Radio Flux Density Monitoring and Imaging of Supernova 1987A
Question. How does a supernova evolve into a supernova remnant?
Background. Since its detection in 1987, Supernova 1987A has provided astronomers with a unique opportunity to study how material ejected during a supernova explosion interacts with the surrounding gas and dust. The ATCA has an ongoing programme to i) monitor the evolution of the spectral index of the radio emission and ii) image the radio emission from the newborn supernova remnant. In particular, astronomers are interested to learn how the radio spectral index of SN1987A will evolve from its initial value (-0.9) to the typical value for supernova remnants (-0.5). Images of the supernova remnant help astronomers to understand how the ejecta is interacting with the surrounding interstellar gas, and the physical processes that are responsible for the radio emission.
Experiment. In this experiment, you will measure the flux density of SN1987A at several different radio frequencies, and image the supernova remnant at high resolution.
1. Use SIMBAD to find the location of SN1987A.
2. Use the COORD tool at:
http://www.parkes.atnf.csiro.au/cgi-bin/utilities/coord.cgi
to calculate the LST range when SN1987A is observable from ATCA. Note that the ATCA dishes can be steered to a limiting elevation of 12.5°, but that the signal-to-noise of high-frequency observations deteriorates rapidly below elevations of about 35° due to moisture in the Earth’s atmosphere.
3. Refer to Figure 2 and Table 1 of Manchester et al (2002). Estimate the flux density of SN 1987A at 1.4, 2.4, 4.8 and 8.6 GHz during July 2010.
4. Refer to Figure 8 and Table 3 of Manchester et al (2002). Estimate the radius of the radio supernova remnant during July 2010. You can check your answer in Gaensler et al (2007).
5. Open the ATCA observing characteristics calculator at:
http://www.atnf.csiro.au/observers/docs/at_sens/
i) What is the size of ATCA’s field-of-view when observing at 1.4GHz?
ii) What is the resolution of ATCA at 1.4GHz using a 6km array? Is this sufficient to resolve the emission structure of SN1987A? Assuming a distance of 50kpc to the LMC, what is the linear scale of SN1987A’s radio emission structure?
iii) What is the continuum RMS noise level of ATCA at 1.4GHz using a 6km array and an integration time of 6 hours? Is this sufficient integration time for a 5-sigma detection of SN1987A? What is the minimum integration time you would need for a 5-sigma detection?
Repeat the above questions using different arrays, e.g. 1.5km, EW352/367, H75.
Repeat the above questions using different observing frequencies (2.4, 4.8 and 8.6 GHz) and different arrays.
What would happen to the shape of the ATCA beam if SN1987A was located at a i) lower declination (e.g. -75 degrees) or ii) a higher declination (e.g. -5 degrees)?
More information about the array configurations can be found here:
http://www.narrabri.atnf.csiro.au/observing/configs.html
You can also use the VRI tool to visualise the array configuration, integration time and the resulting (u,v) coverage:
http://www.narrabri.atnf.csiro.au/astronomy/vri.html
6. In your group, discuss the combination of array(s) and integration times that you should use to obtain reliable flux density measurements of SN1987A at 1.4, 2.4, 4.8 and 8.6 GHz.
7. In addition to sensitivity, there are other considerations for obtaining a “nice” resolved image of an astronomical object with an interferometer:
i) the object must be have a smaller angular size than the field-of-view
ii) the shape of the beam must be roughly circular
iii) the observations need good Fourier coverage i.e. sample many different parts of the (u,v) plane.
iv) the object must have a larger angular size than the beam
In your group, discuss the best choice of array, observing frequency and integration time for imaging SN1987A.
8. You should now have sufficient information to complete the cover sheet and the observations table of your proposal. Go to http://opal.atnf.csiro.au
9. Using the articles in the recommended reading list as a starting point, write a couple of paragraphs that explain the scientific motivation for your observations and why you have adopted your observing strategy.
10. Go to the ATNF online data archive:
and check if ATCA data already exists that you could have used for this project. (Note that this should have been the first step in preparing your observing proposal, not the last!)
Recommended Reading
You will need to refer to papers marked with an asterisk to complete this exercise. Other papers are for interest only and can be found using ADS (http://adsabs.harvard.edu/abstract_service.html) or the astro-ph preprint server (http://xxx.lanl.gov/).
*Manchester et al (2002) “Evolution of the Radio Remnant of SN1987A: 1990-2001”
*Gaensler et al (2007) “Fifteen Years of High-Resolution Radio Imaging of Supernova 1987A”
Manchester et al (2005) “Imaging of the Radio Remnant of SN 1987A at 12 mm Wavelength”
Woltjer, L (1972) “Supernova Remnants”