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:

  1. were not observed previously with VLBI;

  2. galactic latitude |b| < 6°.

  3. declination δ > -30°

  4. have at least two measurements of flux density that allow estimation of their spectral index at frequencies higher than 2 GHz.

  5. have single-dish flux density extrapolated to 22 GHz > 80mJy;

  6. 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

  1. to drop extended sources not suitable as calibrators;

  2. to make estimates of correlated flux densities and compactness of detected candidates at baselines 1–2 thousand kilometers;

  3. 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):

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