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Supernova 1998 bu


Here is an image of Supernova 1998 bu taken at 5 AM on October 30, 1998 from the Sunflower Observatory in Olathe, Kansas.

SN1998bu was probably the most significant supernova discovery of 1998. I had just gotten started in my supernova search efforts when this one was discovered. Ironically I had imaged this galaxy a month earlier and after the announcement of the discovery of the supernova by Mirko Villi in Italy, I looked at my earlier images and found the very faint supernova there but it was so faint that I had over looked it. My image was useful in plotting the earlier stages of the light curve for this significant event.

After early September Leo and M96 were no longer visible due to passage behind the sun. Since most Type 1a supernovae do not last longer than 150 days, no one expected it to still be visible. But I noticed that Leo was rising right before dawn and decided on October 29 to stay up all night and find out for myself.

Sure enough it was there! I sent the image in to VSNET and ISN and the resultant analysis by Steve Lucas is shown below. The light curve for SN1998bu has been revised and we have learned something new about Type 1a Supernovae. This little star will define my first year of operation as a private imaging observatory on the IAUC network.

Here is Steve's report followed by the revised light curve.

SN 1998bu Late-Time Light Curve (Type Ia Supernovae past 150 days)

ABSTRACT:


The number of late time (more than 150 days) photometric or visual measurements involving type Ia supernovae (SNe) have been very limited. Events displaying useful information accessed via the astronomical literature include: SNe 1937C, (Schaefer 1994; Pierce & Jacoby 1995) 1972E, (Ardeberg & deGroot 1973; Branch et. al 1994) 1986G, and 1986N (Turatto & Cappellaro 1990). The latter had inconclusive data due to the complexity of the explosive site. Data on the remaining events will be examined.

On 10/31/98 Dr.Taichi Kato posted a message on the VSNET chat forum regarding a CCD image taken by L.Robinson (Olathe, KS) of the formally bright SN 1998bu in M96 (visual maximum ~11.7). Private Correspondance with Mr. Robinson yielded a jpg image of the fading SN in NGC 3368 taken on 10/30/98 (164 days past maximum light). From this image an "eyeball-estimate" (from sequenced and unsequenced comparison stars) of the SN has been initiated by this author and an extension of the (late exponential tail) light curve, will be presented. A transformation formula to reduce the (single) data point will also be attempted....

INTRODUCTION:

SN 1937C in IC 4182 can possibly be called the "grandfather" of Type I SNe which have displayed extended, or late-time observations. (ED.Note: This event saw Minkowski (1940) divide SNe into appropriate types, hence the initiation of type I and II SNe). Approximately 22 observations were made more than 300 days after maximum light (Schaefer 1994). SN 1972E in NGC 5253 comes in a very close second with at least one data point at 286 days after maximum, and one point at ~day 315 (Younger & van den Bergh 1985). It might be noted that SN 1895B was classified as a type I SN where extensive observations after day 400 were reported, however the photometry "appears to have been unreliable" (Schaefer 1994).

In Branch 1994, Comparisons between both SN 1937C and 1972E have been reported (from other references) to have been almost identical from the spectrum and light curves. Only slight variations between the two occur from possible systematic errors in some of the photographic data, and decline times (SN 1937C declining faster than SN 1972E by 1.1m +/-0.3)....

SN 1998bu IMAGE/OBSERVATION/MAGNITUDE ESTIMATE:

A message from the VSNET (Dr.Taichi Kato, proprietor), on 10/31/98 indicated an L.Robinson had obtained a CCD image of the galaxy NGC 3368, and had imaged the now fading SN 1998bu. "This image was taken without a filter using an ST7 at full resolution on an LX200 10" F6.3 with an F6.3 focal reducer yielding F4.07. It was a 300 second guided exposure with dark frame subtracted and flat field corrected then stretched and saved as a jpg". Private correspondance with Mr. Robinson yielded a copy of the 1998bu image. This author then attempted an "eyeball magnitude estimate" utilizing the Thompson/Bryan Supernova Charts, 1989 (NGC 3368)....Below is the method of estimation and the formula used to derive the single data point at day 164 past maximum light, this point is then added to create a late-time light curve.

When the image was received from Mr. Robinson my information software (compuserve), placed one copy in a download section, and kept the second image as an attachment to the e-mail message. The former image displayed a pristine image of NGC 3368, complete with the halo of the galaxy, and a prominent bar. This image, however, subdued the event (SN1998bu) as if it were acting as a form of intrinsic reddening. The magnitude estimate was rather difficult at that point. On the attachment image the galaxy appeared as if false color image processing had been administered. The starfield and SN event suffered no noticeable extinction....from this image, the estimate was made.

Utilizing the aforementioned Thompson/Bryan Chart for NGC 3368, a comparison was initiated utilizing a 15.7m sequenced star (Birch et. al, Perth Observatory) 13.5"E X 2.25"S of the core, and a 16.0m unsequenced star 8"W X 6"S of the center. Based on the two stars I estimated the magnitude in Mr. Robinson's image at 16.1V +/-0.1. at day 164 past maximum light....

The initial light curve presented to the VSNET and ISN community on 7/4/98 was from estimates in the visual mode, thusly this estimate had to be converted to that band. The first thing to be attempted was a color-index of a SN at such a late epoch....this is where the information on SNe 1937C and 1972E came in handy.

In Pierce and Jacoby 1995 (pg.2891) a visual bandpass photometric table for the entire apparition of the 1937C SN event is displayed. In Younger & van den Bergh 1985, and Ardenberg & deGroot 1973, extensive V-band photometry for SN 1972E is also displayed. If we then take an extrapolated value at about 164 days to conform to the epoch of the SN 1998bu data point from SN1937C (0.40), and the value from SN1972E (0.29) we derive an averaged color excess (B-V value) of 0.345. We can then use this value to determine a visual estimate for placement on the late time light curve.

On VSNET chat message #1356 a value for a transformation formula was introduced for unfiltered CCD magnitude estimates. As far as I can determine this formula is as good as any at the present time, and will be employed here...v = V + 0.2(B-V). Considering an estimate of 16.1V, plugged into the formula, utilizing 0.345 as the averaged B-V, we have a magnitude estimate of 16.169 visual +/-0.1.

COMPARISONS/DIFFERANCES:

A comparison between the SNe 1937C, 1972E, 1986G, and 1998bu and the average visual light curve (Doggett & Branch 1985) are presented. The latter data will be displayed in the attached late time light curve.


EVENT DECLINE/100d DECLINE/DAY

1937C 3.40V 0.0295V (171d) Pierce & Jacoby (1995)
1972E 3.04V 0.0323V (134d) Younger & van den Bergh (1985)
1986G 3.02V(+/-0.06) 0.0302V (100d) Phillips et. al (1987)
1998bu 3.18v 0.0268V (164d) Lucas (1998) this article
Avg.v 3.56 0.0296v (164d) Doggett & Branch (1985)


LATE LIGHT CURVE/DISCUSSION:

On the enclosed light curve at about 30 days past maximum light the last visual observation from the VSNET forum (mirrored to the ISN) is recorded. This estimate was from J.Ripero (M-1 SN Search Team, Spain) on 6/19/98 and was 13.5 visual (updated 11/1/98 VSNET). A "linear connection" has been installed from this point to the magnitude estimate of this author, taken from the image of L.Robinson on 10/30/98 (16.169 +/-0.1v).

There is some information that can be gleaned from the light curve. Firstly, the 100 day decline of the event is inferred to be 3.18v magnitudes from maximum light. Secondly, the average visual mean light curve from Doggett & Branch 1985 follows the linear decline to about day 61. The mean light curve then runs fainter than the estimated linear decline connecting the two data points. At day 150 the differances between the two is 0.48mv, at day 164 this difference is a bit fainter at 0.46mv....there is no way at this time to determine if the slope of the mean light curve would indeed display such differances from this linear decline, due primarily to a lack of available data points....we are dealing with pure supposition, here.

Contrary to earlier reports about SN 1998bu being fainter than the mean average at early times (0-30d). This pattern appears to reverse itself from about day 61 until day 164. Many more events will have to be monitored at late times to determine if this pattern is real. Of course fill-in magnitude estimates would assist greatly...Good Hunting!

Completed November 5, 1998

Steve H. Lucas (77620.1721@compuserve.com) International Supernovae Network


REFERENCES:

A.Ardeberg & M.deGroot: A&A _28_, 295-304, (1973)
D.Branch et. al: ApJ.,_421_:L-87-90, Feb., (1994)
J.Doggett & D. Branch: AJ _90_,(11), November, (1985)
S.Lucas: This article (1998)
R.Minkowski: P.A.S.P., _52_,206, (1940)
M.Phillips: P.A.S.P., _99_,592, (1987)
M.Pierce & G.Jacoby: AJ _110_ (6), Dec., (1995)
B.Schaefer: ApJ.,_426_:493-501, May 10, (1994)
G.Thompson & J.Bryan: Cambridge University Press (1989)
M.Turratto & E.Cappellaro: AJ, _100_,(3) Sept.,(1990)
P.F.Younger & S.van den Bergh: A & A Suppl. Ser. _61_, 365-373, (1985)

Light Curve


This is the revised light curve for SN1998bu in M96.


Further Developments

Here is some further information recieved via email regarding this observation. It is very frustrating that it has been cloudy every night since October 30 and I have not been able to image M96 again since then. I hope someone else gets some more images.

Nov. 8, 1998

SN 1998bu

Steve Lucas has posted an interesting article on the late-stage magnitudes of SN 1998bu. Being considered "subluminious", the present magnitude seems to be challenging to theorists (esp. to our colleages, Dr. Yamaoka).

Being around mag 16, the SN will provide enough chance for both CCD and visual observers to record the present brightness. Is there any plan of getting a fainter CCD sequence?

Regards,

Taichi Kato


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