So some poking about on websites has yielded some results... First, getting information out of NASA is definitely a “wheat from the chaff” kinda operation. NASA documentation is a bewildering blizzard of information presented in a format that probably made sense to the instrument designers, but is not end-user friendly. Eventually I managed to random-walk my way to an exposure-time calculator.
And this is where I typically meet my second challenge. The late-time spectra of supernovae are weird. They are dominated by line emission, but the lines are actually quite wide, something like about 1/10th of the spectral width of broad band photometric filters in the NIR. (Roughly 600 Angstroms or so for a line near 1.64 microns for the gear-heads out there). Now for several years I’ve been estimating the strength of the late-time emission lines in SNe Ia by extrapolating Peter Meikle’s absolute IR light curves from 100 days post max to the epoch of observation at around 1 year. For the extrapolation I assume that the late-time light curve fades at a rate consistent with the 77 day half-life of Cobalt-56, which is the dominant energy source in the ejecta. This gives me a rough H-band magnitude for the late-time epoch, and has been OK at predicting the strength of the emission, though I haven’t rigorously tested how accurate this is. (Probably not that accurate, I’d guess, but I don’t really have any other method).
Since this method gives me a broad-band magnitude, I then have to convert this to a line flux. Now flux points for broadband magnitudes are actually tabulated as flux densities (ie flux per unit wavelength rather than the actual integrated flux in the band pass). Thus I first have to convert the flux point to a total integrated band flux. If the flux calculation requires a total line flux, then I’m home and dry, but if it wants a flux density, then I have to divide the integrated flux by the width of the line before trying to go on with the calculation. I’ve run into both situations for different instruments, but for the NICMOS calculator it’s just integrated line flux, which is easier.
Then its a matter of choosing settings, clicking on radio buttons and filling in web-forms and after a bit of tweaking it appears that a 900 second exposure would detect my emission line at a S/N of about 1 for a supernova with a broad-band H-magnitude of about 23. That’s pretty good actually. It means I can probably go about 2-3 magnitudes fainter than my ground-based observations and still have some hope of getting enough of a detection to measure a Doppler shift. Hooray.
Now the next question: are there enough targets to make this an interesting program to run in the spring?
And this is where I typically meet my second challenge. The late-time spectra of supernovae are weird. They are dominated by line emission, but the lines are actually quite wide, something like about 1/10th of the spectral width of broad band photometric filters in the NIR. (Roughly 600 Angstroms or so for a line near 1.64 microns for the gear-heads out there). Now for several years I’ve been estimating the strength of the late-time emission lines in SNe Ia by extrapolating Peter Meikle’s absolute IR light curves from 100 days post max to the epoch of observation at around 1 year. For the extrapolation I assume that the late-time light curve fades at a rate consistent with the 77 day half-life of Cobalt-56, which is the dominant energy source in the ejecta. This gives me a rough H-band magnitude for the late-time epoch, and has been OK at predicting the strength of the emission, though I haven’t rigorously tested how accurate this is. (Probably not that accurate, I’d guess, but I don’t really have any other method).
Since this method gives me a broad-band magnitude, I then have to convert this to a line flux. Now flux points for broadband magnitudes are actually tabulated as flux densities (ie flux per unit wavelength rather than the actual integrated flux in the band pass). Thus I first have to convert the flux point to a total integrated band flux. If the flux calculation requires a total line flux, then I’m home and dry, but if it wants a flux density, then I have to divide the integrated flux by the width of the line before trying to go on with the calculation. I’ve run into both situations for different instruments, but for the NICMOS calculator it’s just integrated line flux, which is easier.
Then its a matter of choosing settings, clicking on radio buttons and filling in web-forms and after a bit of tweaking it appears that a 900 second exposure would detect my emission line at a S/N of about 1 for a supernova with a broad-band H-magnitude of about 23. That’s pretty good actually. It means I can probably go about 2-3 magnitudes fainter than my ground-based observations and still have some hope of getting enough of a detection to measure a Doppler shift. Hooray.
Now the next question: are there enough targets to make this an interesting program to run in the spring?
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