showed there exist examples of
Fermi-detected blazars with steep spectrum, the majority
of flat spectrum sources are blazars. The angular resolution
at baselines with CA6 antenna reaches 0.9" at 9 GHz, which
will allow us to reject some candidate sources on the basis
of their morphology.
- to find a population of γ-ray
sources
that have no association with radio sources brighter than 2 mJy
(or to confirm that such a population does not exist).
The existence of a population of radio weak(silent) γ-ray
sources has a potential for interesting discoveries.
Independently, a list of 215 sources with δ > 0°
was observed with VLA.
Source list
Map of 2FGL unassociated sources:
Current list of targets. Sources marked
with # in the first column were not associated in the original 2FGL
release, but currently are considered associated owing to new observations.
Results
Early results from this program were submitted for publication to Monthly
Notices of the Royal Astronomical Society:
L. Petrov, E. K. Mahony, P. G. Edwards, E. M. Sadler, F. K. Schinzel,
D. McConnel, ATCA observations of Fermi unassociated sources,
Mon. Not. of Roy. Astr. Soc., 2013, in press. Preprint:
http://arxiv.org/abs/1301.2386.
Summary of results:
We observed our program for 29 hours on 19–20 September
2012 with ATCA in the H214 configuration. The data were recorded at
4.5–6.5 and 8.0–10.0 GHz.
The correlator output
(58 Gb) is available. We performed post-processing with
Miriad package.
Analysis procedure included calibration, imaging, and source analysis.
We found 626 objects in 267 fields. Of them,
210 objects have position offsets with respect to γ-ray
2FGL objects exceeding 2.448σ, i.e. beyond the areas
of 95 per cent probability of their localisation, and they
were discarded.
The rms of images is typically in a range of 0.15–0.25 mJy.
The detection limit is 1.8 mJy for sources in the center of the field
of view and 9 mJy at the edge of the field of view, at 6.5'.
There are 23 fields that have a number of resolved objects.
We found 416 counterparts within 95 per cent probability of
localization of 267 2FGL sources. For 69 2FGL sources more than one
counterpart was found. Among 416 counterparts, 95 have a spectrum
flatter than -0.5, 75 have a spectrum steeper than -0.5, and for 246 objects
no spectral index has been determined. The flat spectrum sources are
considered as tentatively associated with Fermi objects.
For 107 sources the probability of association is more than 10 times
exceeds the probability to find a background source with that flux
density by chance. These flags help to identify a subset of more probable
associations. However, taken alone they do not establish a firm association.
Images
Here a set of 822 radio images at 5.5 and 9.0 GHz
subbands is available. NB: Flux densities written in margins of images
are not for beam attenuation. Point sources that were automatically detected
are circled. They were inspected my analysts who flagged points that were
either spurious detections, or too weak, or far beyond the center of the view.
They are marked with red. Remaining objects
are marked green.
Catalogues
The list of 416 sources falls into three categories:
- Category I: 142 objects detected at both 5 GHz and
9.0 GHz subbands within 2.7 arcmin of the pointing direction.
We provide in Table 1 γ-ray
source name; IAU name
of detected radio source; tentative association
status, estimates of its J2000 coordinates followed by
$1\sigma$ uncertainties in arcseconds (uncertainty of right
ascension is not scaled by cosδ factor),
Δα and Δδ flux densities at 5 GHz
and 9 GHz in mJy, F5 and F9, corrected for beam
attenuation followed by their standard deviations,
Δ5 and Δ9, and spectral index and its error,
Sp and ΔSp. We provide distance of a source from the
pointing direction D, followed by Nσ, the ratio of
this distance to its standard deviation derived from reported
2FGL position localisation errors. If the source was associated
with an object either from NVSS, SUMSS or MGPS-2 catalogues,
its flux density at 1.4 GHz (NVSS) or 0.843 GHz (SUMSS and MGPS-2)
is shown in column F1 followed by the 1 GHz catalogue code
(N for NVSS, S for SUMSS, M for MGPS-2), followed by the
source identifier in that catalogue. If the source was associated
with WISE object, its WISE source ID is shown in the last column.
Column 3 shows two flags: "a" if a source has likelihood
ratio greater than 10, and therefore, considered a likely
association, "e" if a source is extended, and "f" if a source
has the spectral index flatter than -0.$.
- Category II: 210 objects detected only at 5 GHz subband
within 6.5 arcmin of the pointing direction.
We provide in Table2 estimates of flux
density at 5 GHz corrected for beam attenuation. The contents
of Table2 is similar to Table1, except columns F9 and Δ9
and which are missing. The spectral index is computed over
the sub-band [4.58, 6.41] GHz. The spectral index estimates
with uncertainties greater than 0.3 are omitted.
- Category III: 64 objects either detected beyond 6.5'
of the pointing direction or detected only at 9 GHz band. Since
calibration for beam attenuation becomes uncertain at large
distances, we can only provide a low estimate of their flux
density: 10 mJy. Table3 lists these sources.
Its contents similar to Table2 except columns F5 and Δ5 which
are missing.
Dependence of flux density of detected sources of categories I and II
on galactic latitude. Flat spectrum sources are shown with
green hollow circles.
Other sources are shown with filled blue discs.
Future work
We were allotted three 15 hours blocks in September 2013:
2013-09-25, 21:30, 2013-09-26, 12:30, C2624, H214
2013-09-26, 21:30, 2013-09-27, 12:30, C2624, H214
2013-09-27, 21:30, 2013-09-28, 12:30, C2624, H214
We are going to re-observe with ATCA those sources that were detected
at 5 GHz, but are not detected at 9~GHz, and had position offsets
exceeding FWHM of the 9~GHz beam. We will re-observe sources listed
in Table3 with pointing to their positions found from this survey as well.
We are also going to re-observe sources that were detected at the edge
of the beam or by the sidelobe.
In the second phase of the project we are going to re-observe 144
fields where we did not detected a source in the mosaic mode. This will allow
us to detect all the sources in the 95 per cent probability of 2FGL localisation
that are brighter than 2 mJy.
In the third phase of the project we will observe detected sources
with LBA and determine correlated flux densities from regions smaller
than 50 mas. This will allow us to associate detected radio sources
with blazars.
Current status
- 2015.05.03 Made catalogue of detected sources within
3FGL fields and
2FGL fields fields.
- 2015.03.17 Images of radio sources considered as candidates
to association from VLBA observations were made
available.
- 2013.03.04 Three blocks of 15 hours each were allotted
for the follow-up project AOFUS-2.
- 2013.02.10 Exepriment aofus-1.1 for re-observation of
J1417-4404 has been analyzed
- 2013.01.10 Data analysis is finished. Paper has been
submitted.
- 2012.12.19 Ran cross-reference among detected sources
against WISE and NVSS catalogues within 2σ of AOFUS
position errors. Results of the cross-referencing are added
to catalogue of point sources.
- 2012.12.16 2nd version of the preliminary
catalogue of point sources has been
generated. Flux densities and positions were evaluated using
visibilities by fitting in the vicinity of point sources
that were found by image analysis. The catalogue
includes sources that
a) have position differences not exceeding 2.4σ of the 2FGL
position where σ is a reported 2FGL position uncertainty, and
b) were detected at 5.5 GHz.
In total, potential associations were found for 267 Fermi
objects (roughly 2/3 of targets). For a number of Fermi objects
more than one association has been found. The catalogue contains
375 entries. Position uncertainties were re-calibrated using results of
analysis of the differences of 105 matching sources from the NVSS catalogue.
Median position uncertainty is 3".5 in right ascension and 2".0 in declination.
- 2012.12.13 Second proposal of ATCA observations has been submitted to
the ATNF. The goals: 1) to re-observe sources that were detected far from
the pointing direction at one band only; 2) to observe in mosaicing mode
the fields with 95% probability of localization exceeding FWHM of the beam
- 2012.12.13 Proposal for LBA follow-up observations of detected
sources has been submitted to the ATNF.
- 2012.11.22 Preliminary catalogue of point
sources has been generated. In total, potential associations within
a field of view (250" at 5.5 GHz and 400" at 9.0 GHz) were found for
261 sources (roughly 2/3 of targets).
- 2012.11.19 Point sources were identified.
Maps with source identifications were visually inspected and flagged.
Red circles denote points that are deemed
artifacts; Green circles denote points
that are deemed sources or a source component.
- 2012.11.03 Preliminary very coarse results was produced.
See images and
flux table.
- 2012.10.02 The AOFUS correlator is available
at here
(58 Gb). In total, 618 sources have been observed:
411 targets and
207 calibrators.
- 2012.09.21 The AOFUS experiment generally run went very well.
Of the 1589 scans, we lost about 20 scans when a correlator
block dropped out (and observing had to stop to re-install it).
Otherwise the weather was good throughout and no other problems.
- 2012.09.17 The schedule files for AOFUS experiment:
- 2012.02.26 The ATNF Time Allocation Committee
allocated 29 hours, [2012.09.19_05:00, 2012.09.20_10:00] UTC,
for this project (code C2624, at H214 configuration).
- 2011.12.13 The proposal has been submitted to the ATNF.
People
Team members (in alphabetic order)
| Phillip Edwards | ATNF, Australia |
| Yury Kovalev | Astrospace Center, Russia |
| Elizabeth Mahony | the Netherlands Institute for Radio Astronomy |
| David McConnell | ATNF, Australia |
| Tara Murphy | University of Sydney, Australia |
| Leonid Petrov | Astrogeo Center, USA |
| Elaine Sadler | University of Sydney, Australia |
| Frank Schinzel | University of New Mexico, USA |
| Greg Taylor | University of New Mexico, USA |
References
- Abdo A. A., 2012, ApJS, 119, 31
- Ackermann M., 2012, ApJ, 753, 83
- Kovalev Y. Y. 2009, ApJ, 707, L56
- Murphy T. et al. 2010, MNRAS, 420, 2403
- Petrov L., Kovalev Y. Y., Fomalont E. B., Gordon D.
2008, AJ, 136, 580
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This web page was prepared by Leonid Petrov
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Last update: 2016.03.09_15:09:15