OT and the Systematic Automated All-sky Search for Bright Optical Transients Lior Shamir & Robert J. Nemiroff Abstract Real-time detection of bright optical transients has an important role in modern astronomy, and many robotic telescopes have been built and operated for this purpose. However, since traditional narrow- angle telescopes cover only a very small portion of the sky at a given time, one can reasonably assume that some bright short-timescale transients may not be recorded by the available narrow-angle sky surveys. In this study we applied a systematic all-sky automated search for bright short-timescale optical transients. The search used two panoramic all-sky cameras located in the northern and southern hemispheres, and could automatically alert on optical transients brighter than ~5.5th magnitude that lasted for 12 minutes or more. After three years of operation, we recorded a seemingly ~5th magnitude optical transient detected for approximately 12 minutes by two CONCAM all-sky cameras located in Cerro Pachon -- Chile and La Palma -- Spain. The data were also used to deduce upper limits to the frequency of short-timescale bright transient events.

The Night Sky Live 2  all-sky images X 1024 pixels. 180-Second Exposure. Picture taken every 3:56 minutes. Up to visual magnitude 6.8 near the image center.

Transient Detection Fuzzy Logic-Based Cosmic Ray Hit Rejection Bright Planets and Variable Stars (dM>1) Rejection Canonical Image Database Comparison with a Set of Canonical Frames Taken at the Same Sidereal Time All-Sky Image Transients

Canonical Image Database Images are added to the canonical image database based on the following algorithm: 1.Find all point spread functions in the image 2.Search the point spread functions of all stars brighter than 5.2 (V-mag) that should appear in the sky at the time the image was taken. 3.If the PSFs of 96% of the stars are found, the image is added to the database.

Transients are alerted if they persist for 2 consecutive frames (~8 minutes), or detected By more than one CONCAM station at the same time. Geocentric coordinates of transients from different stations are compared, and an alert is triggered in case of a match. Transients 40s brighter than their local background are alerted. This criterion is comparable to ~5.5 visual magnitude in CONCAM3 systems. Alert Criteria Limiting Detection Magnitude ColorMagnitude A4.3 ± 2 F4.5 ± 2 G4.7 ± 2 K4.9 ± 2 M5.2 ± 2

Image recorded at Cerro Pachon on 4/20/06 at 00:19:43 UT (180 second exposure) Image recorded at Cerro Pachon on 4/20/06 at 00:23:39 UT (180 second exposure) OT060420

Image recorded at Cerro Pachon on 4/20/06 at 00:19:43 UT (180 second exposure) Image recorded at Cerro Pachon on 4/20/06 at 00:23:39 UT (180 second exposure)

Image recorded at Cerro Pachon on 4/20/06 at 00:27:35 UT (180 second exposure) The peak magnitude of the flash ColorMagnitude A4.3 ± 2 F4.5 ± 2 G4.7 ± 2 K4.9 ± 2 M5.2 ± 2

Image recorded at La Palma on 4/20/06 at 00:19:43 UT (180 second exposure) Image recorded at La Palma on 4/20/06 at 00:23:41 UT (180 second exposure)

Image recorded at Paranal on 4/20/06 at 18:44 (a), 21:38 (b), 24:27 (c), and 28:54 (d).

Exposure No. StartEnd 100:19:4300:22:43 200:23:3900:26:39 300:27:3600:30:36 Exposure No. StartEnd 100:18:4400:20:16 200:21:3800:23:08 300:24:2700:25:57 400:27:2400:28:54 Start and end of the exposures taken by CONCAM on 4/20/2006 Start and end of the exposures taken by MASCOT on 4/20/2006 Exposure No. StartEnd 100:20:1600:21:36 200:23:1000:24:30 300:29:0000:30:20 possible scenario of several flashes coming from the same location The peak magnitude of the flash ColorMagnitude A4.3 ± 2 F4.5 ± 2 G4.7 ± 2 K4.9 ± 2 M5.2 ± 2

Each CONCAM all-sky camera cover ~1500 deg 2, and the two CONCAMs together cover ~2300 deg 2, which are ~5.69% of the sky. Each CONCAM camera takes ~140 images per night, but since only 25% of the images are clear, each camera produces ~35 clear images per day, or ~ images per minute. Assuming, for instance, that a 480-minute transient brighter than 5.5th magnitude appears once every 24 hours, we would expect to record ~0.66 transient images per day, or ~727 in three years. Since no transient images were recorded, we can conclude with certainty of ~26.96  that based on the CONCAM images, no 480-minute transient brighter than 5.5th magnitude appears once every 24 hours. In the same fashion we can analyze the probability that shorter-timescale transients appear every day and every year. Setting Upper Limits to the Number of Transients The probability that a transient event brighter than 5.5th magnitude does not appear every night. The probability that a transient event brighter than 5.5th magnitude does not appear every year.

By using the frequency of optical transients of different timescales, we can deduce an upper limit (2  ) to the number of optical transients brighter than 5.5th magnitude that are visible in the entire sky at any time. For instance, if we have no more than one 30- minute optical transient brighter than 5.5th magnitude every 11 days, than we have no more than 30/(11*24*60) = 1.89*10 -3 optical transients brighter than 5.5th magnitude at any given time. The same analysis for transients of other timescales shows that there are no more than ~2*10 -3 optical transients brighter than 5.5th magnitude at any given time, anywhere in the sky. Upper limit (2  ) of the frequency of transients brighter that 5.5th magnitude. Upper limit (2  ) of the number of transients brighter that 5.5th magnitude at any given time.