Optical Light Curve of SN2000cx
Abstract:
We have measured the light curve of SN2000cx (Fig.1), a Type Ia supernova
(SN) located in the outer part of NGC 524 (an S0 galaxy), using observations
made at CTIO telescopes (10 nights UBVRI at 36inch telescope and 20
nights BVRI at the YALO telescope). Using the standard analysis,
we derive a substantial negative reddening in the host galaxy. Implication
of this result is that relations between colors and decline rate for type
Ia SNe are not universal.
Observations and reductions
From the 36 inch telescope observations we measured aperture photometry of
the SN and Landolt standard fields. Seven of these nights were photometric
and we used them to determine the transformation equations between instrumental
and library magnitudes. With these equations we determined the magnitude
of the SN every night and the brightness of others stars in the field,
"local standards".
With the YALO observations we measured PSF photometry of the SN field
and used the local standards magnitudes to find the brightness of the SN
each night (Fig. 2). For a more detailed description, see the procedure
described by Suntzeff et al.(1999,
AJ 117:1175)in sections 2.2 and 2.3.
Then we added Kevin Krisciunas's data, observed and reduced independetly
of ours. We see that the two data sets aproximately agree, but in some
phases there are big differences (worst case ~0.2mag). However, in the
bands and times that we used in the following analysis they are very close
(Fig. 3).
Analysis
We used Mark Phillips's programs to compute the reddening in the host galaxy
(Phillips
et al. 1999, AJ 118:1766), obtaining the surprising results:
-
The SN's (B-V) color at tail is bluer than we expected even for an unreddened
SN. We are computing a physically impossible negative reddening: E(B-V)=-0.235+-0.051
(Fig. 4).
-
Using the "colors" defined by the differences between the maximum magnitudes
of each curve we find with Bmax-Vmax a big reddening,
E(B-V)=0.176+-0.104, but with Vmax-Imax we
again find a negative one, E(B-V)=-0.128+-0.88 (Fig. 5). All results are
corrected by Galactic reddening.
Also we used the Tonry et al. distance to the galaxy (2001,
ApJ 546:681), scaled to H0=65 km s-1 Mpc-1,
and the Galactic absorption to compute the absolutes magnitudes at maximum
light of this SN. The results are B=-19.093, V=-19.201, I=-18.709 (uncertaints
~0.2mag). This SN is also fainter than we expected (Fig. 6).
Conclusions
This SN is in the outer part of an early type galaxy. We expected it to
have little reddening, but Bmax-Vmax implies a big
one.
Others methods show a physically impossible negative reddening.
Finally, the light curve of this SN looks normal but we found this
object having intrinsic colors different that we expected. This tell us
that the relations between colors and decline rate are not as universal
as we thought.
Figures
Figure 1: Color image of the SN field, the SN is below and right
of the brightest star. JPG (65 kb)
Figure 2: CTIO light curve.
GIF (5 kb)
or Postscript (30 kb)
Figure 3: CTIO + Kevin's light curve.
GIF
(5 kb) or Postscript (26 kb)
Figure 4: (B-V) color at tail, the blue line is what we expect
for an unreddened galaxy.
GIF (7 kb) or Postscript
(30 kb)
Figure 5: Bmax-Vmax and Vmax-Imax
"colors" of SNe vs. dm15 (taken from Phillips et al. 1999), the blue circle
is SN2000cx.
GIF (10 kb) or Postscript
(26 kb)
Figure 6: Maximum magnitudes vs. dm15 (taken from Phillips et
al. 1999), the red circle is SN2000cx.
GIF (68 kb)
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