Supernova 2002ap: the first month Page: 76
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L76 A. Gal-Yam et al.
-1.5 -- -
0.2 I I I
4500 5000 5500 6000 6500
Rest wavelength [Angstrom]
Figure 4. Comparison of Wise spectra of SN 2002ap (solid lines) with
contemporary spectra of SN 1998bw (dotted line) at ages 7 days before
peak (top panel) and at peak magnitude (middle panel), from Patat et al.
(2001). The bottom panel demonstrates the resemblance of the spectrum of
SN 2002ap at age 20 d (solid line) to SN 1997ef at age 40 d (dotted line;
from Matheson et al. (2001); the superposed narrow lines are from the
underlying galaxy; earlier spectra of SN 1997ef were not available in digital
(unless otherwise noted, from here on we refer to our best estimate
B-band peak date, February 7.1 UT, as the time of peak brightness),
and up until the date of our last spectrum (-20 days after
The spectral evolution of SN 2002ap displays several
characteristic trends also identified in 1997ef (Matheson et al.
2001, Iwamoto et al. 2000) and 1998bw (Stathakis et al. 2000,
Patat et al. 2001, and references therein). These include the
reddening of the continuum, a redward shift and narrowing of
prominent 'emission-like' and 'absorption-like'3 features, and
decreasing values of the expansion velocities. Following Patat et al.
(2001) we have calculated expansion velocities from the Si i
A6355 absorption, and find a decrease from ~ 38 000 km s- 7 days
prior to peak to ~15 000 kms -1 at peak, and to -6000 km s-1 14
days later. This is similar to the results obtained for 1998bw, and
consistent with the measurements of Motohara et al. (2002;
- 16000km s- around peak from 1.083 m Het) and Filippenko
& Chornock (2002; ~9000 km s-- four days after peak from
7774 A 01). The prominence of the 4600 A emission peak relative
to the one bluewards of 5000 A (both tentatively attributed to Fe i
blends, e.g., Patat et al. 2001 and references therein) in spectra of
SN 2002ap at and after peak resembles spectra of SN 1997ef but
differs somewhat from those of SN 1998bw. A notable feature in
the spectra of SN 2002ap is a narrow, unresolved absorption,
consistent with He it A4686. This feature first emerges in the
spectrum taken near peak brightness and is stronger still seven days
later. It does not appear in later spectra of the object, nor in spectra
of either SN 1997ef or 1998bw.
3 Model synthetic spectra show that at early times, the spectral shape of
these objects is determined by the wavelength dependence of the absorption
optical depth, so that features in the spectrum cannot generally be attributed
to a certain emission or absorption line (e.g. Patat et al. 2001, and references
3 DISCUSSION AND CONCLUSIONS
We have presented the results of our optical photometric and
spectroscopic monitoring of SN 2002ap, during the first month
after its discovery. Our best estimate for the date of B-band
maximum is 2002 February 7.1, and our derived peak magnitude of
MB - 16.9 is significantly lower than the one measured for the
'hypernova' SN 1998bw. However, except for the lower peak
magnitude, the characteristics of the light curves, as well as the
spectral evolution of this object, are strikingly similar to those of
SNe 1997ef and 1998bw. Our work shows that, at least to some
degree, one may use the well studied properties of SN 1998bw as a
tentative basis upon which to plan future observations of SN
2002ap, after scaling the peak magnitude.
In light of its similarity to SNe 1998bw and 1997ef, one of the
intriguing questions about SN 2002ap is whether it was also
associated with a GRB. Using our most conservative estimate for
the date of the B-band peak, February 7.1 7 6d (Table 2), and
assuming that the lag between a hypothetical GRB and the time of
B-band maximum is similar to that measured for SN 1998bw
(14.3 d, Galama et al. 1998), we would expect the GRB trigger to
have occurred around January 23.87.6 d. However, if we use
instead our estimated U-band peak date, February 4.7ti.h d, which
is best constrained by our photometry, along with the appropriate
U-band time lag from Galama et al. 1998 (13.7 d), the resulting
GRB trigger time is January 22.0 - d. Hurley et al. (2002) found
no candidate GRB that might be associated with SN 2002ap in an
intensive search of gamma-ray data from all available sources,
starting January 21. Our results suggest that the GRB trigger may
have occurred outside the time frame searched. If the GRB-peak
magnitude time lag for SN 2002ap was just one day longer than the
lag measured for SN 1998bw, the trigger is likely to have been
missed by Hurley et al. We conclude that in order to detect, or set a
secure upper limit to the fluence of a GRB associated with SN
2002ap, a search similar to the one reported by Hurley et al. should
be extended to include data taken several days prior to January 21.
Finally, there remains the question of the classification of SN
2002ap and its look-alikes, SNe 1998bw and 1997ef (and possibly
also SN 1998ey, Garnavich, Jha & Kirshner 1998, and the recent
SN 2002b1, Filippenko, Leonard & Moran 2002). Various authors
have used various types for these events, usually some combination
of Ib, Ic and 'peculiar'. In view of the fact that these SNe are
spectroscopically similar to each other, but well distinguished from
prototypical SNe Ib (e.g. SN 1984L) or Ic (e.g., SN 1987M or
19941), we suggest that they be defined as a new SN sub-type, and
provisionally designate them as Type Id SNe. This should replace
the often used nickname 'hypernovae', initially used to describe
the unusually energetic and luminous SN 1998bw (Iwamoto et al.
1998). This term has since become somewhat misleading as it was
later applied to events that were modelled as highly energetic
explosions, but were not especially luminous, such as SN 1997ef
(Iwamoto et al. 2000). Currently, all events that are spectro-
scopically similar to SNe 1998bw and 1997ef, e.g., 2002ap and
2002b1, are sometimes referred to as 'hypernovae' for lack of a
better name, even though they are not known to be very luminous
or exceptionally energetic. Admittedly, these events are probably
closer to Type Ic SNe than to any other sub-type, and the physical
differences between them and other Type Ic's are not fully
understood. However, the current SN classification scheme is
purely observational and based mostly on optical spectroscopic
properties (see Filippenko 1997, for a review). In our opinion, the
difference between the early-time spectra of the three well-studied
2002 RAS, MNRAS 332, L73-L77
Royal Astronomical Society - Provided by the NASA Astrophysics Data System
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Gal-Yam, Avishay; Ofek, Efran & Shemmer, Ohad. Supernova 2002ap: the first month, article, March 27, 2002; Oxford, UK. (digital.library.unt.edu/ark:/67531/metadc861691/m1/4/: accessed November 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.