AUTOMATED SEARCH FOR TRANSIENT OPTICAL POINT SOURCES

P. Boynton, R. Kennicutt, and D. Tomandl

Department of Astronomy University of Washington

Seattle, Washington 981 95

INrRODUCrION

A variety of short time-scale optical phenomena may be associated with the formation and subsequent behavior of collapsed objects. Recent detection of y-ray bursts suggests a t least one population of transient sources, whose roughly isotropic distribution suggests a local origin (within a few kpc).’

Possible classes of events that range from nuclear goblins2 to neutron star quakes3 could be responsible for various kinds of burst emission susceptible to optical detection. However, few attempts have yet been made to actively sesrch for such phenomena.’6

In this paper, we describe a proposed system with background-limited per- formance that has a I-ster field of view, is capable of detecting a 10th-magnitude event with duration of 1 sec, and that specifies its position within 20 arcmin. The system consists of two vidicon units separated by several kilometers. Parallax and coincidence conditions discriminate against atmospheric phenomena and hold the accidental event rate to less than one per year. System sensitivity is greatly im- proved for a restricted field of view appropriate to monitoring specific candidate regions.

GENERAL REQUIREMENTS

Although we consider the problem of narrow field (down to I ”) burst detection, because of the possibility that detectable sources are local and infrequent, a wide- field long-term search seems desirable. Such a n effort is interesting to the extent that it yields information on the spatial distribution and time structure of transient events. The sensitivity, in terms of noise-equivalent flux density, for a background- limited system depends on measurement resolution in both space and time. In ad- dition, spatial and temporal resolution must be compatible with real-time image processing. In the following discussion, the resultant compromise between various aspects of system capability has been influenced by the characteristic milliseconds- to-seconds time-scale anticipated for collapse, accretion, and related phenomena. Thus, guidelines for design may be summarized as follows: wide field of view with “interesting” angular resolution, time resolution appropriate to anticipated time structure of events associated with collapsed or collapsing objects, real-time image processing and event identification, maximum detection sensitivity consistent with the first three guidelines, and an automated system dedicated to a long-term ob- serving program.

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210 Annals New York Academy of Sciences

S Y S T E M DESIGN

These guidelines have led us to formulate a system with an SIT/EBS vidicon (silicon vidicon with pretarget gain) as an image-recording detector. The largest practical field for - f / l imaging optics is roughly 60” (1-ster coverage). We have chosen 25,600 image elements (160 x 160) as the largest manageable number for storage and processing while allowing image integration times down to 0.1 sec with negligible dead-time. This choice results in 20-arcmin angular resolution. Image processing consists of storing and continually updating averages of both the mean and variance of the signal in each of the 25,600 image elements. Concur- rently, on each scan, an element-by-element test tags those that deviate from their updated mean by more than five standard deviations. With the assumption of a gaussian distribution for these deviations and requiring coincidence between two independent detectors, this 50- threshold implies an “accidental” event rate of less than one per year for continuous operation.

Only modest pretarget vidicon gain (< 100) is required to ensure that statistical fluctuations in sky background counts dominate various system noise contribu- tions. Under background-limited conditions, system sensitivity in terms of noise- equivalent flux density (NEFD: flux equal to one standard deviation noise level) is given by

where D is the linear dimension of a picture element on the photocathode, d de- notes the linear size of the limiting optic aperture, and f represents the focal length. Bsky is the sky brightness, and 7 is the integration time. The detector format size (80 mm) and number of picture elements specify D. Given the practical limitation that the focal ratio cannot be less than unity, performance is completely specified by 8 (the field of view), 7 , and B“:,. For a 1-sec integration period and a nomi- nal dark sky (8 x erg sec- ~ r n - ~ ster-l or 3 x lo4 photoelectrons/sec/pic- ture element),

NEFD = 2 x tan2(8/2)erg sec-’ Hz-’.

This corresponds to about a 12th-magnitude sensitivity limit for a 60” field, which is roughly a 10th-magnitude limit a t the 50- level. A 100-fold increase in sky brightness results in a 2.5-mag reduction in sensitivity.

Pretarget gain is actually chosen as a compromise between noise performance and target saturation. For dark sky and I-sec integration, a gain of 20 allows op- eration a t about 10% of saturation. For a two-order of magnitude increase in sky background, g = 1 is adequate to provide background-limited performance at about 50% of saturation.

An additional difficulty is scintillation noise. A 10th-magnitude or brighter star in an image element will probably dominate the sky noise contribution through scintillation fluctuations, which reduces the sensitivity in those elements. For views out of the galactic plane, approximately 30% of the elements will be ad- versely affected. Under brighter sky background conditions, this effect becomes

Boynton el al.: Transient Optical Point Sources 21 I

less important. At the 8th-magnitude level, even observations in the galactic plane are not seriously affected.

Clearly, photographic film has two major advantages over electronic image recording. From the expression

D f * d ’

N E F D a -

it is clear that the much-improved spatial resolution and larger possible format of photographic schemes can give a manyfold improvement in sensitivity. However, the use of the Prairie Network camera system’ seems practical only as a passive monitor, checking for optical signatures of events with previously determined position and time. As an example, modifying the Prairie Network system to give a 12th-magnitude sensitivity limit would require 50- 100 exposures per camera per night. Although this amount of exposures is not necessarily a drawback for a pas- sive monitoring program, it does constitute a time-consuming data reduction problem for an active search.

In addition to questions of sensitivity, angular resolution, and response time, there are further design considerations.

Automated Operation

To achieve maximum observance time, these units must be automated to the extent that only brief daily or weekly intervention is required. Automatic tracking with zenith angle less than 15” (except for moon avoidance) seems desirable.

Pa railax

Two stations operated on a 3-km baseline provide adequate parallax to dis- criminate against atmospheric phenomena. Furthermore, by scanning the two images “out of step,” the system becomes largely immune to electrical transients.

Pattern Recognition

Meteor trails are the largest single transient background. Because meteors can produce false coincidences, even within the requirements of the parallax test, ele- mentary pattern recognition must be attempted for each pair of frames to detect possible confusion. The same procedures eliminate lightning, aurorae, and other effects. The computing hardware already required for image processing is ade- quate to meet this task within the context of real-time processing.

NARROW-FIELD PERFORMANCE

Access to two 24-in. f/.35 Schmidt optical systems has led us to ask about the performance of this vidicon system for active searches in specific candidate loca-

212 Annals New York Academy of Sciences

tions, such as compact x-ray sources (which lack optical identification), globular clusters, along spiral arms, external galaxies, and so on. For a fixed aperture, the sensitivity of this system (for 0 < 20") can be expressed as

NEFD = 1.2 x tan (0/2) erg sec-' cm-' Hz-I.

For a 5" field, this implies a 15th-mag 50 sensitivity or 17th mag for a I " field (about the practical limit in terms of tracking resolution and vidicon gain). Also, for narrow fields, the resolution time can be greatly reduced by scanning only a portion of the full raster. The 1" sensitivity of erg sec-' cm-* Hz-' is a considerable improvement over previous search attempts, although a comparison based solely on NEFD is somewhat simplistic. In addition to transient phe- nomena, there are obvious applications of this equipment in studying periodic light variations.

SUMMARY

The following plot of NEFD (FIGURE 1) summarizes the capabilities for wide and narrow field-of-view operation with this vidicon system.

APERTURE (mm) 200 150 100 00

I I I

5

F I X E D APERTURE 24 INCH SCHMIDT OPTICS

10'270 10 20 30 40 50 FIELD OF VIEW (deg)

FIGURE I .

1

W n

V :'p 17 2

ACKNOWLEDGMENT

We thank Paul Zucchino, Princeton University, for helpful discussions.

Boynton et al.: Transient Optical Point Sources 213

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