Calculations

The database stores unabsorbed flux measurements, observation dates, an approximate explosion date, and the distance of the supernova. Lightcurve data are calculated from these values, using some basic computations.

Ages

Because dates are stored in modified julian form, the age in days is a simple subtraction: age = date observed - date exploded . For dates spanning multiple days, the calculated date observed is instead the middle date, becoming age = (date observation starts + date observation ends)/2 - date exploded .

Luminosities

Only fluxes are ever stored in the database; this means all displayed luminosities are calculated using the stated distance. This comes from the standard equation, flux * 4 π distance2 = luminosity . Some unit conversion is necessary, since fluxes are stored in 10-13 erg cm-2 s-1, distances are in megaparsecs, and luminosities are given in 1039 erg s-1. This works out to luminosity = 4 * flux * π * (distance * 3.08567758)2 X 10-4.

Fluxes

In some cases papers only list luminosity measurements, instead of their measured unabsorbed fluxes. When this is the case we reverse the luminosity calculation as above, only being sure to use the distance mentioned in the reference paper. When the more recent accepted distance differs from the distance used in the cited paper, this can lead to listed luminosity differing from the luminosity given in the cited paper.

Energy Conversions

When possible, measurements have been converted into a .3 - 8 KeV band; these converted values are given in a separate column. How these data are calculated is indicated by a superscript number attached to the value: 0 indicates it was taken straight from the paper, 1 indicates it was automatically converted by SNaX using PIMMS, and 2 indicates the data was re-reduced from the original observational data.

When the data is automatically converted using PIMMS, it is done by using the model information associated with the original calculation. Commands are generated and executed in command-line pimms; currently we use PIMMS 4.8. For example, for a measurement of 6.3 x 10-13 ergs cm-2 s-1 in .1-2.4 coming from data fitted to a bremsstrahlung model that predicted a temperature of 9.1 KeV, and NH of 7.80 x 1020 cm-2 would generate:

When uncertainty is known in the flux measurements the estimated uncertainty after the energy conversion comes from running the calculation on the maximum and minimum values, and finding a differences. For example, for a flux measurement of 6.3 ± .3 x 10-13 ergs cm-2 s-1, the pimms conversion would also be run for 6.0 x 10-13 and 6.6 x 10-13. The results of these calculations are then compared to the conversion of 6.3 x 10-13..

Selecting values to use

The data given as general supernova reference information is primarily chosen to be in accordance with the most recent paper from which we use data. It is worth noting that, as above, this means that luminosities given for a certain flux may not be the same as the luminosities given in the paper that presents the flux measurements.

Explosion Date

In some cases the explosion date is not well known. The date listed, which is used to calculate ages for the lightcurve, is the date suggested for use by the most recent paper from which we cite flux measurements. We strongly recommend that you check the cited paper if you wish to use this date for your own calculations.

Fitting

The plotting tools include the ability to fit data to a power law. To do this, a linear least-squares regression is used. By taking the natural logarithm of each age and luminosity value, a linear fit can be used to find m and b such that ln(y) = m (ln x) + b. Raising e to the value of each side, this is the same as y = em ln(x) eb, or (with C = eb) y = C xm. Fitting assumes symmetric gaussian error; when error is assymetric SNaX calculates as though the lower bound is representative of a standard deviation.