We continue the description of the experiments realized with the radiotelescope SPIDER230 illustrating the recording of the transit of Cassiopeia A, a supernova remnant among the most interesting objects for radio astronomy, the brightest extrasolar radio source of the sky in the microwave band.
In the Visible, Cassiopeia A is very weak, since its radiation is absorbed by interstellar dust in the plane of the Milky Way. This radio source was identified in 1947 (one of the first recorded by a radio telescope) and its optical counterpart was discovered in 1950. It is thought that the supernova that gave rise to Cassiopeia A exploded 11,000 years ago and that the light of the explosion reached Earth about 300 years ago. There is no news of a sighting of this supernova, but it is possible that the star of the sixth magnitude 3 Cassiopeiae, John Flamsteed cataloged by August 16, 1680, was just Cassiopeia A.
Evocative images of Cassiopeia A in the visible (left) and in the radio band (right). The image was taken in visible light by the Hubble Space Telescope (NASA, ESA and the Hubble Heritage STScI/AURA-ESA/Hubble Collaboration. Acknowledgement: Robert A. Fesen-Dartmouth College, USA and James Long-ESA/Hubble), while the radio image was taken from the VLA radio telescope (Image courtesy of NRAO/AUI).
Cassiopeia A is one of the "sample radio sources" often used by radio astronomers to calibrate instruments and to determine the diagram of the antenna. In fact, the procedure provides for the registration of the transit of a radio source with apparent diameter much smaller than the width of the main lobe of the antenna. This item is ideal to verify the performance of SPIDER230 and derive the amplitude of the lobe receiving our tool: in addition to being the brightest source sample is characterized by a straight line of the spectrum (in bi-logarithmic scale) over almost the entire the radio band, with a secular decrease in the flux density of the order of 1.1%/year. To get the value of the flux density in the CasA band from 20 MHz to 30 GHz using the expression:
where the value of the constant A is obtained taking into account that S(1 GHz) = 2723 Jy with spectral index n=-0.77 (period 1986). The following figure shows the calculations that were performed to obtain the spectrum of Cassiopeia A and the corresponding emission intensity at a frequency of 11.2 GHz, which is worth about 423 Jy (it is considered the secular variation of the flow).
Calculation of the spectrum of Cassiopeia A (flow pattern as a function of frequency) in the radio band from 20 MHz to 30 GHz (data source: "The Absolute Spectrum of Cas A: An Accurate Flux Density Scale and a Set of Secondary Calibrators" JWMM Baars, R. Genzel, IIK Pauliny-Toth, A. Witzel-Astron. Astrophys. 61.99-106 (1977)).
Spectrum of the most intense "radio sources sample" used by radio astronomers to calibrate their instruments. The small angular size of these objects (usually no more than 4 minutes of arc) and the relatively intense flow make them very useful as sources of evidence in order to verify the performance of radio telescopes and antennas.
Using these data, we simulated the CasA transit registered with the SPIDER230 radiotelescope. The estimates are theoretical and consider an ideal behavior of the receiving system. The brightness temperature of the radio source, of the order of 210 K, produces an increase in the temperature of the antenna equal to about 0.68 K, very "diluted" because of the difference between the apparent size and the width of the lobe of receiving radio telescope.
The calculations show that the power variation "view" at the RAL10PL receiver input is of the order of 0.1 dBm -52.7 dBm with a value corresponding to the point of maximum intensity during transit. Registration simulated also shows the drift of the radiometric base line to facilitate comparison with the experimental data shown subsequently.
Recording simulated transit of radio source Cassiopeia A (3C461) in front of the antenna of the SPIDER230 radio telescope.
The verification of the theoretical evaluations was performed by Filippo Bradaschia, CEO of PrimaLuceLab, RadioAstroLab business partners in the implementation of the radio telescope SPIDER230 that we thank for cooperation. The receiving station installed at the Polo Tecnologico di Pordenone includes the RAL10PL 11.2 GHz receiver Total-Power made by RadioAstroLab for SPIDER230 and the antenna system (circular parabolic reflector 2.3 meters in diameter) with equatorial mount, motor and dome protection made by PrimaluceLab. The instrument is fully controllable through an Ethernet line, by software RadioUniverse.
Taking advantage of a day of " dry air " were organized observations to record the transit of Cassiopeia A. By using an equatorial mount , the antenna of the radio telescope SPIDER230 is very precise but, being small apparent size of the radio source , find the object can be difficult. To facilitate this task has been exploited SPIDER230 an interesting feature: the ability to shoot radio-images of a specific area of the sky using the RadioUniverse database software.
By orienting the antenna of the radio telescope toward the region of the sky where there is Cassiopeia A there were two consecutive images at low resolution, with a size of 10 x 10 pixels resolution radio and 0.5° in order to frame an area of sky wide 5 x 5° within which one could be reasonably sure of finding the radio source. The amplification factor of the receiver RAL10PL was set to the maximum value.
The SPIDER230 radio telescope used to record the transit of Cassiopeia A.
The two images obtained show the gradient of the signal caused by the contrast between the background of the sky (pixels of blue color) and the flux emitted from the ground and from the atmosphere near the horizon (pixels of red color). The angle at the lower right represents the closest point on the horizon. The presence of the CasA seems to be confirmed by the weak increase in brightness of the pixels shown in the following with respect to the boundary.
Images radio consecutive recorded by SPIDER230 in the area of sky of Cassiopeia A.
These measures have served to align the region in the radio map where you assumed the presence of the radio source, so as to require the software RadioUniverse automatically record 5 consecutive transits of the CasA, large 8 degrees. In all the recordings it was observed a signal peak, with sufficient contrast, exactly at the point occupied by the radio source. You are then exported the data for further processing with a spreadsheet.
Check the transit of Cassiopeia A with the software RadioUniverse.
Transits of the radio source Cassiopeia A.
A simplified model of the diagram of the receive antenna of the SPIDER230 (parabolic reflector with circular symmetry and a diameter of 2.3 m) was approximated as a uniformly illuminated circular aperture and is given only the azimuthal variation.
As previously noted, Cassiopeia A (object "almost punctiform") is often used as radiosource sample to verify the characteristics of the diagram of the receiving radio telescope, characterized by a lobe receiving much broader. In particular interesting obtain the HPBW parameter (Half Power Beam Width) which represents the amplitude at half power of the main lobe of the antenna (expressed in degrees). It uses the following formula:
in agreement with the value HPBW=0.8° obtained from the model of the antenna of SPIDER230 used in the simulations.
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