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Radio patrol survey by Gregory and Taylor Back to CATS manual (English) (Russian) [ CATS home ] [ Back to CATS list ] [ ftp ]

1986AJ.....92..371Gregory & Taylor
corrected by H.Andernach 2/97
Astronomical J., v.92, p.371 (1986).

Radio patrol of the northern Milky way: A catalog of sources.II

P. C. Gregory and A.R. Taylor

Abstract


We persent a catalog of 1274 discrete sources detected during a 5 yr, 6 cm, radio-patrol survey of the galactic plane for variavle sources. Approximately 20 % of the volume of the galactic disk was repeatedly observed on 16 occations. The survey is capable of detecting both short-term (few days) and long-term (> 1 yr) variablility. Seventy-five percent of the sources are weaker than 75 mJy, and as a consequence reliable sources and 27 possible variables were detected, of which 64 % exhibited shirt-term variability. We propose two new identifications for the short-term variables GT 0116+622 and GT 2318+620, with the transient gamma-ray source Cas Г-1 and the x-ray source 4U 2316+61, respectively. The absence of gaint outburst ( > 5 Jy) leads to an upper limit of three on the number of objects in the syrvey area with radio properties like Cyg X-3. A comparison with previous centimeter variability studies suggests the possible existence of two distinnct variability components.

Introduction

Between 1977 and 1981 we carried out a radio patrol of the northern galactic plane in a systematic survey for contunuum variable sources using the NRAO 91 m, transit telescope at wavelength of 6 cm. During this period, a 500 squre degree area along the galactic plane, from l = 40^o to 220^o and |b| <=2^o, was repeatedly surveyed on approximately 16 times to provide a record of variability on time scales ranging from a few days to 5 yr.
...

II. Survey characteristics, calibration, and analysis

Full ditails of the methods of observation, calibration, and data analysis were contained in the first catalog (Taylor & Gregory, 1983 A.J. 88, 1784), and only a breaf summary is given here. The 91 m telescope was equipped with a two-channal, 6 cm receiver connected to a rotatable, dual-beam-feed system at the prime focus. Each channel had a system temperature of 70 K and bandwidth of 580 MHz centered at 4.76 GHz. Beam-swotched in antiphase between the two feeds, yielding two independent outputs. The feeds were sensive to right-hand circular polarization, and produced two 2.8' beams separated by 7.2'.

The observations are carried out by exicuting a sequence of alternating north- and south-bound declination scans across the galactic plane with two beams aligned at 11^o to the sky track. Since the declination scans are a fixed length (4.3^o in 1977-1979 and 4.0^o in 1980-1984) the galactic latitude coverage varies with the inclination of the galactic plane to the meridian, from +2.15^o at the northern extreme (DEC= 62^o) to +-1.2^o at DEC=0^o. Coverage of the observing region is achieved by interleaving 14 scan sequences. Short-term variability information is obtained by repeating a particular sequence five to six times during one observing session. The same sequences is repeated two to three timesd a year later to provide long-term variability information. In general, each region of sky has been repearted surveyed eight times with a northbound scan in one sequence and a further eight times with a southbound scan from another sequence, for a total of 16 repeats over 5 yr. ...

The analysis of the 1984 observations is incomplete and will not be discussed further in this paper. In three longitude intervals 40^o < l < 55^o, 87^o < l < 86^o and 111 <l < 114^o, the galactic confusion was too large to permit the detection of discrete sources. These regions are not included in the analysis presented here but will the subject of another paper describing a new analysis technique.

III. Survey results

a) The Catalog of Discrete Sources
The catalog of the 1274 discrete sources with variability information is presented in table II.

Column 1 gives the source name in the 8-digit "Parkes" system.
Columns 2-3 and 3-4 give source coorditanes 1950 with 1sigma errors. The mean flux density and estimated 1sigma error are listed in columns 5-6.
Column 7 is size (diam) in arcmin in direction of the scan throgh the source.
Column 8-14 present the results of the search for variability.
Column 8 gives the number of observations of the source in that observing epoch which exhibits the largest short-term variability. This was the epoch used tio calculate the short-term variability indices, V1 and V2, which are listed in colimn 9 and 10. The V1 index is the ratio of the measured rms variation to the expected 1sigma variation due to the noise and instrumental effects combined. For nonvariable sources, the index V1, by definition, has an expectation value of one. This index is effective in detecting variability that occurs continuously. However, for sources that vary only a small fraction of the time, or when there are only two or three mesurements, V1 will considerably underestimate the degree of variability. To ovewcome this problem we calculate a second index, V2, which measures the largest deviation from the mean signal strength, in units of the expected 1sigma variation (see Taylor & Gregory 1983).

Sources are classified as short-term variables if the probability of occurrence, by chance, of the measured values of V1 and/or V2 is less than 0.1 %. Sources for which the probability of V1 and/or V2 is less than 1%, but greater than 0.1% are classified as possible variables.

b) The variable Source
The catalog lists a total of 1274 sources. Of these, 1190 were aexamined for short-term variability, resulting in 20 variables and 18 possible variables. 801 sources were examined for long-term variability resulting in 12 additional variables and nine possible variables for a total of 59 sources variable on a time scale between one day and 5 yr. The number of possible variables found, 27, should be compared with the number of false possible variables expected, which is 11.
A catalogue not completely identical with the published one was provided by J.A. van Leeuwen (jvl@bear.ras.ucalgary.ca) to H.Andernach on 19-Oct-1994. Its format is displayed below: 
GTname     RA_1950+-s DEC_1950+-" S4.79+-mJy  size' M   V_1   V_2     ?       ?       ?    ?       ?   long_ind term_var
----+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8----+----9----+----0----+----1---
GT0001+625 000100 10 +623129  43   200   23   1.2  14  0.70  1.44   63.16  57.28   3.58 238.40   1.00   0.78  0 1