EGaPS: EVN Galactic Plane Survey -- search for more K-band
calibrators using EVN
Text of the proposal in PDF format,
Objectives of the campaign
We request a 48 hour session with the EVN for observing 350
candidates in order to dramatically increase the number of calibrators
at K-band near the Galactic plane for supporting
VERA and
VLBA
experiments focused on the study of galactic objects. Previous
observations with the VLBA in 1994--2009 in S/X produced
a list of 4337 sources, with position accuracy better
than 5 mas and with S/X images. However, the density of this catalogue is
not sufficient to provide enough calibrators closer than 2°
near the Galactic plane for VERA and VLBA observations of many interesting
galactic objects. VERA observations, in particular, critically depends
on availability of calibrators within 2°.2 of a target source.
Our approach
In the past, the list of bright compact sources detected at VLA and/or
MERLIN served as a pool for candidates for VLBI calibrator surveys.
Almost all these objects have been already surveyed with the VLBA. Since
the VLBA Calibrator list is complete at the 150–200 mJy level, new
calibrators are relatively faint objects. Most all-sky surveys at
frequencies higher than 2 GHz are either incomplete and/or not deep
enough. As a result, information about spectral indexes is sparse and
unreliable. For this reason, it is more difficult to find remaining flat
spectrum good candidate sources.
Our approach for a massive calibrator search is to use dedicated VLBI
experiments. On 2007.11.03 a 12 hour pilot VLBI experiment, project
EP062, was carried out.
The goal of that EVN project was to find more calibrators at C-band observing
flat spectrum sources at
Ef–Jb–Tr–Mc–Nt–On.
We used advantage of the high sensitivity of the EVN network that includes
Effelsberg. Among 375 target sources, 330 objects have been detected,
i.e. the detection rate was 88%. During 70s integration time
the detection limit at the baseline Ef–Mc was 5–10 mJy
depending on the elevation angle. A report which presents preliminary
results from this project (Petrov & Kovalev, 2009, paper in preparation).
Encouraged by success pilot project EP062, we are going to exploit
the technique of calibrator search surveys that was successful tested
in 2007, and propose a dedicated 48 hour EVN experiment for observing a list
of 350 sources at 22 GHz at
Ef–Yb–Jb–Mc–Nt–On.
New calibrator candidates were found by analyzing 197 radio astronomical
catalogues using the CATS database.
The database includes NVSS, CLASS, CRATES, and others. We have also used
results of the recent ATCA survey at 20 GHz. Initially, we selected
1088 sources which satisfy the following criteria:
- were not observed previously with VLBI;
- galactic latitude |b| < 6°.
- declination δ > -30°
- have at least two measurements of flux density that allow estimation
of their spectral index at frequencies higher than 2 GHz.
- have single-dish flux density extrapolated to 22 GHz > 80mJy;
- have spectral index α flatter than -0.53
(S ∼ να ).
1260 sources satisfy these criteria. Then the list was screened and matched
against the catalogue of known planetary nebulae (Kohoutek, mash1, mash2 ).
62 sources which are within 30" from the known planetary nebulae were
removed from the candidate list.
Then the list was matched against the catalogue of known planetary Hii
regions. 155 sources which are within 30" from the known Hii were removed
from the candidate list.
Remaining sources were scrutinized
by an analyst and ∼200 sources manually flagged out. These are the
mainly the sources with unreliable spectrum.
Finally, 559 candidate source remained in
the list.
Goals
Our goal is increase the number of available phase calibrators
in the Galactic plane by 60%. To date, 522 compact extragalactic sources within
|b| < 6 ° were observed with VLBI. For majority of them positions were
determined with the milliarcsecond level of accuracy and for some of them
images are available. The proposed survey will add up to 300 more sources.
This will significantly improve the probability to find a calibrator within
several degrees from any target near the Galactic plane and
δ > -30°. New found strong sources can be used as phase
calibrators for detecting weak targets, while weak ones will provide
a reference position for sufficiently strong targets, such as masers,
in a differential astrometry type of observations.
The purpose of this experiment is the community service: to provide
more calibrators for phase reference observations of Galactic targets,
mainly water masers at the K-band.
Results will also be of a particular importance for the VSOP-2 project
which is expected to have (i) fast switching capability within two degrees,
(ii) highly sensitive K-band receiver.
Results of these observations will allows us
- to drop extended sources not suitable as calibrators;
- to make estimates of correlated flux densities and compactness
of detected candidates at baselines 1–2 thousand kilometers;
- to improve coordinates of VLBI component to the 1–10 mas level.
Proposed observations
We request one 48 hour observing session. We are going to observe at 7
antennas, including Effelsberg, Yebes and Robledo. We request
Robledo in order to have an extra highly sensitive antenna and ensure
successful outcome of this endeavor. We include Onsala for improving
the baseline coverage. Each source will be observed in two scans of
120 second long separated by approximately 6 hr in a sequence that
minimizes slewing. Every 1.5 hr a burst of 4 strong compact
sources with known maps will be observed: two objects at elevation angles
7–30°, and two objects at elevations 50–90°. The purpose
of scheduling these calibrators is a) to allow estimation of the troposphere
path delay in zenith direction; b) to evaluate the atmosphere opacity;
c) to improve the amplitude calibration using results of the
VLA calibrator
monitoring program and our RATAN-600 monitoring which includes
K-band.
Data are requested to be recorded at 16 channels of 16 MHz each at the
total bandwidth 256 MHz centered around 22 GHz, single LCP polarization,
1024 Mbit/s with 2 bit sampling. We request the data to be correlated
at the Bonn correlator to allow for astrometry solutions. We request
correlation with the shortest accumulation periods and the maximum number
of spectral lags that the Bonn correlator is able to handle. This will
provides us the largest possible fringe search window.
The expected detection limit at baselines Ef–Ro will be 15 mJy,
at Ef–Yb 20 mJy, and at Ef–Mc,
Ef–Nt will be 40 mJy, i.e. 5–10 times deeper than the
VERA 22 GHz fringe search survey. The accuracy of position determination
in this experiment is estimated to be in the range of 1 to 10 mas.
Post-correlating analysis will be done with HOPS, VTD/post-Solve and in
parallel with AIPS.
People
Team members (in alphabetic order):
- Yuri Kovalev Astro Space Center LPI, Moscow
- Leonid Petrov ADNET Systems Inc./NASA GSFC
List of candidate sources
The list of targeted sources has 369
objects. Among them, 344 sources will be observed in 2 scans and 25 sources
are observed in 1 scans.
In addition to target sources, 69 sources
will be observed as tropospheric calibrators.
Current status
2009.09.30:
The schedule file has been prepared
and submitted. Statistics of the schedule are
computed.
2009.01.30:
Proposal was submitted to the Programme Committee.
The code assigned: EP066.
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This page was prepared by Leonid Petrov
(
)
Last update: 2009.10.13_15:30:43