[vsnet-chat 1016] Light Curve and Analysis of SN 1998bu

["Steve H. Lucas" <76620.1721@compuserve.com>]

Supernovae Enthusiasts and Friends:

I have completed a light curve and analysis of the bright supernova (SN)
1998bu discovered by Mirko Villi of Italy. The results were gleaned from
submissions by the supernovae community. The light curve will appear on the
ISN Home page shortly. Many thanks to all who participated.....

THE BRIGHT SUPERNOVA IN M96 (SN1998bu)

ABSTRACT:

A light curve and an "eyeball" analysis of the bright supernova in M96 (NGC
3368), discovered by amateur SN hunter and co-webmaster of the
International Supernovae Network (hereafter ISN) Mirko Villi is presented.
The magnitude estimates were gleaned from the supernovae chat and alert
forums of the VSNET (variable star network/Japan), and the ISN
(International Supernovae Network/Italy). Worldwide magnitude estimations
assisted in making this amateur (professional?) endeavor a meaningful one
involving a very serendipitious event. 

The light curve is an assemblage of submissions of the event's rise and
decay in the R, V, and Visual band. Visual observational estimates were
compared to sequencing provided by M.Funada (ISN) and Brian Skiff (Lowell
Observatory) [see vsnet alert message #1756], plus sequencing was available
utilizing the Thompson/Bryan SN Charts(1989) . All in all their were ~127
data points that were used, which were binned into 0.5 day averages.

This event presented some interesting rise and decay trends which are
evident in the submitted light curve. SN1998bu appears to be more rapid and
sub-luminous in the visual, V and R-band than mean averages and theorized
estimates indicate. An analysis and comparison data are discussed.

key words: supernova, amateur (professional?) observations, light curve,
analysis.

INTRODUCTION:

NGC 3368 is an Sab(s)II type galaxy, and is a member of the "M96 Group
which is a major condensation of the Leo I Cloud; it is centered on a dense
core (3 degrees X 1.5 degrees). As the nearest of the well-mixed groups
(types of galaxies), the M96 group is one of the best fields for the
calibration of distance indicators applicable to E and L systems (N3377 and
3379), which are rich in globular clusters"(1). Residing at 10h 46m 45.2s,
+11d 49' 16"( epoch 2000) this galaxy has an apparent blue magnitude of
9.49. Combining this with an absolute magnitude of -21.41 this entity then
has a theorized distance modulus of 30.9, which yields an estimated
distance of 15.13 Mpc(H/59km/s/Mpc)(2). A Type Ia SNe at this distance
theoretically should diplay a maximum brightness (if -19.8 SNe absolute
brightness is inferred) of 11.1. The observed magnitude brightness of this
event was ~11.7v. 

Supernova 1998bu was a truly serendipitious discovery by Mirko Villi of
Forli', Italy. He was testing out a new CCD camera in the light of an
almost full moon and came across the then suspect star on May 9.9, 1998. A
note went out to the supernovae community (IAUC#6899) shortly thereafter,
and a call for verification was requested via the ISN and VSNET alert
forums.

SN 1998bu was located 1' north of the nucleus with a discovered magnitude
of 13.5.  Visual confirmation was made by B. Skiff, at Lowell observatory
on May 10.33 UT at 12.5v (or possibly brighter considering local sky
conditions). Days later the suspect had an identity...SN 1998bu. It might
also be noted that there was a pre-discovery image of this galaxy taken on
3/3 which would indicate that this event was fainter than ~16.5.

VISUAL and V-BAND OBSERVATIONS:

Viewing the light curve (which has a tendancy to appear flatter than most
published light curves due to the 1:1 ratio used for magnitude verses epoch
data, has been favored [by this author] to obtain a more difinitve and
detailed presentation of the available information), we can see that the
events pre-maximum slope is somewhat more rapid than the average mean
visual rise curve (3) from the onset. However the visual observations then
fall into the mean curve trend about 0.5 magnitudes BELOW visual maximum
light (~5/21), and follow this slope for the first 8.3 days AFTER maximum
light. The visual observations then fall away from the mean average from
that point onward to a more fainter posture. This visually noted trend from
the mean seems to indicate a much more rapid decline than the mean and
could indicate a sub-luminous scenario for this event(Pskovskii-Branch
effect). In a note sent to the VSNET and ISN chat forums, Dr. Hitoshi
Yamaoka points out that: "As noticed in IAUC# 6902, the SN is affected by
reddening in both the host galaxy and our Galaxy to an estimated A_V ~ 0.6
magnitude". Could this be one of the reasons for the sub-luminous nature of
this event, visually? 

Maximum light estimates in the V-band were at (5/19) and R-band at 5/16)
respectively. In type Ia SNe a V-band decline decay parameter was initiated
in 1993 by M.M. Phillips (4) known as: delta m[subset] 20 V. Which is the
decay magnitude of a Type I event at 20 days after maximum light (another
parameter of delta m [subset] 60 V is also used for determining the
brightness of the linear tail after the events inflection point).

I will list some referenced V/20(m) parameters. In addition I am
experimenting with a decay parameter for visual observers that will also
use the decay decline for 20 days and will be noted as: v/20(m).It is the
hope of this author to use this excercise to obtain a useful parameter
which can be used by visual observers of bright type I SNe and for any
subsequent research associated therein. (Note: this is just an experiment
and time will determine if indeed this parameter can be useful to the
visual observer).Table 1 is an example of the V/20 used in association with
a small sub-sample (5) which includes an estimation of this event from the
light curve along with the new v/20(m) parameter.
--------------------------------------------------
Table 1: Decay Decline Parameter
--------------------------------------------------
SN      delta m[subset]20V parameter    delta m[subset]20v parameter

1992bc          0.85
1991T           0.88                            0.90*
1992al          0.92
1992A           1.23
1992bo/1993H    1.26
1991bg          1.68
1990N           0.90
1994D           1.14
(1998bu         1.00)                           (1.52)**

* This estimate was gleaned from a light curve created by this
author(available upon request).

** This estimate was applied utilizing an eye measurement involved in the
general decay trend of visual observations for this event. It can be seen
in table 1 that theoretically this event was a much faster visual decliner
than SN 1991T. And once again, much more data is required to see if this
parameter will be of any use.

Considering that there was limited information in the V-band. An attempt
has been made to compare these to mean V-band averages (6). V-band maximum
appears to lagged behind the visual maximum by ~1.8 days. The trend from
maximum light versus the mean average V curve appears to be slightly
fainter than average (but not as drastic as the visual). The data and mean
would seem to converge about the 30 day epoch. Below (Table 1a) are
statistics reflecting the differance (by eye).
---------------------------------------------------------------------------
-----------
Table 1a: MEAN V-BAND DECLINE VERSUS SN 1998bu DATA
---------------------------------------------------------------------------
-----------
Magnitude       Mean Decline            SN 1998bu V-band data
                per (6) in days

0.5             14.16d                  9.6d
1.0             24.58d                  20.8d
1.5             31.66d                  32d (estimated)


R-BAND OBSERVATIONS

In the R-band some differances occur for this event as compared to
templetes for type Ia SNe (7). Table II infers an R-band Max. of 11.96 on
6/16/98 for SN 1998bu (averaged). As a disclaimer I must caution that the
R-band data templete (7) was selected from a limited sub-sample of events.
---------------------------------------
Table II: R-band Behaviour
--------------------------------------
Templete(7)     SN 1998bu: All Observations
(Mag below              (thru 6/19/98)
Max. in days)           
-4.4d   1.85            0.66 (6.2d)
3.6d    0.00            0.03
9.0d    0.06            0.18
16.0d   0.60            0.48
22.6d        0.64               0.76
28.0d   0.80            0.66

It can be seen that the R-band photometry for this event indicates a faster
decline speed as compared to the mean average templete before, and after
maximum light to well before day 16. A secondary hump (or plateau) is
evidenced on the light curve, which agrees nicely with what occurs on the
referenced templete data at about this epoch. This infers some similarities
along with the  visual and V-band data trend of a "fast decliner". One
other note: In the templete by (7), the R-band appears to be brighter than
the V-band decay slope, where in the case of this event (the SN 1998bu
light curve) it does not? It is more faint! Also the 2.4 day lag behind
Vmax for Rmax is a bit less than templete indications (7). 

DISCUSSION:

With the limited time that this event (SN1998bu) was monitored by the
supernovae community some interesting results are evidenced (all in all). 

1.) The visual trend indicates a much fainter and rapid posture than the
mean average.

2.) V-band photometry indicates a more rapid rise and (slight) decay
decline than the mean average. 

3.) R-band photometry indicates a much faster rise and decline posture than
a mean average templete, this corresponds also with what occurs in the
visual and V-band mode. A nearly similiar decline scenario as SN 1994 is
closely inferred.

Why does the visual, V-band and R-band estimates indicate such a fainter
posture from the mean averages? Is the absorption of this event along our
line of sight a factor (0.6m)? Is a color-related involvment regarding this
SN a primary consideration, that needs to be accessed in the future? Or is
this event to be regarded as a peculiar type Ia SN? These questions will
remain unanswered until professional data is presented and many more events
are monitored to help determine a difinitive pattern to other events. Many
thanks to all individuals who contributed to this event in the form of
visual, V, and R-band photometry.

--------------------------------------------------------------------
References:
1.) Galaxies and the Universe, Vol. 9 of Stars and Stellar Systems; Kuiper
(1975).

2.) A Revised Shapley-Ames Catalog of Bright Galaxies; Sandage and
Tammann(1981).

3.) Doggett and Branch; AJ _90_,(11), November (1985).

4.) M.M. Phillips; ApJ,_413_,L105 (1993).

5.) Hamuy and Phillips et. al, AJ,_112_(6)(2438-2447), Dec., (1996).

6.) B.Leibundgut: Astron. & Astrophy., _89_ (1991)

7.) E.M. Schlegel; AJ.,_109_(6), June, (1995)
--------------------------------------------------------------------

Steve H. Lucas
International Supernovae Network
(76620.1721@compuserve.com)
July 1, 1998

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