North Pole Transient Search

Julie Zuber, Oberlin College

Glen Langston, NRAO


During 1996 May, June and July, we monitored the sky north of 62 degrees declination at 8.35 GHz, searching for transient radio sources. After extensive editing to reject data taken during bad weather, a total of 119 hours of observations were obtained. During these times all sources brighter than 11.4 Jy were detected (10 sigma limit). A total of 3 known radio sources are found in this region, and 5 transient radio source candidates were detected. The nature of these sources is discussed.


The first radio source surveys have shown that the universe is evolving (Ryle 1968 and references therein). It is well known that although the radio sky is mostly constant, some radio sources vary on short time scales. It is observed that sources vary earlier and to greater degree at higher frequencies (Kellerman and Pauliny-Toth 1968, Clements et al. 1995). However, the duration of extragalactic radio sources variations observed is much longer and smaller in magnitude than is seen in higher energy events, such as gamma-ray bursts. Detection of bright but short lived, non-repeating, astronomical radio transient sources is limited by primarily by weather and by man-made interference. Reliable detection of transient events requires careful examination of the data to exclude a wide variety of man made effects.

We have started a series of observations intended to place limits of the number of short lived radio transients. The north polar region of the first survey is shown in the figure above. The SVA survey presented here is the first of a series of all northern sky surveys to detect sources variable on the time scale of weeks. The SVA survey is also the first complete survey of the northern sky at 8.35 GHz (X-Band), 14.35 GHz (Ku-Band) flux densities are given for sources detected at X-Band. This survey uses the NRAO/NASA/JPL Green Bank Earth Station, recently re-ferbished to track space radio telescopes, as a part of a orbiting VLBI (OVLBI) network. The tracking station in Green Bank is funded by NASA, to support the Japanese VSOP and Russian RadioAstron satellites to be launched in 1997/1998 (Hirabayashi, 1995, Burke, 1995).

Initially, it was planned that the SVA survey would report flux densities at both X and Ku bands simultaneously, however bad weather and hardware problems have made it difficult to obtain simultaneous Ku data for all times when X band data were obtained.


D'Addario etal. (1992), described the design of the the OVLBI Green Bank Earth Station (GBES). Here, the properties relevant to astronomical surveys are described. The GBES is required to support satellite links in both X- and Ku-band in support of the Radioastron and VSOP missions. The tracking station utilizes cryogenically cooled front ends with dual circularly polarized feed horns at both X- and Ku-bands. The antenna optics are arranged to allow simultaneous observations at both bands of a single location on the sky.

The optics geometry is sketched in the figure 2, below.

The primary reflector is a 13.7m parabola (M1), the secondary is a hyperbolic subreflector of approximately 2\,m diameter (M2). The X-band feed horn illuminates the subreflector directly; ideally all its energy passes through the tilted Frequency Selective Surface (FSS, labeled M3) which is placed in front of it. The Ku-band horn illuminates the subreflector via a double reflection off of an offset ellipsoid (M4) and the FSS.

The magnification (M) parameter of a Cassegrain antenna relates the focal length of the primary reflector to the equivalent focal length of the Cassegrain system. The relatively high value of M=5.70, in the optics system reduces spurious beam squint to a negligible level.

The radome sits above the vertex ring, and encloses the feeds, FSS, and ellipsoid. The radome is a sheet metal hexagon with an outer diameter of approximately 1 meter. The top of the radome is covered with a sheet of woven-Teflon based radome material. Due to the fact that there are four surfaces that could get wet, the ellipsoid, FSS, and the two horn windows, it was decided that a radome covering of the optics would provide many benefits. First, loss will be reduced in wet weather conditions, since there will be only one layer of water for the signal to pass through. The loss is not comparatively worse in dry conditions, since the radome material can be made thin and have extremely low-loss (< 0.01 dB). Also, Teflon-based radome laminates are naturally hydrophobic, so the effect of rain will be lessened. Finally, enclosing the vertex optics has the benefit of allowing environmental control, which reduces the effects of temperature on the FSS and ellipsoid.


The observations presented here were made during between 96 May 01 and 96 June 30, but were frequently interrupted due to weather and development work in preparation for the space VLBI mission. The survey observations are carried out with the telescope in ``transit mode'', with azimuth near 0 or 180 degrees from north. The sky is scanned in a ``raster mode'' common for single dish surveys (eg. Langston et al. 1990), taking data while moving the antenna up or down in elevation.

The declination range is scanned continuously moving the telescope in elevation at a constant rate of 28o per minute north and south. At the end of each 1 minute scan, 1.5 seconds are used for calibration of the system gain. The second declination range is scanned in 2 days, again taking data in 1 minute scans, while scanning moving the antenna in elevation at a rate of 26.8\degree\ per minute. The start local sidereal time of the scans two days are offset by 30 seconds, so that regions missed in one day are scanned in the next. The third declination range is observed in 7 days, with the antenna at 180 degrees azimuth. Each scan in the third region has a duration of 160 seconds, with calibration performed when the antenna is pointed near zenith. During all scans in all declination ranges, the antenna moves slightly in azimuth, to follow a constant right ascension for each scan. Data are recorded every 0.125 seconds for sky frequencies of 8.35 and 14.35 GHz for both left and right circular polarization. The intensities were measured. for 500 MHz bandwidths.


The data are written in ``RAW archive'' FITS format tables at the end of each scan. The raw data recorded are the azimuth, elevation, local sidereal time, and 4 channels of antenna temperature in Kelvins. The ``RAW archive'' table also records the offset between station and GPS time, and weather information. The data were written to FITS files in 1 hour segments. The raw archive data intensities were recorded in Kelvins of system temperature as a function of antenna elevation. The system temperature measurements were calibrated by injecting a calibration signal into the front ends, and synchronously detecting this signal at the detectors.

After a day of observations, the ``Raw archive'' data are calibrated and the angular coordinates are converted to Right Ascension and Declination (J2000). The calibration process also includes extensive data flagging, to remove the

(1)} sun and moon,

(2) jumps in the baseline data due to intermittent hardware problems,

(3) tests on the sky and system temperature values to reject bad weather and,

(4) rejection of entire scans if the RMS signal level exceeded tolerances.

The process of removing the sky and system temperature contributions to the measured signal is illustrated in figures 3, 4, 5. The raw X band LCP signal level for a scan containing radio source 3C58 is shown in figure 3. The sky and system temperature were removed from the input data by fitting a simple system temperature plus sky model to the raw data and subtracting that model from the data. The fit was made for each band and polarization separately. The result of subtracting the sky model is shown in figure 4. Next, for all data in each scan, the median was calculated for all data points with 30 arc minutes of the central point. This median was subtracted from the data. The result of this processing was to leave a nearly flat scan of the sky with occasional radio sources, as is shown in figure 5.

A number of attempts were made to automatically flag data. The entire scan was discarded if the RMS noise in the scan exceeded 0.01 Kelvins after this processing. If the sky + system temperature value in any data point in a scan exceeded 100 Kelvins, the data point was discarded.

Antenna pointing corrections are calculated offline. During the data calibration process, the data are converted into ``Single Dish FITS'' format so that further processing may be done using the Astronomical Image Process System (AIPS).

Each day of observation is gridded onto a image using the AIPS task SDGRD, which allows a variety of image projections and data convolution options. The SVA data were grided onto images with 3.4 square arcminute pixels, using a Gaussian convolving function with 7.46 arcminute full width at half maximum.

The data are visually examined for uniform quality; bad weather data are easily recognized by ``stripes'' of high noise along strips of declination. If any data were bad in a one hour segment, that segment was discarded.

After all one hour segments of the survey are completed, the final images were produced. These images were examined by eye in AIPS to select all sources with flux density per beam greater than 11.4 Jy. A Gaussian fit is made to each maxima in the images and the coordinates and peak flux densities are recorded. The flux density scale was set by observations of the bright radio source 3C286, with a Baars et al. flux density of 5.20 Jy at 8.35 GHz.

After the source list was completed, the sources were identified by comparison with an updated version of the Dixon (1970) source list and Becker et al. 1991 source list based on the Condon et al. 1989 survey of the sky at 6cm.

For all sources detected in this manner, the raw data scans were examined for baseline problems and other signs of hardware problems. Sources were kept only if they visible in both X and Ku bands and both Left and Right circular polarizations of these bands. Otherwise the data for the entire hour was discarded.


The SVA source list consists of 6 columns, (1) Source Name, (2) Right Ascension, (3) Declination (J2000), (4) Flux Density (Jy) at 8.35 GHz and (5) Notes.

If the source is previously detected in the 3CR, Westerhout (1958), or Dixon (1970) catalogs, that source name is listed. If the source is unidentified, the source is given the IAU style, name with the prefix SVA.

The sources are identified based on their location measured in the SVA survey being within 9 arc minutes of a published radio source location. For a few sources near the galactic plane, when the source appeared extended, the angular distance criteria was extended to 15 arc minutes.

      RA           DEC    Intensity   Name   Date/Type  Note
    (J2000)      (J2000)    (Jy)             (DDMMYY)   

00 00 40.605   67 24 58.70   28.1  W1         HII region
00 25 18.797   64 09 10.63   26.1  3C10       SNR      Remnant of 1572 
02 05 44.950   64 47 33.45   49.4  3C58       SNR      Remnant of 1181 
09 33 38.405   87 34 33.94   29.5 SV=155.3    280596   Transient
10 48 22.903   74 02 14.35   18.3 SV=96.5     240596   Transient
10 56 39.065   76 50 48.39   12.3 SV=64.8     310596   Transient
22 33 55.976   82 52 19.04   23.8 SV=125      040596   Transient
23 13  5.111   75 06 13.38   79.2 SV=417      110596   Transient

Transient Events

During the SVA survey observations, a large number of transient events were detected, but few events met all selection criteria. No events were selected unless the events were detected in both X and Ku band, and unless the events showed similar brightnesses in both polarizations of each band. This selection criteria should remove all known satellites and many types of hardware errors intrinsic to the tracking station electronics.

All events were rejected unless the data were taken during good weather, and all scan data taken immediately before and after the events were of good quality.

Initially it was planned that all transient events would be rejected unless the measured points of the event within the scan were well matched to the antenna beam shape, as would be expected for transient radio sources which varied slowly during the 1 second the antenna beam crossed the source. However, this criteria rejects very rapidly varying sources and sources seen in the near sidelobes of the antenna beam. Since the sidelobes cover a few times the angular area that primary beam covers, it is much more likely to detect a short lived transient source in a sidelobe than in the main beam.


The first large area survey of the sky at 8.35 GHz has been presented. The data from this survey is useful for placing limits on the number of short lived radio sources. Several intriguing transient radio sources were also detected in this survey. Further observations are needed to determine the nature of these sources. The requirement that the sources be detected in both polarizations and both bands strongly suggests the sources are external to the antenna and electronics. However a number of terestrial explanations are possible. These include birds, planes or satellites. Assuming that the 5 transients represent an upper limit to the number of transient radio soures, then transient rate for sources with total time duration of 1 day is N( S>11.4 jy, tau = 24 hours) < 1.34 /str/day.

This research was supported by the National Radio Astronomy Observatory operated by Associated Universities Incorporated under a cooperative agreement with the National Science Foundation. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA-Goddard Space Flight Center.


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