SN 2002cx
Encyclopedia
SN 2002cx is a peculiar type Ia supernova
. It was discovered in May 2002 by a team of researchers from LBL
. It behaved differently than normal type Ia supernovae, and different from several other previously observed peculiar type Ia supernovae including SN 1991T and SN 1991bg.
at Palomar
. On 2002 May 17.2 a spectrum taken by T. Matheson, S. Jha, P. Challis, and R. Kirshner of the CfA with the 1.5-m telescope at FLWO
suggested it was a peculiar SN 1991T like type Ia. SN 2002cx had photometric follow up taken at Lick
using KAIT
and the Nickel telescope
, and further spectra were taken at FLWO and Keck.
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. The B-band light curve of SN 2002cx before 15 days after max evolves in a similar manner to SN 1999ac, brightening faster than SN 1991T but slower than SN 1994D
or SN 2000cx. SN 2002cx declines in brightness faster than SN 1991T and SN 2000cx in the B-band. In the V-band SN 2002cx is similar to SN 1999ac until 30 days after max. Again SN 2002cx declines faster in the V-band than SN 1991T, but slower than is typical for a type Ia.
SN 2002cx is peculiar in the R-band, as it brightens very fast in a manner wholly different from SN 1999ac. It has not secondary maximum in R-band as expected if it were similar to SN 1991T, but instead has a plateau after max. The R-band also declines more slowly than normal. The I-band behaves similarly to the R-band, with a quick brightening, a plateau and slow decline. While a plateau in I-band is expect for sub-luminous supernovae, the following slow decline is not.
The first spectrum of SN 2002cx was obtained with FLWO on 2002 May 17, 4 days before B-band max. 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. 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. This spectrum for SN 2002cx has a low expansion velocity measuring only ~6400 km s−1. 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. At the time of measurement SN 2002cx's expansion velocity was the lowest measured for an early time type Ia, 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.
Four later spectra were taken on June 2, June 6, June 12, and June 16 at FLWO. The spectrum of SN 2002cx has evolved by this point to have a redder continuum. It has also lost the Fe III absorption lines and its Fe II lines have gained prominence at λ4555 and λ5129. The Ca II H & K lines continue to remain weak, a further departure from SN 1997br. Unlike another type of sub-luminous type Ia, SN 1991bg, SN 2002cx does not show Ti II
lines around 4100-4400 Å
.
Spectra were taken at Keck corresponding to 20, 25, and 26 days after maximum light in the B-band. 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. Fe II still dominates the spectrum. 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. 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. 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. SN 2002cx has absorption and emission lines between 6400 Å and 7000 Å that are unique among previously discovered type Ia supernovae.
A final spectrum of SN 2002cx was taken on July 16, 56 days after B-band maximum. SN 2002cx was by then in the nebular phase, with emission lines dominating over absorption lines. The lines were far narrower than previously observed type Ia supernovae and are less pronounced as well. 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. 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.
SN 2002cx did not evolve much between 4 days and 1 day before max, not did it evolve much from 12 days after max to 27 days after max, However, it underwent dramatic evolution during the two weeks after maximum light in the B-band.
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. 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.
by 0.034 magnitudes from dust in Milky Way
.
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. 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. The color of SN 2002cx is consistent with Lira-Phillips law at late times.
The V-R color of SN 2002cx evolves similar to other type Ia supernovae before 5 days after max, although it is somewhat redder. After 5 days after max SN 2002cx gets progressively redder, although still slightly bluer than SN 1991bg. At 25 days after max SN 2002cx continues to redden, while SN 1991bg starts to get bluer.
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.
Type Ia supernova
A Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...
. It was discovered in May 2002 by a team of researchers from LBL
Lawrence Berkeley National Laboratory
The Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...
. It behaved differently than normal type Ia supernovae, and different from several other previously observed peculiar type Ia supernovae including SN 1991T and SN 1991bg.
Discovery
SN 2002cx was discovered on 2002 May 12.21 UT by W. M. Wood-Vasey, G. Aldering, and P. Nugent of LBL with the Oschin 1.2-m telescopeSamuel Oschin telescope
The Samuel Oschin telescope is a 48-inch aperture Schmidt camera at the Palomar Observatory in northern San Diego County, California. It consists of a 49.75-inch Schmidt corrector plate and a 72-inch mirror. The instrument is strictly a camera; there is no provision for an eyepiece to look...
at Palomar
Palomar Observatory
Palomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...
. On 2002 May 17.2 a spectrum taken by T. Matheson, S. Jha, P. Challis, and R. Kirshner of the CfA with the 1.5-m telescope at FLWO
Fred Lawrence Whipple Observatory
The Fred Lawrence Whipple Observatory is an astronomical observatory owned and operated by the Smithsonian Astrophysical Observatory and is their largest field installation outside of their main site in Cambridge, MA...
suggested it was a peculiar SN 1991T like type Ia. SN 2002cx had photometric follow up taken at Lick
Lick Observatory
The Lick Observatory is an astronomical observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton, in the Diablo Range just east of San Jose, California, USA...
using KAIT
Katzman Automatic Imaging Telescope
The Katzman Automatic Imaging Telescope is an automated telescope used in the search for supernovae.The KAIT is a computer-controlled reflecting telescope with a 76 cm mirror and a CCD camera to take pictures. It is located at the Lick Observatory near San Jose, California.KAIT can take close to...
and the Nickel telescope
Anna L. Nickel telescope
The Anna L. Nickel telescope is a 1 meter reflecting telescope located at Lick Observatory in the U.S. state of California.The smaller dome on the main building at Lick had originally held the second hand 12 inch Clark refracting telescope, the first telescope to be used at Lick. In 1979 it was...
, and further spectra were taken at FLWO and Keck.
Light curve
SN 1994D
Supernova 1994D was a Type Ia supernova on the outskirts of galaxy NGC 4526. It was discovered in 1994 by Treffers, Filippenko, Van Dyk, and Richmond using the automated 30-inch telescope at Leuschner Observatory....
or SN 2000cx. SN 2002cx declines in brightness faster than SN 1991T and SN 2000cx in the B-band. In the V-band SN 2002cx is similar to SN 1999ac until 30 days after max. Again SN 2002cx declines faster in the V-band than SN 1991T, but slower than is typical for a type Ia.
SN 2002cx is peculiar in the R-band, as it brightens very fast in a manner wholly different from SN 1999ac. It has not secondary maximum in R-band as expected if it were similar to SN 1991T, but instead has a plateau after max. The R-band also declines more slowly than normal. The I-band behaves similarly to the R-band, with a quick brightening, a plateau and slow decline. While a plateau in I-band is expect for sub-luminous supernovae, the following slow decline is not.
Spectra
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
from Fe III
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
λ4404 and Fe III λ5129. Si II
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
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. This spectrum for SN 2002cx has a low expansion velocity measuring only ~6400 km s−1. 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. At the time of measurement SN 2002cx's expansion velocity was the lowest measured for an early time type Ia, 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.
Four later spectra were taken on June 2, June 6, June 12, and June 16 at FLWO. The spectrum of SN 2002cx has evolved by this point to have a redder continuum. It has also lost the Fe III absorption lines and its Fe II lines have gained prominence at λ4555 and λ5129. The Ca II H & K lines continue to remain weak, a further departure from SN 1997br. Unlike another type of sub-luminous type Ia, SN 1991bg, SN 2002cx does not show Ti II
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
lines around 4100-4400 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
.
Spectra were taken at Keck corresponding to 20, 25, and 26 days after maximum light in the B-band. So little evolution was observed between these spectra that they were all combined in order to increase the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
and study less pronounced features. Fe II still dominates the spectrum. 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. Unlike other type Ia supernova, SN 2002cx has double-peaked emission lines
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....
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. 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. SN 2002cx has absorption and emission lines between 6400 Å and 7000 Å that are unique among previously discovered type Ia supernovae.
A final spectrum of SN 2002cx was taken on July 16, 56 days after B-band maximum. SN 2002cx was by then in the nebular phase, with emission lines dominating over absorption lines. The lines were far narrower than previously observed type Ia supernovae and are less pronounced as well. 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. 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.
SN 2002cx did not evolve much between 4 days and 1 day before max, not did it evolve much from 12 days after max to 27 days after max, However, it underwent dramatic evolution during the two weeks after maximum light in the B-band.
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. 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.
Color
SN 2002cx does not suffer from much host related reddening, as evidenced by its very blue spectra. It is extinct in B-V colorB-V colour
B-V colour is one of the two colour indices in the UBV photometric system.Astronomers measure the amount of light a star gives off in different colours. Hot stars give off more blue light than red; cool stars give off more red light than blue. Coloured filters are used to measure different...
by 0.034 magnitudes from dust in Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
.
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. 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. The color of SN 2002cx is consistent with Lira-Phillips law at late times.
The V-R color of SN 2002cx evolves similar to other type Ia supernovae before 5 days after max, although it is somewhat redder. After 5 days after max SN 2002cx gets progressively redder, although still slightly bluer than SN 1991bg. At 25 days after max SN 2002cx continues to redden, while SN 1991bg starts to get bluer.
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.