Supernova 1999aa in NGC 2595

SN 1999aa - A Third Supernova Discovery


Just 9 weeks after the discovery of SN 1998ey a suspect was found 1" east and 28" north from the centre of NGC 2595 on Feb.11.0166 U.T.. Unlike the previous 2 discoveries, this one was well placed in altitude, near the meridian and in a very transparent sky. At magnitude 15.5 it was very obvious in a 30 second exposure using the f6.3 30cm SCT and Starlight Xpress ccd camera.

Further inspection of 6 images of NGC 2595 taken during Feb. and March 1998 showed no object at the position of the suspect. A Digitised Sky Survey image of the galaxy was downloaded and showed no corresponding object.

From its brightness it was obvious that if it were a minor planet then it should be well known and a search for one was carried out on MegaStar and the Central Bureau for Astronomical Telegram's (CBAT) Minor Planet Checker website which proved negative. Blinking 2 images taken 30 minutes apart should show an obvious motion of 10 to 20 arc seconds for a main belt asteroid as the suspect was near the opposition point.

At this point confirmation was requested from Mark Armstrong who willingingly stopped his own patrolling to take an image of NGC 2595 which duly showed the suspect as described by position and brightness. Having crashed 'Astrometrica' Mark was able use his version and make precise measurments of position on 2 of his images taken some 2 hours 14 minutes apart without detecting any motion of the suspect.

It was now time to notify CBAT and an email was forwarded giving all the relevent information.

Although this was a very strong supernova candidate, a second night's observations were suggested by Tom Boles and as rain was forcast for the U.K., Micheal Schwartz was contacted for help to obtain an astrometric observation of the object a day after discovery. His positional measure was indentical to Mark's and was forwarded to CBAT.

Within 24 hours a spectrum was obtained on Feb 12th by A.V.Filippenko, W. D. Li, and D. C. Leonard, University of California at Berkeley using the Lick Observatory 3-m Shane reflector. From the ccd spectra, the suspect was confirmed as a peculiar type-Ia supernova, and about 6 days before maximum brightness.

This all happened very qickly, as the desingnation (SN 1999aa) and spectoscopic confirmation appeared together on IAUC 7108 less than 2 days since the actual discovery.

This supernova was reported by several other observers after IAUC 7108 had been issued. Beijing Astronomical Observatory recorded SN 1999aa on Feb. 11.6 and 12.6 UT, the first obervation being only 14 hours after the discovery. R.Kushida (Yatsugatake South Base Observatory) recorded SN 1999aa on Feb. 13.663 U.T., 2.5 days after discovery.

M. Villi, Forli, Italy, reported that an image of NGC 2595 obtained on Feb. 6.91 UT showed nothing at the position of SN 1999aa to a limiting magnitude of 18. The Lick Observatory Supernova Search also reported that SN 1999aa was not visible on an image taken on Feb. 5.3 with the 0.8-m Katzman Automatic Imaging Telescope to a limiting mag. of about 19 in the course of the Lick Observatory Supernova Search. All in all, this was another lucky one.

Ron Arbour

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David Bishop
Last modified: Fri Jul 2 12:11:59 EDT 1999