Sunday, January 17, 2016

Direction isn't important IF the assumption of Hubble's Law Holds

Charlie's Answer
Charlie Kilpatrick
Charlie KilpatrickI've authored refereed publications on supernovae and supernova remnants
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A supernova is a transient astronomical object whose intrinsic brightness is several (~seven to nine) magnitudes brighter than a classical nova.
Where supernovae lie in luminosity-timescale space.
That's it.  The definition is purely in terms of observational characteristics with no physical interpretation overlaid to confuse the issue. 
This point is one that's underappreciated by the public and especially astrophysicists.  You should never, ever mix classification with physical mechanisms.  The former always informs the latter.  If you start working physical mechanisms into your definitions (e.g., a supernova is the explosion of a high-mass star), then you'll start biasing the interpretation of your observations and have a much harder time dealing with new trends in your data.
In addition to the above definition of supernovae, several other classifications have been developed, such as "Type I" (supernovae without hydrogen in their spectra) and "Type II" (supernovae with hydrogen in their spectra).  Only through continued observation of trends in the lightcurves (variation in brightness with time) and spectra of supernovae were astrophysicists able to predict that a particular subclass of supernovae come from white dwarfs (so-called "Type Ia supernovae") while the remainder come from high-mass stars. 
Jonathan Doolin
Do they have a global standard for reporting information on supernova data?
I have seen reports of "redshift" and "magnitude" but to measure a redshift, one must first be sure that they have properly identified the colors that have been redshift.  That's fine.
But then I see reports of "magnitude" being listed through locally defined colors... Red, Green, Blue filters that exist on the telescope itself.
Astronomers are very smart, so they surely take into account that if the colors have shifted, then surely whatever they measure as magnitude should shift, too. 
However, looking through the IAUC (International Astronomical Union Circulars) data on supernova, many report different magnitudes based on filters, but when I read papers supporting the lambda Cold Dark Matter universe based on Type I supernova data, they do not get into the nitty gritty of how the actual magnitude of the supernova is calculated.  In fact, if I recall, correctly, these papers often don't report the magnitude at all; but rather simply report the redshift and distances to the supernova. (And they tend to ignore right-ascension and declination)
To assign the supernova a "distance" is a physical interpretation of the observational characteristics... The observational characteristics are just the Right ascension, Declination, and the spectral analysis. 
Do amateur astronomers sending in International Astronomical Union Circulars have sufficient equipment and training to provide an accurate measurement of magnitude, and be sure they are all measuring and communicating the same quantity?
Or does the information about magnitude through two or three filters give all that is needed to reconstruct the spectral analysis?
Also, when they make a measure of redshift, how confident are they that they have found, for instance, the hydrogen alpha line?  Many IAUC cirucluars report precisely what line they are using to find the redshift, but many just say z=2.1 or whatever, without giving any indication of how it was calculated.
Charlie Kilpatrick
There's no formal standard, but it's generally expected that you should report the magnitude of the observation for photometry and the redshift, magnitude, and a guess at the spectral type and epoch (i.e., how many days relative to the supernova reaching maximum brightness) for spectroscopy.
Redshifts are exact.  They're based on comparison to known spectroscopic lines in supernova spectra - usually hydrogen or helium features, although silicon features are used for Type Ia supernovae (which lack hydrogen and helium).
Magnitudes are measured in a specific filter (V, R, B are commonly used filters for visible, red, and blue) or sometimes in unfiltered light for faint objects.  These filters have been very precisely engineered and measured so we know exactly how much light at each wavelength can pass through.  There are also hundreds of standard stars measured in each filter for comparison, which sets a magnitude scale.  Whenever you see a supernova with a magnitude measurement, that's based on comparison to a bright, photometric standard star.
Astronomers who study Type Ia supernovae for cosmology don't much care about the intrinsic brightness of their supernovae.  They only want an object with a distance and redshift they can measure simultaneously so they can put it on the Hubble diagram.  If you're reading papers about cosmology, you'll also need to read the papers cited within to get a better understanding on how the actual supernova photometry is performed.  Also, Hubble's law is assumed to be isotropic based on the Cosmological principle , so it doesn't really matter where in the sky a supernova is located (as in RA and dec) - as long as it has a distance and redshift you can measure then you should be able to model Hubble's law.
Assigning Type Ia supernova distances is also based on an assumption.  The peak, bolometric, intrinsic magnitude of Type Ia supernovae has been observed to be roughly the same to within a couple tenths of a magnitude.  Therefore, if you can measure the relative magnitude (at peak) of a Type Ia supernova and assume it's similar to all other Type Ia supernovae you've observed (ymmv), you'll have a rough idea of the distance to that supernova.
I've never been on an IAU Circular (I generally use Astronomer's Telegrams, for example: , Spectroscopic Classification of ASASSN-15rw as a Type Ia ).  As I understand it from reading their guidelines on submissions (How to Submit Scientific Items for Publication in the IAUCs), they prefer submissions with CCD-quality data.  Most amateurs don't own that kind of equipment as it can run into the tens or hundreds of thousands of dollars.  But there aren't any rules or prejudice against amateur submissions.
It would be very difficult to reconstruct spectral analysis based on photometry, especially in terms of resolving lines such as H-alpha.  IAUCs that report spectral lines to two or more degrees of precision are fitting a spectral line profile to the error bars in their data in order to measure the uncertainty in their redshift measurement.  It's a form of non-linear error propagation.  Usually, specific codes are used to perform the spectral fitting, such as SuperNova IDentification (SNID) by Blondin and Tonry.  That code has extensive documentation on how redshifts, magnitudes, types, etc. are evaluated:

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