(click on image for full resolution)
4/22/98 color composite image from
HÅkon Dahle's supernova page. Used by permission.
Exploding Star Fans:
The analysis (my own) of SN 1998S, along with light curve(s) are presented. Needless to say this was an interesting event to catalog, and displayed some peculiar behavior that I have not witnessed for any event I have studied. The intial light curve will be attached to this message (.gif format), with a second curve (blueprint) following under separate cover. The latter will connect all mean value points to display a "cyclic" behavior that appears to have existed in the descending branch of the light curve. My heartfelt thanks are offered to all who had monitored this event, and provided the data to make this analysis possible. Thanks also to the individuals who offered their input and expertise. This material may be used freely by anyone who deems the material worthy of presentation.
Steve H. Lucas
The Bright Supernova 1998S In NGC 3877; An Identity Crisis?
ABSTRACT:
A visual light curve and "eyeball analysis" of the bright supernova in NGC 3877 (SN 1998S) discovered by Chinese professional astronomer Zhou Wan of the Beijing Observatory (BAO) is presented. The visual magnitude estimates were gleaned from the supernovae chat and alert forums of the VSNet (variable star network/Japan), the ISN (International Supernovae Network/Italy), French astronomers (Site Aude des Supernovae [tabs98s.html@www.visuanet.com]) courtesy of Jean-Marie Llapasset, and The Norweigan Astronomical Society, Variable Star Section, courtesy of Bjorn Granslo ( http://www.astro.uio.no/~bgranslo/sn1998s.html). 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 bright supernova (SN) in NGC 3877 has displayed some interesting behaviour which is presented in the accompanying light curve.
The light curve is an assemblage of submissions in the Visual band. Observational estimates were compared to magnitude sequencing provided by various organizations. Initial information involving comparison magnitude sequencing was provided by B. Granslo in vsnet chat message #1324 which displayed information from a variety of modern reference sources. There was sparce data in other photometric bands, some will be touched on lightly or will not be addressed at all.
This event presented considerable full rise (~3.3 magnitudes) and decay information (~3.2 magnitudes). SN1998's signature, via a spectrograph (IAUC# 6830), indicated a type II-n event, however in viewing the light curve it appears to mimic a type Ia event (verses a visual mean trend), and is somewhat fainter than a mean SN type II-L (Linear) event. A subtle cyclic appearance, with a amplitude of ~0.5mv exists in the overall shape of the descending branch of the light curve and can only be speculated upon as to its true nature. (first implied on 4/19/98 by Dr. T. Kato in a VSNET message)
Of notable interest is a pre-maximum "glitch" (on the the rising branch of the light curve) which was observed in the visual, on 3/14/98 from various observers (six days before estimated maximum light [3/20/98]), and also in the V-band estimates 9.4 days before maximum light (3/10.6/98). Professional analysis will hopefully determine if this anomoly is real.
An analysis and discussion of various mean average trends are discussed, plus notable fluctuations during the events evolution. These are apparent on the light curve, and will be brought to light with some conjecture applied (by this author)(see Appendix A). It is the hope of this excercise to define the shape of the light curve and hence the event, rather than any detailed analysis of the data points....
key words: supernova, amateur observations, light curve, mean average templetes, analysis.
INTRODUCTION:
In VSNET alert message #1519 the following notice was posted, courtesy of Dr. Yamaoka: "According to IAU Circular 6829, Supernova 1998S was detected in NGC 3877 on Mar. 3 UT by BAO Supernova Survey. It was about 15.2 mag (unfiltered) on the discovery frame, and then rose to 13.5 mag (also unfiltered) on Mar. 4.3 UT. The location of SN 1998S is: R.A. =3D 11h46m06s, Decl. =3D +47o29'.0 (2000.0), which is 16" west and 46" south of the nucleus of the host galaxy. The spectroscopy shows a featureless continuum but probably with a broad H-alpha emission, which indicates that this SN is of type-II in a very early phase. The location of SN 1998S is embedded on the host galaxy. So the photometry is rather difficult; but it is very challenging and the results should be very useful....."
NGC 3877 is a ScII.2 type galaxy shining at 11.02 (Apparent blue magnitude in the Bt system corrected for galactic and internal absorption) and has an absolute magnitude of -20.35 (absolute magnitude in the Bt system). A supernovae at the estimated distance of NGC 3877(1)[~18.79 Mpc, or ~61M light years](Distance Modulus - 31.37 [Ho - 50+/km/sec/Mpsec]) indicate an estimated peak brightness for a type I event of ~11.57, and a type II event at ~13.57 (utilizing a -19.8, and -17.8 absolute value [author's estimate]). This event's estimated maximum of 11.9mv on 3/20/98 is then similiar, by observable inferrance, (observations) to a type Ia event?
The weighted mean observed recessional velocity for NGC 3877 is ~887(H/50+) which places this entity a bit closer to us than Cluster B of the Virgo Cluster (M49) which has a velocity of ~963+/-81. (1) and (2). Being that this event displayed a preliminary type II signature there is another consideration, by implication of the light curve, that 1998S MIGHT? be a bright linear type II event ~1.67 magnitudes brighter than expected. Reference (3), displays some similarities that exist with other bright type II events which have been studied and analyzed.
VISUAL OBSERVATIONS
The amateur/professional community began visual and photometric monitoring of this supernova (SN) almost immediately after its discovery. The initial estimates were provided by French observers (Site Aude des Supernovae [tabs98s.html@www.visuanet.com]), the Spanish Supernova Group M-1 (http://personales.jet.es/drodrig/sn98) (from private communication), and individuals associated with The International Supernovae Network/Italy and VSNET/Japan. The latter provided extensive magnitude estimates for this event (over 400). The light curve is a reflection of the abovementioned teams and their visual observational estimates and are displayed as a mean value per day.
On the light curve a separate set of information is rendered for the French observers, who (as far as I can determine) performed their own magnitude sequencing from their own photometric calibrations.
Detailed magnitude sequencing (VSNET message #1324) (with noted catalog magnitude uncertainty were taken from: "The Hipparcos Catalogue, Tycho Catalogue, Tycho Input Catalogue, and Guide Star Catalogues. The Colour indices (b-r) were based on the PPM USNO-A1.0 catalogue", were presented by Bjorn Granslo (Norwegian Astronomical Society - Variable Star Section). Visual estimates from various observers were also presented by Mr. Granslo and were made accessible on VSNET message #1344, ISN chat forums, sci.astro amateur, sci.astro research usenet groups and later on VSNET message #1376 (part 2).
The French observers presented magnitude estimat