[vsnet-chat 1084] Analysis and Light Curveof SN 1998aq

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

Supernovae Enthusiasts:

I have completed a light curve and analysis of the bright supernova (SN)
1998aq discovered by Mark Armstrong of the UK.. The results were gleaned
from submissions by the amateur supernovae community. The light curve can
be currently viewed on the ISN Home page. Many thanks to all who
participated.....

THE BRIGHT TYPE Ia SUPERNOVA IN NGC 3982

ABSTRACT:

A light curve and "eyeball analysis of the bright supernova in NGC 3982 (SN
1998aq) discovered by Mark Armstrong, Assistant Co-ordinator of the UK
Nova/Supernova patrol 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 cooperation assisted in making this endeavor a meaningful one in
that the extensive data on the rise and decay behaviour of this event could
be cataloged. 

The light curve is an assemblage of submissions in the Visual band.  Visual
observational estimates were compared to sequencing provided by Dr. Tachi
Kato (VSNET) and presented in a hand-drawn chart by Stefano Pesci on the
home page of the ISN. There was sparce data in other photometric bands and
will not be addressed here.

This event presented considerable full rise (~3 magnitudes) and decay
information (~2+ magnitudes) which are evident in the submitted light
curve. SN1998aq, as a whole, appears to be a normal type Ia event even
considering the rapid pre-maximum rise in brightness. The event then
followed mean averages from ~0.5m magnitudes before maximum light and thru
the very early event decline. After day ~20, the event decay estimates
began to "break-away" from the mean toward a fainter than mean average
trend. 

An analysis and discussion of mean average rise trends before maximum light
and a newly considered behavioral trend are discussed.

key words: supernova, amateur observations, light curve, trend behaviour,
analysis.

INTRODUCTION:

According to IAUC #6875, discovery was made on Apr 13.05UT when the SN
was 14.9 mag. The host galaxy is Sbc galaxy NGC 3982. The location of the
new star is R.A. 11h56m26s.00, Decl. +55o07'38".8 (2000.0), which is 18"
west and 7" north of the center of the host. The event then rose almost a
full magnitude (0.9m) in <2 days (G.Hurst [priv. com., forwarded by Dr.
Yamaoka Hitoshi, VSNET and ISN Alert forums]).

NGC 3982 is also considered a member of the Ursa Major I North Cloud, a
lesser of four clouds (South, X, and Z) (1). The V (subset t) photoelectric
magnitude for the entity is given as 11.1, the B-V (subset T) Color Index
is 0.7, the surface brightness per square arc minute at the major isophotal
diameter is 12.7.

This Cloud "has a membership of 37 galaxies of which 32 are spirals, 2 are
lenticular, 1, is irregular, 1 is elliptical and 1 is peculiar. Some
noteworthy galaxies residing within this cloud are: NGC's 3631 (SNe 1964A,
SN1965L), 3733 (SN1980D), 3756 (SN1975T), 3913 (SNe 1963J, SN 1979B), 3992
(SN 1956A), Anon.1156+5259 (SN 1964E), and 4102 (SN 1975E). The Distance
Modulus on an average for six galaxies is 31.13, which yields a distance of
~16.8 Mpc (The major diameter of the Cloud is 11.1 degrees or 3.3 Mpc).This
yields an estimated Hubble parameter of 72.5 km/sec/Mpc in the vicinity of
the Cloud by implementing the formula V avg.-1218 km/sec/Mpc (based on an
average of 37 radial velocites) / the distance (Dmpc)-16.8 Mpc". A
supernova in this Cloud (per the abovementioned paper) would be magnitude
~12.6 for a type I SNe.
-------------------------------------------
(Authors Note: In keeping with a Hubble parameter (Ho/50+) discussed in
other event information: In (2) NGC 3982 has a B (subset t) magnitude of
11.59, and an absolute magnitude of -20.30, thus yielding a distance
modulus of ~31.89. A type Ia event is estimated (utilizing the authors
-19.8 value) to occur at ~12.09m, which is good agreement with this events
estimated visual maximum of ~12.0).
--------------------------------------------
Mark Armstrong is a very ardent supernovae hunter, here is an excerpt of a
message sent by Mark to the ISN chat forum dated 3/30/98, explaining his
devotion to the task....mere days before his discovery of SN 1998aq: 

"Hello friends:

I have just discovered my second supernova 1998V people may be interested
in a few of my observing statistics. I started patrolling in June 1995 with
a 10inch LX200 and starlight xpress ccd. I made my first discovery on 23
October 1996 (sn 1996bo) after 3148 patrols in 285 hours on 81 nights.
1998V was discovered on March 10 when the grand totals were 18038 patrols
in 985 hours on 231 nights. 1998 has been very good so far with 3378
patrols in 150 hours on 27 nights. I have just topped 1000 hours in total".

VISUAL 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 SN
1998aq had an extremely rapid rise trend from discovery to maximum light
(~4/28.9). The trend then caught up with mean average rise estimations
(3)(see discussion section for a proposed adjustment), at ~the 0.5m (below
maximum) mark. 

The visual trend to maximum light, then followed the mean value rather
closely remaining slightly fainter than the average (~0.1mv). After the
estimated visual maximum (4/28.9), the decay posture then followed mean
values rather nicely with very slight dips towards the fainter extreme at
various points in the epoch decline, and are within the projected error bar
on the light curve.

At ~5/24/98 the information became sparce, with only single estimates
providing information on this event. It might be noted that on, or about
5/20/98 the trend of the visual observations began a separation towards a
fainter posture than the mean average visual curve (3), studies at this
time can not determine if this trend is real, or other factors were
influencing the estimates. 

Estimates in the V, and R-band were sparce, and no attempt to completely
evaluate these estimates is considered. The R-band estimates are displayed
on the light curve for the short period of time this event was monitored.
The R-band did show a linear decline trend, abruptly changing its profile
at about the 30 day mark, into what can be theorized as the beginning of a
secondary "hump", although no evidence for a primary "hump" is apparent
unless the inferrance for R maximum is taken to have occured at ~4/25 (per
the light curve).

DECLINE PARAMETERS:

I have been experimenting with a decay parameter (see 1998bu analysis on
the ISN Home page) for visual observers that will use the decay decline
value for 20 days after maximum light 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. Table 1 displays four amateur
discovered and monitored supernovae where statistics have been gleaned by
this author. A larger sample will have to be analyzed (at least 10 events)
to determine if this value has any merit...(NOTE: this technique was
originally initated by professional astronomer M.M. Phillips (4) Utilizing
the B, and V band).
-----------------------------------------
Event   Visual Magnitude decline 
         from Maximum to 20 days

1989B           1.50
1991T           0.90
1998bu          1.52
1998aq          0.96
Mean Visual(3)  0.94
-----------------------------------------

ANOTHER RISE/DECLINE TREND:

In an analysis of SN 1991T available for view on (5), a measured 0.5m
([FWHM]-Full Width Half Maximum) and a 1.0 magnitude rise/decline value
([FWFM]-Full Width Full maximum) (in days) was mentioned. This value is
determined by measuring the amount of days from 0.5m and 1.0m respectively
below maximum light on the rise and decline branch of a presented light
curve. This utility was favored by R. Barbon et. al (6) to determine "fast
and "slow" supernovae. Here are the same four events (as above) with these
values implemented. 

-----------------------------------------
Event           FWHM            FWFM
                (0.5m)          (1.0m)

1989B           17.8d           25.6d
1991T           20.0d           29.2d
1998bu          17.4d           31.4d
1998aq          17.6d           31.0d
Mean Visual (3) 21.2d           35.4d
-----------------------------------------

DISCUSSION:

SN 1998aq was discovered by amateur astronomer Mark Armstrong under a
controlled supernovae search program. The amateur community responded with
vigor on monitoring this event in the visual mode, and was freely published
on the ISN and VSNET chat forums. 

The event seems to mimic the behaviour of a normal type Ia SN at ~0.5m
below maximum light on the rising branch of the light curve and follow this
trend in the early decline phase of its evolution (~1.0 magnitude below
maximum at ~day 20). 

From ~day 20 in the decline phase, the estimates seem to indicate a
"break-away" from the mean value towards a more fainter posture decline. Is
this phenomena REAL? Was extinction a factor? Is a color-related individual
event involvment regarding type Ia SN a consideration? In SN 1998bu the
"break-away" occured at a much earlier epoch (day 8.3), however all things
being equal the two events did exhibit some behavorial similarities at
later epochs (~30 day mark). 

An adjustment to the mean average visual light curve should be considered
at this time, however many more events will have to be monitored to
determine if this adjustment has merit. The proposal encompasses moving
(closing) the 1.0 magnitude value on the average mean light curve value by
3.2 days. This adjustment would accommodate a smoother transition of visual
observations that have been noticed on the four events monitored
extensively in the visual mode by amateur astronomers (89B, 91T, 98bu, and
98aq). In ALL cases the visual estimates yielded a faster rise than the
mean average on the light curve. Thusly, instead of the mean average at 1.0
magnitude below maximum on the rising branch of -15.7 days (by eye), a
value of -12.5 days before maximum would be inserted. The submitted light
curve reflects this change....any comments?

Many thanks and appreciation to all folks who contributed to the study of
this event from the VSNET and ISN community.

Steve H. Lucas
International Supernovae Network
(76620.1721@compuserve.com)
July 19, 1998

---------------------------------------------
REFERENCES:

1.) From an unpublished(?) paper by James T. Bryan and Harold G. Corwin
Jr., _Atlas of the Ursa Major I North Cloud_, released in 1989).

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.) David Bishop's SNe Home Page: (http://vsnet.ggw.org/asras/supernova.html)


6.) Barbon et. al: Astron & Astrophys. _25_, 241 (1973) displayed in "SN I"
proceedings of the Texas Workshop on type I SNe, March 17-19, 1980.
---------------------------------------------

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp