Light curve

SN 2002cx hit maximum light in the B-band at 2452415.2 JD (2002 May 20.7) at 17.68, and in the V-band on 2452417.5 JD (2002 May 23) at 17.57.^[12] The B-band light curve of SN 2002cx before 15 days after maximum evolves in a similar manner to SN 1999ac, brightening faster than SN 1991T but slower than SN 1994D or SN 2000cx.^[9] SN 2002cx declines in brightness faster than SN 1991T and SN 2000cx in the B-band.^[9] In the V-band SN 2002cx is similar to SN 1999ac until 30 days after max.^[9] Again SN 2002cx declines faster in the V-band than SN 1991T, but slower than is typical for a type Ia.^[9]

SN 2002cx is peculiar in the R-band, as it brightens very fast in a manner wholly different from SN 1999ac.^[13] It has no secondary maximum in R-band as expected if it were similar to SN 1991T, but instead has a plateau after max.^[13] The R-band also declines more slowly than normal.^[13] The I-band behaves similarly to the R-band, with a quick brightening, a plateau and slow decline.^[13] While a plateau in the I-band is expected for sub-luminous supernovae, the following slow decline is not.^[13]

Spectra

The first spectrum of SN 2002cx was obtained with FLWO on 2002 May 17, 4 days before B-band max.^[14] At this point SN 2002cx is similar to SN 1997br as both have a blue continuum, with absorption lines from Fe III λ4404 and Fe III λ5129.^[14] Si II λ6355 though is not apparent in SN 2002cx at this point, and it has very weak Ca II H & K lines suggesting that SN 2002cx is similar to SN 1991T which also lacked such lines.^[14] This spectrum for SN 2002cx has a low expansion velocity measuring only ~6400 km s⁻¹.^[14] This marked one way in which it was different from SN 1997br, as SN 1997br's expansion velocity was ~10,400 km s⁻¹ at the same point relative to its own B-band maximum.^[14] At the time of measurement SN 2002cx's expansion velocity was the lowest measured for an early time type Ia,^[14] Another spectrum taken on 2002 May 20, 1 day before B-band maximum light, showed little evolution from the one taken on 2002 May 17.^[15]

Four later spectra were taken on June 2, June 6, June 12, and June 16 at FLWO.^[16] The spectrum of SN 2002cx has evolved by this point to have a redder continuum.^[16] It has also lost the Fe III absorption lines and its Fe II lines have gained prominence at λ4555 and λ5129.^[16] The Ca II H & K lines continue to remain weak, a further departure from SN 1997br.^[16] Unlike another type of sub-luminous type Ia, SN 1991bg, SN 2002cx does not show Ti II lines around 4100–4400 Å.^[16]

Spectra were taken at Keck corresponding to 20, 25, and 26 days after maximum light in the B-band.^[17] So little evolution was observed between these spectra that they were all combined in order to increase the signal-to-noise ratio and study less pronounced features.^[17] Fe II still dominates the spectrum.^[17] The Ca II infrared triplet is weak in SN 2002cx as compared to other type Ia supernovae, as expected since the Ca II H & K line is also weak in SN 2002cx.^[17] Unlike other type Ia supernova, SN 2002cx has double-peaked emission lines which may be due to jet-like emissions or rotating ejecta, or may simply only be seen in SN 2002cx because its low expansion velocity does not wash them out.^[17] The emission or ejecta hypothesis is considered less likely because if it were the case all the peaks should share equal separation, which they do not.^[17][18] SN 2002cx has absorption and emission lines between 6400 Å and 7000 Å that are unique among previously discovered type Ia supernovae.^[18]

A final spectrum of SN 2002cx was taken on July 16, 56 days after B-band maximum.^[19] SN 2002cx was by then in the nebular phase, with emission lines dominating over absorption lines.^[19] The lines were far narrower than previously observed type Ia supernovae and are less pronounced as well.^[19] SN 2002cx most clearly differs from other type Ia supernovae in the region between 6500 Å and 8500 Å where it has a primarily flat continuum and weak Ca II infrared triplet absorption.^[19] Because this is the region covered by the R and I-bands, it may explain the odd color evolution of SN 2002cx in these bands.^[19]

SN 2002cx did not evolve much between 4 days and 1 day before max, nor did it evolve much from 12 days after max to 27 days after max,^[16] However, it underwent dramatic evolution during the two weeks after maximum light in the B-band.^[16]

Based on the odd behavior of SN 2002cx's spectrum there are some questions as to whether it is a type Ia supernova or not.^[20] Although it does not show Si II lines neat 6150 Å as is required of a type Ia, SN 2002cx's evolution is explainable using the paradigm of other type Ia observations and so Li et al. consider their classification as a type Ia as secure.^[20]

Color

SN 2002cx does not suffer from much host related reddening, as evidenced by its very blue spectra.^[4] It is extinct in B−V color by 0.034 magnitudes from dust in Milky Way.^[4]

SN 2002cx is similar to SN 1999ac in B−V at max, but also at times as late as 50 days after max which is unexpected as the two supernovae evolve differently at late times.^[4] SN 2002cx has a B−V color of −0.04±0.04 at 4 days before max, and 0.04±0.05 at the time of max in the B-band which is bluer than SN 1991bg at the same time.^[4] The color of SN 2002cx is consistent with Lira-Phillips law at late times.^[21]

The V−R color of SN 2002cx evolves similar to other type Ia supernovae before 5 days after max, although it is somewhat redder.^[21] After 5 days after max SN 2002cx gets progressively redder, although still slightly bluer than SN 1991bg.^[4] At 25 days after max SN 2002cx continues to redden, while SN 1991bg starts to get bluer.^[4]

SN 2002cx's V−I color is red for all times, only slightly bluer than SN 1991bg before 25 days after max, and redder after 25 days after max.^[4]