OT060420 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.

• 1024 X 1024 pixels.• 180-Second Exposure.• Picture taken every 3:56 minutes.• Up to visual magnitude 6.8 near the image center.

http://concam.net/ha/

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

Color Magnitude

A 4.3 ± 2

F 4.5 ± 2

G 4.7 ± 2

K 4.9 ± 2

M 5.2 ± 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

Color Magnitude

A 4.3 ± 2

F 4.5 ± 2

G 4.7 ± 2

K 4.9 ± 2

M 5.2 ± 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.

Start End

1 00:19:43 00:22:43

2 00:23:39 00:26:39

3 00:27:36 00:30:36

Exposure No.

Start End

1 00:18:44 00:20:16

2 00:21:38 00:23:08

3 00:24:27 00:25:57

4 00:27:24 00:28:54 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.

Start End

1 00:20:16 00:21:36

2 00:23:10 00:24:30

3 00:29:00 00:30:20

possible scenario of several flashes coming from the same location

The peak magnitude of the flash

Color Magnitude

A 4.3 ± 2

F 4.5 ± 2

G 4.7 ± 2

K 4.9 ± 2

M 5.2 ± 2

Each CONCAM all-sky camera cover ~1500 deg2, and the two CONCAMs together cover ~2300 deg2, 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 ~0.0243 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.