(click on image for full resolution)
4/22/98 color composite image from
HÅkon Dahle's supernova page. Used by permission.
Some notes on SN1998aq:
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.
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(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).
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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).
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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
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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.
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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
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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://www.RochesterAstronomy.org/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. ---------------------------------------------
Another SN was discovered by English amateur astronomer M. Armstrong. This SN is very remarkable!
According to IAUC 6875, discovery was made on Apr 13.05UT when 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. Type definition has not been reported yet. On Apr. 13.8 exposure, this new star was about 14.5 mag, which indicates that this SN should be on the brightening phase (very early phase).
The distance module of the host galaxy NGC 3982 is m-M = 31.15 (from Turry's nearby galaxy catalog), which shows that this galaxy is on the same distance with Virgo cluster. NGC 3982 = UGC 6918 = IRAS 11538+5524 = PGC 37520 is Seyfert 2 galaxy, and also detected by ROSAT. Typical SNeIa on Virgo distance would be about 12 mag or so at their maximum (without reddening), and a some (~2mag) dimmer if it is of other types. Following photometries and spectroscopic observations are strongly urged. Especially it can provide the nature of SN on very early phase.
Sincerely Yours,
Hitoshi Yamaoka, Kyushu Univ., Japan
yamaoka@rc.kyushu-u.ac.jp
Back to Bright Supernovae.
David Bishop