Cassiopeia A, one of the brightest radio sources in the Universe, it’s a supernova remnant that is possible to study with Radio2Space SPIDER radio telescopes. As you can see in the graph below, Cassiopea A radio emission at 1420 MHz is second only to the Sun and it has a flux of 2400 Jansky at 1420 MHz: this way it’s possible to detect and study Cassiopea A also with the compact antennas of the SPIDER radio telescopes, thanks to the high sensitivity of the H142-One receiver and the advanced features of the RadioUniversePRO software. In this article I will write how we detected Cassiopea A and how I used the SPIDER 300A advanced radio telescope to detect Cassiopea A: starting with antenna alignment, we detected interferences and performed RFI mitigation, then we recorded Cassiopea A transists, spectra and radio maps.
Cassiopea A is a very interesting object to study since, in visible frequencies, it’s extremely weak since it’s hidden behind the interstellar dust of the Milky Way plane which absorbs the visible radiation. That’s why Cassiopeia A (also called Cas A) was identified in radio wavelengths only in 1947 (Cassiopea A was one of the first radio sources to be identified) and the optical counterpart was identified in 1950. Most probably Cassiopea A has been generated by a supernova event (exploded about 11,000 years ago and that reached the Earth about 300 years ago) from sixth magnitude star 3 Cassiopeiae, that John Flamsteed cataloged by August 16, 1680. Cassiopeia A .
When we want to record weak radio signals coming from Cassiopea A (this is valid for all the radio sources except for the Sun) we have to be sure that the radio telescope is correctly aimed to the right sky area: in fact we can’t directly see the object we want to study. In order to do so, we took advantage of the high pointing precision of the WP-100 mount of the SPIDER 300A radio telescope and we used the Offset Alignment feature of RadioUniversePRO to perform a precision alignment on the Sun. Since the Sun is the strongest radio source in the sky at 1420 MHz, it’s easy to use it and calibrate the pointing position of the SPIDER radio telescope.
The Source Visibilities tab of RadioUniversePRO allows you to check the visibility of many radio sources and we used this feature to verify that Cassiopeia A had a good elevation from the ground. In fact we recommend to study radio sources that are least at 30 degrees from the ground. Then we made a double-click on Cassiopea A row and SPIDER radio telescope automatically pointed at it. Before starting to record data, we used the BBC Tool feature of RadioUniversePRO to check for interferences in the 50 MHz bandwidth (centered at 1420 MHz) recorded by the SPIDER radio telescope. As you can see in the image below, the neutral Hydrogen line (1420 MHz) is clearly visible along with some artificial interferences. By using the BBC Tool feature of RadioUniversePRO we can easily remove the interferences and allow the SPIDER radio telescope to record valid data without artificial radio signals.
Having correctly pointed the radio telescope and removed interferences, we started recording many results on Cassiopea A. First of all we started capturing a Cross-Scan: this technique involves recording a transit in both Elevation and Azimuth, centered on the object. This way we get a graph of the intensity of the radio emission along a cross centered on the object and that allows to determine the maximum radio emission of Cassiopeia A. In order to perform this operation, we select the “TPI Plot” tab in RadioUniversePRO and we use the Cross-Scan feature. Here you can set the length of the scan, the separation of each record point and the integration time of each record point. The SPIDER radio telescope mount moves the antenna and the software creates a graph like the one you can see in the image below.
RadioUniversePRO saves the data in various formats that can then be processed with different softwares. In this case we used a simple editor to generate a graph that shows both transits 15 degrees long with the variation of the radiometric datum recorded both in elevation and azimuth. We clearly noticed the increase in the recorded radio value caused by Cassiopea A. In this way we also verified that the SPIDER radio telescope mount is perfectly aligned on the radio sources in the sky and that Cassiopea A was perfectly framed.
Then, in order to highlight the 1420 MHz neutral Hydrogen emission line, we recorded the Cassiopea A spectrum. We used the calibrated spectrum On-Off feature of RadioUniversePRO: the software automatically records data from the radio source (“on” position) and then calibrates with a point in the sky away from the radio source (“off” position): the result is a calibrated spectrum as you can see in the image below where the emission of the hydrogen line at 1420 MHz is clearly visible.
One of the most advanced features of RadioUniversePRO is the Mapping one and we used it to record different radio maps of Cassiopea A, by setting a capture area of 15×15 degrees, with 30 seconds of integration for each point and a separation between the points of 1 degree. By recording different maps we checked the consistency of data: as you can see in the image below, all of them show an increase in the signal right close to the map center, just where we expected to find Cassiopea A.
By saving recorded data in FITS format (just like the professional radio telescopes) we extracted the data that can be processed with different softwares. Then we used the NASA FITS Viewer software to process one of the radio maps, better highlighting Cassiopea A signal from the sky background. The radio map captured by the SPIDER was then compared to an optical image, as you can see in the image below. It is easy to see how, in an area apparently empty of particular objects, the SPIDER radio telescope instead records an important object, just the supernova remnant known as Cassiopea A in the nomenclature used for radio sources.
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