Abstract
We examine the absolute magnitudes and light-curve shapes of 14 nearby(redshift z = 0.004--0.027) Type Ia supernovae (SNe~Ia) observed in the ultraviolet (UV) with the Swift Ultraviolet/Optical Telescope. Colors and absolute magnitudes are calculated using both a standard Milky Way (MW) extinction law and one for the Large Magellanic Cloud that has been modified by circumstellar scattering. We find very different behavior in the near-UV filters (uvw1_rc covering ~2600-3300 A after removing optical light, and u ~3000--4000 A) compared to a mid-UV filter (uvm2 ~2000-2400 A). The uvw1_rc-b colors show a scatter of ~0.3 mag while uvm2-b scatters by nearly 0.9 mag. Similarly, while the scatter in colors between neighboring filters is small in the optical and somewhat larger in the near-UV, the large scatter in the uvm2-uvw1 colors implies significantly larger spectral variability below 2600 A. We find that in the near-UV the absolute magnitudes at peak brightness of normal SNe Ia in our sample are correlated with the optical decay rate with a scatter of 0.4 mag, comparable to that found for the optical in our sample. However, in the mid-UV the scatter is larger, ~1 mag, possibly indicating differences in metallicity. We find no strong correlation between either the UV light-curve shapes or the UV colors and the UV absolute magnitudes. With larger samples, the UV luminosity might be useful as an additional constraint to help determine distance, extinction, and metallicity in order to improve the utility of SNe Ia as standardized candles.
Highlights
Brightness decline, making them excellent standardizable candles
MX = mX − rcX − KX − AMW,X − Ahost,X − μ, where MX is the absolute magnitude of the SN viewed through filter X, mX is the apparent magnitude at maximum light in filter X, rcX corrects for the red tails of the uvw2 and uvw1 filters, KX is the K correction, AMW,X and Ahost,X are the foreground (Milky Way, MW) and host–galaxy extinction, and μ is the distance modulus
R1,X and R2,X are the extinction coefficients such that AX = R1,XE(B − V ) + R2,XE(B − V )2. They were calculated using the SN 1992A spectrum at ∼5 days after maximum brightness, extinguished by the MW extinction law with RV = 3.1 (Cardelli et al 1989) or the circumstellar Large Magellanic Cloud (LMC) extinction law from Goobar (2008). They should not be used to calculate the extinction to arbitrary sources observed with Ultraviolet Optical Telescope (UVOT), and these terms can vary even for SNe Ia
Summary
Type Ia supernovae (SNe Ia) are among the most luminous of astrophysical events, making them useful probes of the distant universe. Brightness decline, making them excellent standardizable candles (see Branch & Tammann 1992, Branch 1998, Leibundgut 2001, and Filippenko 2005 for reviews of the subject) This standardizing is done by calibrating the peak luminosity with distance-independent observables such as the light-curve shape. Recent observations by Wood-Vasey et al (2008) strengthen this claim, Garnavich et al (2004) and Krisciunas et al (2009) have shown that some SNe that are subluminous in the optical are subluminous in the near-IR, possibly indicating a continuous or bimodal distribution of the near-IR absolute magnitudes of rapidly declining SNe. At shorter wavelengths, Jha et al (2007) have recently shown that the U-band light curves of SNe Ia are standardizable, similar to the optical, and can be used to determine the extinction and distances. Swift has been used to monitor spectra in the UV, and the satellite has obtained the best UV spectral sequence of an SN Ia to date (SN 2005cf; Bufano et al 2009)
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