| We present the second epoch images of the galactic plane ( |b| < 5 degree, l = -15 degree to 255 degree) at 8.35 and 14.35 GHz. These observations used the NRAO/NASA Green Bank Earth Station to survey the sky simultaneously at these frequencies. These are the second results from the Galactic Plane (GP) survey observations, a program to monitor the sky at 8.35 and 14.35 GHz. The GP survey series is intended to detect short lived radio sources. We present an additional four independent observations of the galactic plane, combined to provide a set of reference images of the galactic plane. |
Galactic plane surveys (color) overlayed on the optical (black and white) image of the full sky. |
Observing plan
The GP transient survey is intended to find short lived transients by making rapid observations of the galactic plane. A complete epoch of observation takes less than 2 days, if the weather co-operates. The GPB images are each produced from four independent observations. The GPB median images are reference images without transients due to astrnomical sources, weather or hardware problems. The GPB median images are produced by independently processing the four epoch of observation independently, then median filtering the image data. This process is shown graphically in the below. |
 |
| Example of 4 epochs of model scans of the sky, containing three types of sources, 1) static source (left), 2) linearly decreasing source (middle) and 3) transient source (right). The scans are offset for calarity. | Example of processing of the four scans to produce 1) a median of the data (top), 2) average of the data (middle), 3) deviation scan (bottom), which is the average minus the median. |
The transients are detected in the deviation images, which are described below.
Galactic Emission
The transient radio sources are expected to be compact, so that GP survey data reduction was carried out in a manner to remove extended galactic emission, leaving only structures smaller than 45 arc-minutes. The removal of extended emission is achieved in several steps. The first is fitting and removing a model of the system temperature and sky brightness using a Tsys + Tsky*sec(elevation) model. The model is fit using only the 1.4 degrees at both ends of the 10 degree scans in galactic latitude. This process preserves the flux density of sources smaller than 7.2 degrees, if they lie on the galactic plane.The next step of GPB processing is removal of a baseline model produced by median filtering all data in a window of angular window of 90 arc-minutes, along the scans of galatic latitude. There is no median filtering in the galatic longitude direction.
The GPB image is produced from four separate independent images of the galactic plane. The GPB image pixels are computed on a pixel by pixle basis, by the median pixel of the 4 independent images. (In the case of 4 values, the median is defined as the average of the two middle valued pixels. The lowest and highest valued pixels are discarded.)
However, the 8.35 and 14.35 GHz (radio wavelength) emission of the galaxy is produced by an extended halo of emission combined with embedded compact radio sources. In the figures below, the galatic plane is shown in the range -15 to 75 degrees longitude, between -5 and 5 degrees latitude.
| Below is the 8.35 GHz survey image after subtraction of the sky model fit to the data. For good weather sections, the sky model fit is good. For bad weather regions, the sky model is poor. No median filtering in galactic latidute is applied, but the four epochs are combined. The FITS format image is available. |
|
|
|
| Above is the 14.35 GHz survey image after subtraction of the sky model fit to the data. For good weather sections, the sky model fit is good. For bad weather regions, the sky model is poor. No median filtering in galactic latidute is applied, but the four epochs are combined. The FITS format image is available. |
Deviation Images
| Below is the 8.35 GHz image produced by averaging the two outlier pixels of the 4 pixels of the 4 epochs of the GPB survey. The GPB 8.35 GHz median image (above) is subtracted, to show the outlier pixels, which are the top transient candidates. For good weather sections, the sky model fit is good. For bad weather regions, the sky model is poor. No median filtering in galactic latidute is applied, but the four epochs are combined. The FITS format image is available. |
|
|
|
|
| Below is the 14.35 GHz image produced by averaging the two outlier pixels of the 4 pixels of the 4 epochs of the GPB survey. The GPB 14.35 GHz median image (above) is subtracted, to show the outlier pixels, which are the top transient candidates. For good weather sections, the sky model fit is good. For bad weather regions, the sky model is poor. No median filtering in galactic latidute is applied, but the four epochs are combined. The FITS format image is available. |
| 14.35 GHz | 8.35 GHz |
| GPBA-KCAL-1.FIT | GPBA-XCAL-1.FIT |
| GPBB-KCAL-1.FIT | GPBB-XCAL-1.FIT |
| GPBC-KCAL-1.FIT | GPBC-XCAL-1.FIT |
| GPBD-KCAL-1.FIT | GPBD-XCAL-1.FIT |
Extended Emission Near 15degrees longitude
| Zoom in on the region centered of 15 degrees galatic longitude. Below are the 8.35 GHz (left) and 14.35 GHz (right) survey images after subtraction of the sky model fit to the data. |
|
|
| Zoom in on the region centered of 15 degrees galatic longitude. Above are the 8.35 GHz deviation image (left) and 14.35 GHz deviation image (right), after subtraction of the sky model fit to the data. The FITS format files are found in the links above. |
Transient Source Candidates
Transient source candidates were selected by two means, 1) comparision of the source lists produced independently from the GPA and GPB surveys and 2) subtraction of the GPA survey image from the GPB image and identification of differences in the two surveys.| Note | |
 |
The image at left shows the GPB image of the Galactic center at
8.35 GHz. The X axis is galactic longitude and the Y axis is
galatic latitude. The intensity range is -10 Jy (white) to 10 Jy (black).
Circles show the locations of source found in the
GPB survey not found in the GPA survey.
Contours are shown in order to allow comparison with the GPB-GPB image below.
There were a total of ?? transient and variable source candidates identified from the sample of sources in the two surveys brighter than 4 Jy. The majority of these candidates are found to be explainable as different minor components of much larger and brighter radio sources. |
 |
The image at left shows difference of the GPB and GPB images of the
Galactic center at 8.35 GHz.
The X axis is galactic longitude and the Y axis is
galatic latitude. The intensity range is -1 Jy (white) to 10 Jy (black).
The region within 0.5 degrees shows high noise level due to the complexity of the very high brightness region at the galactic center.
Tucker's Transient, located at |
| GPB Obs. Date | Spx (Jy) | RMS1 | RMS2 | RMS3 | Sp 14.35 (Jy) | RMS1 | RMS2 | RMS3 | |
| **** | 16.1 | 1.5 | 3.6 | 0.5 | 21.7+/-0.4 | 2.2 | 5.0 | 0.9 | |
| 98Sep24 | 28.8+/-0.3 | 2.9 | 4.4 | 1.4 | |||||
| 98Sep26 | ***** | 5.4 | 2.2 | 1.8 | |||||
| 98Oct03 | 22.1+/-0.3 | 2.0 | 4.2 | 0.9 | |||||
| 98Oct12 | 23.2+/-0.3 | 2.3 | 4.7 | 0.7 |
Conclusions
|
We see only one true transient between the surveys at a lower flux limit of
4 Jy.
Looking at the individual sections of the survey,
the transient has a flat spectrum
with a peak intensity of ~22 Jy.
The Galactic Plane Survey seems to be limited
more by instrumentation than by processing.
Problems with the pointing models for the 45'
NASA/NRAO GBES result in pointing offsets that are as large as
half a beam width and are
sometimes difficult to correct in the image processing routine.
Calibration was easily
corrected, and therefore not a major contribution to error.
To report transients at a
flux limit lower than 4 Jy,
the pointing offsets must be greatly reduced. Image alignment
with the GPB and GPA surveys, though, was successful around
the Galactic Center. We can
report this transient, refered to as Tucker's Transient,
at a high level of confidence.
The transient is about a 20 sigma event above an off source RMS
in the residual image
from the GPB-GPA subtraction. The off source RMS, though,
was measured in the Sgr A complex.
Unfortunately, the whole survey cannot be report to such a high level of confidence. The pointing offsets make it difficult to subtract the two epochs of the survey around bright and/or extended sources, the Cygnus Region for example. The pointing offsets must be corrected before another epoch of the survey is made. A third epoch should be made to monitor and confirm Tucker's Transient and to continue the search for transient sources in the Galactic Plane. Intraday Transients The first search for intraday transients in the Galactic Plane "B" Survey was primarly based on looking at the deviation maps from both the X and Ku Band, and recording the coordinates of sources that were in both bands. The intraday transients are rather bright sources (>75 Jy) after scaling. No lower flux limits were established while searching for the transients. |