[vsnet-alert 1751] suspected SN and GRB 980425

[Yamaoka Hitoshi <yamaoka@rc.kyushu-u.ac.jp>]

Additional notes to Dr. Skiff's comment: 

>     IAUC 6895 contains interesting news about an eruptive object
>possibly associated with the recent gamma-ray burst GRB 980425.  The
>new object appears in a spiral arm of the small galaxy ESO 184-G82.
(snip)
>     The new object is presently about mag. 15 and has brightened
>considerably in recent days.  

The same IAUC informs that this new object was photomated by ESO
group as brightening about delta_V = 0.8 mag during May 3 - 6 (only
three days!) and delta_R = 1.7 mag during Apr. 28 - May 6.  On May
6.4, it was observed that V = 14.00 and R = 14.04, which can be
observed by not-so-large telescope.  

>Apparently the spectrum is not that of a supernova and remains
>indecipherable for the moment. 

The spectrum is also available at
http://sc6.sc.eso.org:80/~ohainaut/SN/spec.ps .  It shows some broad
absorption lines, which indicates that this is an explosive event. 
Also, this new object was detected (as very intence) in radio (IAUC
6896), which can be the evidence that it is interacting with a dence
circumstellar matter. 

The heliocentric recession velocity of the galaxy ESO 184-G82 was
measured as 2528 km s-1 (ESO group, IAUC 6895), from which this galaxy
is about 2-2.5 times further than Virgo.  Normal SNe Ia at this
distance would be 14 mag or so at maximum, though at this moment this
object shows a very different spectrum than SN Ia near the maximum. 

As a gamma-ray source, GRB 980425 was not so bright.  The first GRB
whose optical counterpart was certainly detected (GRB 970228) was
about ten times brighter (apparently) than GRB 980425.  Also, the
gamma-ray light curve of GRB 980425 appeared to be single-peak, on the
other hand that of GRB 970228 was structured by many peaks.  

There remain many puzzles: whether this new star is SN or not; whether
this new star is associated with the galaxy ESO 184-G82 or the
background or foreground event; whether it is related with GRB 980425
or not.  The photometric followup are definitively useful to
understand the nature of this stupid object. 

Unfortunately, the sequence provided by ESO group and Dr. Skiff was
constructed by 3 mag or more dimmer than the new star. Could you
prepare a brighter sequence, Taichi or others?

Sincerely,
Hitoshi Yamaoka, Kyushu Univ., Japan
yamaoka@rc.kyushu-u.ac.jp

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