[vsolj-alert 109] suspected SN and GRB 980425

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

山岡＠九大理 です。

ガンマ線バーストGRB 980425の光学対応天体候補について、IAUC 6895,6896に
かなりの分量の情報があります。非常に興味深い天体なんですが、いかんせん
南天なので、日本から見ることができません(SN 1987Aみたいだ)。

撮像やスペクトル観測がESOのグループによって行なわれています。
http://sc6.sc.eso.org/~ohainaut/SN
で見られます。スペクトルを見ると爆発天体ではあるようなのですが、通常の
どのタイプの超新星のものとも似ていません。爆発後少なくとも8日は経って
いるので(最初にこの位置に可視光で見える天体が確認されたのは4月28.4日、
最も最近スペクトルを撮ったのは5月6.4日)、普通の超新星ならどのタイプで
も観測例がある時期ではあります。

この天体がGRB 980425の光学対応天体なのかどうか、また重なって見える銀河
ESO 184-G82の中の天体なのかどうか、さらにこの天体の正体は一体何なのか、
さまざまな謎があり、非常に興味深いものです。

以下に、vsnet-alertに投げたarticleを(引用部を省略して)添付します。

			九州大学(六本松地区)理学部物理学教室  山岡  均
			〒810-8560 福岡市中央区六本松4-2-1
			yamaoka@rc.kyushu-u.ac.jp

Additional notes to Dr. Skiff's comment: 

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.  

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

VSNET Home Page

Return to Daisaku Nogami

---

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