help

How to Calculate the Flux of a Supernova Remnant

-- | August 29, 2014

Question: Good day Sir. How is work?
Please I kinda need help. I have a question disturbing my mind.
This is question.
There is a radio source with the following attributes:

Object name: 3C461

RA (hh mm ss): 23 23 24
Dec (dd mm ss): 58 48 54
Epoch: 2000
Flux Density (Jy): 2477
Frequency(MHz): 1420
Other name : SNR-Cassiopeia A.

My understanding from this is that, when this radio source is observed at 1420MHz, the flux density will be 2477Jy.

My question now is this; if i decide to change my frequency of observation to say 4GHz, will flux density still be 2477Jy?
And if it changes, how am i suppose to calculate the flux density at this new frequency? — Peter

Answer: Cas A is a supernova remnant and thus its radio flux is due primarily to relativistic synchrotron emission. This kind of emission has a spectral index of about -0.5 to -0.75, meaning that if you know the flux at one radio frequency (nu_1) you can calculate the flux at a second radio frequency (nu_2) as follows: Flux(nu_1)/Flux(nu_2) = (nu_1/nu_2)^alpha, where alpha is the spectral index. The spectral index from Cas A is measured to be about 0.77 (the reason why it is so large, by the way, is kind of complicated, and not completely understood). Therefore, Flux(4 GHz)/Flux(1 GHz) = (4/1)^0.77 = 0.34. So the flux of Cas A at 4 GHz should be about 1/3 that at 1 GHz.

Jeff Mangum

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