Diversity of Decline-Rate-Corrected Type 1a Supernova Rise times:One Mode or Two? Page: 1 of 15
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DIVERSITY OF DECLINE-RATE-CORRECTED TYPE Ia SUPERNOVA RISE TIMES:
ONE MODE OR TWO?
MARK STROVINK
Physics Department and E. O. Lawrence Berkeley National Laboratory
University of California, Berkeley, CA 94720
Received 2007 May 4; accepted 2007 August 31
ABSTRACT
B-band light-curve rise times for eight unusually well-observed nearby Type Ia supernova (SNe) are
fitted by a newly developed template-building algorithm, using light-curve functions that are smooth,
flexible, and free of potential bias from externally derived templates and other prior assumptions.
From the available literature, photometric BVRI data collected over many months, including the
earliest points, are reconciled, combined, and fitted to a unique time of explosion for each SN. On
average, after they are corrected for light-curve decline rate, three SNe rise in 18.81 0.36 days, while
five SNe rise in 16.64 0.21 days. If all eight SNe are sampled from a single parent population (a
hypothesis not favored by statistical tests), the rms intrinsic scatter of the decline-rate-corrected SN
rise time is 0.96+0.2 days a first measurement of this dispersion. The corresponding global mean
rise time is 17.44 0.39 days, where the uncertainty is dominated by intrinsic variance. This value is
2 days shorter than two published averages that nominally are twice as precise, though also based
on small samples. When comparing high-z to low-z SN luminosities for determining cosmological
parameters, bias can be introduced by use of a light-curve template with an unrealistic rise time. If
the period over which light curves are sampled depends on z in a manner typical of current search
and measurement strategies, a two-day discrepancy in template rise time can bias the luminositycomparison by X0.03 magnitudes.
Subject headings: supernova: general cosmology:
1. INTRODUCTION
Within a few minutes of explosion, Type Ia supernov
release most of their energy, but due to self-absorption
they reach peak luminosity only after 2-3 weeks. During
this period of ballistic expansion, while the photosphere
grows in radius but shrinks in characteristic velocity as
slower, heavier ejecta are revealed, basic properties of
the explosion become evident. Spectroscopic signatures
of elements intermediate between carbon-oxygen fuel and
iron-group ash (Filippenko 1997; Branch et al. 2006) re-
veal that burning is incomplete, with deflagration likely
playing an early role (Mazzali et al. 2007); nonvanishing
polarization measures the progenitor's asphericity (Wang
et al. 2007). After peak brightness, when iron features
blanket the spectrum and polarizations wane, SNe be-
come more homogeneous. For SN science to progress,
therefore, it is crucial to study the period of rising lumi-
nosity.
In cosmological studies, SNe Ia are prized for their use
as standardizable candles to trace the history of cosmic
expansion (Riess et al. 1998, 2007; Perlmutter et al. 1999;
for a review, see Perlmutter & Schmidt 2003); in this
context, periods of greater SN uniformity are of greater
value. Indeed, post-maximum luminosity indicators do
yield low-dispersion Hubble diagrams (Wang et al. 2003;
Wang, X. et al. 2005); post-maximum color measure-
ments do solidify the corrections made for absorption by
host-galactic dust (Lira 1995; Phillips et al. 1999; Jha
et al. 2007). Nevertheless, as ever more ambitious cam-
paigns to chronicle the Universe's expansion history are
planned (e.g. Aldering et al. 2002), the fundamental issueElectronic address: strovinkelbl.gov
observations distance scale
of high z -> low z evolution (Howell et al. 2007) keeps
SN science in focus. For example, more than one SN
Ia progenitor or explosion mechanism might be at work,
with progeny neither equally bright nor equally abundant
at high vs. low redshift. To secure such understanding,
continued study of the rise-time period is essential.
The subject of this report is a basic property of this
period the light-curve rise time itself. The B-band rise
time is quite sensitive to the main-sequence mass of the
white dwarf progenitor and to its carbon/oxygen ratio
(see e.g. Dominguez et al. 2001). It is less sensitive to
the progenitor's metallicity.
Pskovskii (1984) published the first rise times for
classes of type I SNe. In retrospect their range is reason-
able, but, oddly, the reported correlation of rise time with
decline rate was positive. For individual SNe, the earliest
rise-time measurements were made by Leibundgut et al.
(1991) (SN 1990N) and by Vacca & Leibundgut (1996)
(SN 1994D). For a group of SNe, the earliest measure-
ment of the average decline-rate-corrected rise time was
reported by Groom (1998) and Goldhaber (1998). They
used a model described e.g. by Arnett (1982), in which
the initial rise of B flux with time is parabolic. Within
;z2 days, these early determinations agree with current
values. Soon thereafter, in a definitive paper, Riess et
al. (1999b) (henceforth Rie99b) established the presently
accepted average rise time to B maximum of 19.5 0.2
days. This was accomplished by ferreting out early unfil-
tered photometry for ten nearby SNe and transforming
it to standard passbands, to which the parabolic model
was applied. Also using this model, Conley et al. (2006)
(henceforth ConO6) recently confirmed the Rie99b low-
redshift average, in addition measuring an average rise
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Strovink, Mark. Diversity of Decline-Rate-Corrected Type 1a Supernova Rise times:One Mode or Two?, article, May 1, 2007; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc893020/m1/1/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.