VGaPS: Galactic Plane Survey — search for more K-band calibrators suitable for VLBA and VERA


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Objectives of the campaign

We request three 24 hour sessions with the VLBA for observing 500 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--2005 in S/X produced a list of 2834 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 4o 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.

In October-November 2005, VERA is observing a list of 3000 potential candidates for fringe searching at K-band. Any source with a detectable fringes will then be followed up by the proposed VLBA observations.

Previous Observations

VLBA calibrator surveys, BB024, BF071, BP110, BP118 and BK124, were designed to expand the list of calibrator sources suitable for phase referencing and as target for geodetic observations. Twenty one VLBA Calibrator observing sessions, each of 24 hours, were made between 1994 and 2005. Each source was observed 2-3 times at different hour angles for 1.5-5 minutes. In this way up to 200 sources could be observed in each 24 hour session with sufficient sensitivity and i-v coverage to determine their suitability as phase-reference calibrators. Analysis of these observations produced images and milli-arcsecond accurate positions of observed sources.

The results from the above observations were published (Beasley et al, 2002; Fomalont et al, 2003, Petrov et al, 2005a, Petrov et al, 2005b, Kovalev et al, in preparation) and were made available on the Web at http://magnolia.nrao.edu/vlba_calib/index.html and http://astrogeo.org/calib/search.html. In the web sites, all sources within a specified distance from a selected target position can be found, images, u-v plots and coordinates for each potential calibrator can be scrutinized.

A small subset of VCS sources, 374 objects, was re-observed at K-band in 2002--2004 for future support of space navigation, proposals BR076, BL115, BL122, Jacobs et al, 2003. Among 252 detected objects, 14 are are within |b| < 5o. These observations showed that

  1. a majority of bright S/X calibrators were detected at K-band;
  2. the difference in position from K-band observations and X-band observations did not exceed 1 mas;
  3. the effect of ionosphere at K-band did not significantly degrade estimates of positions.

Impact of expected results

The dual-beam four station radio interferometer VERA is designed for direct measurements of parallaxes of hundreds of radiosources, such as H2O, SiO masers and late type stars, perhaps with accuracy as good as 10 muas. These measurements involve simultaneous observations of a pair calibrator-target separated within [0o.3, 2o.2] and recorded with independent receivers at K or Q bands. One of the primary goals of VERA mission is to improve the accuracy of the scale of distances used in astronomy, since at the 10 muas precision level trigonometric parallax of almost any object in the Galaxy can be directly determined. This scale of distances has a fundamental importance for various astrophysical applications.

VLBA phase-referencing observations at high frequency of galactic objects is also a significant part of its proposal load. Often, the limitation above 15 GHz for image quality and positional accuracy is caused by the loss of coherence in the image because of the relative large distance of the calibrator from the target. Thus, the new list of calibrators in the Galactic plane will significantly expand the list of VERA target sources and will improve the phase-referencing VLBA projects studying weak galactic objects.

Proposed Observations

Because the area near the galactic plane is not well surveyed, the list of suitable candidates for VLBI calibrators is difficult to compile. First, a list of 3000 sources will be selected from several catalogs (NVSS, CLASS, etc.) utilizing the CATS database capabilities (Verkhodanov, et al., 1997) and using a rigorous procedure based on evaluation of a priori probability of detection developed by Petrov et al., 2005b. Applying this procedure in BP118 observing campaign resulted in an impressive detection rate of 65% in the zones where no compact sources were previously found. Second, all 3000 of the sources will be observed by VERA in October-November 2005 in one scan each for 2 minutes at K-band recorded with 240 MHz bandwidth. Third, those sources for which fringes are detected, will be on the list for the VLBA observations we propose here. We estimate that up to 500 sources will be detected by the VERA observations. This estimate is based on extrapolating the number of sources in the VCS sample, which is complete at the 200 mJy level of the correlated flux density, to 60 mJy, the detection limit of VERA fringe searching observations. VERA system does not have capabilities to provide good maps and determine absolute position of sources with 1-mas accuracy which is required for support of dual-beam observations for measurements of parallaxes of galactic objects with 10 muas accuracy. Thus, the VERA fringe searching survey is designed only to detect sources at K-band. But VERA has the sensitivity comparable to the VLBA, and the pre-selection of sources will significantly increase the yield of VLBA observations.

We, thus, request three 24 hour observing sessions with the VLBA to observe about 500 candidates sources from the VERA fringe-search in the zone declination > -40o, |b| < 5o as well as within 10o from the Galactic center. We will use eight IF channels, spanned over 500 MHz around 24 GHz, with a total bandwidth of 64 MHz at 2 bit sampling, 256 Mb/s in order to reach the maximal sensitivity to reach the approximate VERA sensitivity. We the following correlator setup: 16 spectral channels in each IF and integration time 0.131 sec. This will give us an extra-wide search window needed for fringing sources with poorly known a priori positions. The calibration and imaging will be made in Charlottesville and Green Bank by using packages AIPS and difmap. Positions will be determined with Calc/Solve at GSFC.

The observation plan will be similar to the previous VCS schemes. Each source will be observed in two scans in different hour angles for 150 seconds. Every 1.5 hours, three to four troposphere calibrators will be observed. These calibrators are selected from the list of sources from the 2 cm VLBA survey (Kovalev et al., 2005), which are brighter than 0.5 Jy and have compactness index greater than 0.5 at 15 GHz. During a burst of tropospheric calibrator observations, the sources are selected in such a way that each antenna would observe at least one source at elevation below 20o and at least one source at elevation 50o above the horizon. Results of the VLBA Calibrator survey showed that observing tropospheric calibrators and estimation of troposphere path delay in zenith direction together with other parameters reduces the contribution of the unaccounted tropospheric path delay to less than 0.5 mas.

People

Team members (in alphabetic order):

List of candidate sources

List of candidate sources for bk125a, bk125b, and bk125c: gaps_all.key

Current status

2010.06.16  —  The 2nd version of the preliminary solution was made.

The catalogue of positions of 327 targeted sources was produced.

The name table provides information about names, a priori and a posteriori coordinates of observed sources.

Statistics:

  Observed Detected Not detected
Target 492 327 165
Trop. cal. 51 51 0
Total 543 378 165

Detection rate: 66% targets. Sources with less than 3 detections are not counted.

Probability of false detection as a function of SNR in BP125:

PFD SNR
0.3 4.95
0.1 5.18
0.01 5.61
0.001 5.99
10-5 6.70

2006.10.20_21:01  —  2006.10.21_21:04 :     The third session BP125C was observed. Observation logs are here.

2006.06.21_14:40  —  2006.06.05_14:38 :     The first session BP125A was observed. Unfortunately, station HN-VLBA did not observe due to mechanical problems.     :-(     Observation logs are here.

Tapes/disks are waiting for correlation.

2006.06.11_14:16  —  2006.06.12_14:18 :     The second session BP125B was observed. Observation logs are here.

2006.02.28:  —  Schedules files for two observing sessions, were uploaded to the array operation center.

2006.01.06:  —  The Programme Committee approved for dynamic scheduling, high priority.

2005.09.30:  —  Proposal was submitted to the Programme Committee. The code assigned: BP125


Key files:
  bp125a.key bp125a.sum
  bp125b.key bp125b.sum
  bp125c.key bp125c.sum

References

VERA K-band fringe searching survey

Current status: Processing of 14 observing sessions is finished. Results are available here. (NB: the page is loaded slowly, be patient).

Summary: the number of sources observed: 2462, The number of sources reliably detected: 397, marginally detected: 158. Among them, the number of new sources reliably detected: 103, the number of new sources marginally detected: 122. All new detected sources will be observed in GaPS.

Back to VLBA Calibrator Survey web page.

This page was prepared by Leonid Petrov ()
Last update: 2010.06.16_17:34:17