[vsnet-alert 2885] Possible supernova in NGC 3198

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

cf. vsnet-chat 1802
http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-chat/msg01802.html
cf. vsnet-chat 1803
http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-chat/msg01803.html
cf. vsnet-chart 148
http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-chart/msg00148.html


Dear SN watchers,

  Yesterday, IAUC 7149 announced the discovery of a possible supernova
in very nearby galaxy by Lick team.  However, the nature of this new
object is very uncertain, so it is very interesting.

  The location of the new object is: R.A. = 10h19m46s.81, Decl. =
+45o31'35".0 (2000.0), which is about 86" west and 84" south of the
nucleus of NGC 3198.  This SBc galaxy is apparently very large, so
this location is the inside of the spiral arm.  There is a foreground
star with magnitude about 17 (U1350_07913851), from which the new
object is about 28" east and 16" south.  Turry's "Nearby Galaxy
Catalog" tells that the distance module of NGC 3198 is 30.17, which is
about 1 mag nearer than M 87 (Virgo center).

  The reported magnitudes up to now are:

980324.2    >190:C KAI
990329.2    >175:C KAI
990403.2    >175:C KAI
990415.2     184:C KAI
990419.2     180:C KAI
990420.2     178:C KAI

, which indicates that this object is in (relatively) slowly
brightening phase.  

  The mystery is that these magnitudes are very faint as a supernova
in such nearby distance.  Typical SNeIa without absorption will be
11.3 mag at their maximum, and even the gravitational collapse-driven
SNe will be at least 15 mag or so around maximum.  The initial
brightning phase of supernovae (of any type) are usually more rapid
than this object.  The rare example, SN 1987A, had shown a slow
brightning after a short plateau phase (about 1 week, which is
explained as being due to helium recombination), and at this plateau
phase its absolute V magnitude was about -13.5.  If SN 1987A is on NGC
3198, this initial plateau would be observed as 16.7 mag or so, which
is still brighter than the new object.  Before the initial plateau, SN
1987A has also rapidly brightened.

  Then the nature of this object can be one of below:

a. an intrinsically very dim supernova,
b. a normal supernova affected by the extreme absorption,
c. a foreground (Galactic) variable such as a dwarf nova.  

It is needless to say that the spectroscopy is extremely desired, and
also the followup photometry and magnitude estimates.

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

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