Publications Department
Presented at IAF 2000, Rio de Janeiro, Brazil, 3 October 2000.
Brief beamed transmissions can be more intense than continuous omni-directional signals but require knowing when (and where) to look. For SETI, various phenomena including supernovae and novae have been proposed as "synchronizers" but no major SETI program has yet used any synchronizer. One reason for this is the poor properties of these synchronizers. However, in the last few years our knowledge of gamma-ray bursts (GRBs) has exploded and I propose that GRBs now appear to be the best synchronizers known. Their useful properties include: short duration, large luminosities, high occurrence rate, isotropic sky distribution, and large distances. For targeted SETI, precise synchronizer positions and target distances are required to calculate the synchronizer/transmission time delay. Precise GRB positions are now starting to be obtained from GRB optical afterglows and Hipparcos distance measurements are available. In the next few years two satellites are scheduled for launch (Swift and FAME) which are directly relevant to the use of GRBs as synchronizers. Swift is expected to detect one GRB per day with the capability to localize optical emission to better than 1" and FAME should provide astrometry 20 times better than Hipparcos. Coincidentally, these spacecraft should be launched as the 1hT is being completed.
Keywords: extraterrestrial intelligence - gamma-rays - methods: observational
1. Introduction
In SETI it would be much easier to detect a beacon intentionally broadcasting for others to find rather than "leakage" emission. However, it can be expensive in terms of energy use to operate a powerful beacon for the long periods of time that may be required. One way to consider this is to compare the energetic equivalent of accelerating interstellar probes. While somewhat simplistic, this has the advantage that it is essentially independent of a civilization's total energy generating capacity. i.e. a civilization that is capable of capable of operating a more powerful beacon could instead choose to use the energy for accelerating additional mass.
For example, if the Arecibo 1MW planetary radar were to be operated continuously then the energy expended every 50 days would be sufficient to accelerate a mass of approximately 1 kg to 1% of the speed of light. While it is not possible to know what mass of vehicle may be used by an advanced civilization, note that a team at Johns Hopkins University's Applied Physics Laboratory have proposed a "realistic interstellar explorer" that would have a mass of about 50 kg (McNutt et al. 2000). Thus the advantage of beacons over probes merely on the basis of energy requirements is not as clear cut as sometimes assumed as long as modest mass probes are assumed. This is particularly true if very long transmission periods, such as millions of years, are considered.
For a frugal civilization operating a beacon it would clearly be advantageous if energy saving strategies can be used. One potential technique is to transmit only a relatively brief and highly beamed signal. However, for such a transmission scheme to be feasible, the problem is for a transmitter and a recipient, one or both unknown to the other, to find a strategy that will enable the transmitter and receiver to transmit and observe at the right time and direction.
A strategy to achieve transmitter/receiver synchronization that has been considered by a number of authors is to utilize natural astronomical events, see, for example, Pace & Walker (1975), Tang (1976, 1981), McLaughlin (1977, 1986), Makovetskii (1978, 1980), Pace (1979), Gruber & Pfleiderer (1982), Siebrand (1982), Hilton & Almar (1988) and Lemarchand (1994). In the simplest scheme omnidirectional signals would be transmitted at the occurrence of some particular event such as a nova outburst, maximum flux of a long period variable, specific binary phase, or supernova occurrence. A signal would then be detected at the Earth delayed by a time corresponding to the difference between the event/Earth distance and the event/transmitter + transmitter/Earth distances. This time delay is thus given by:
where Rs is the distance to the synchronizing
astrophysical event,
D is the
distance to the transmitter,
and theta is the angular separation
as viewed from the Earth.
For an event located at a very large distance from both the transmitter and receiver, the time delay between the detection of the event and the transmitted signal is given by the simplified expression:
To simplify further still, for small angles and theta in radians:
The accuracy with which Delta T can be calculated thus depends on both the distance to the object and theta. The smallest errors on the value of time delays are produced for the nearest targets and the smallest angular separations.
In addition to simply using the event as a time marker directionality is introduced if a beamed transmission is made in a direction either exactly or approximately away from the event.
In spite of these various suggestions of using astrophysical synchronizers, only limited observations using this technique have been made with Forbes & Westpfahl (1988) and Colomb et al. (1992) among the few exceptions. Some reasons for this lack of observations may include: (i) Observations must be made at a specific time and dedicated SETI observatories are not common, (ii) the times at which observations must be made have not been well defined as the proposed synchronizers do not give well defined times and accurate target distances (parallaxes) also required to define the observation time have not been available, and (iii) skepticism and misunderstanding within some portions of the SETI community.
In this paper the use of one particular type of natural synchronizing signal is considered - the phenomenon of gamma-ray bursts (GRBs). These appear to posses a number of important advantages over other possible astrophysical events and their use in SETI is advocated for targeted observations of relatively nearby stars. Observations facilitated by the launch of the GRB observatory Swift and the next generations of astrometry satellites are considered in particular. Additional information on this topic can also be found in Corbet (1999).
2. Targeted Compared to Exactly 180 degrees Transmissions
In principle a beamed transmission program could use astrophysical synchronizers in two different ways. A transmission could either be made directly away from the event or else stars "close" to the opposite direction of the event could be targeted. Although the first technique will eventually result in complete coverage of the entire sky a significant problem is that there are two competing constraints for the beam width of the transmitted signal. If a narrow beam width is used then it will take a very long time to illuminate the entire sky. However, if the beam width is large then the signal is weaker and, in addition, the time delay between a receiver's detection of the astrophysical event and the transmitted signal will be larger if the receiver is not at the center of the beam.
For transmissions exactly 180 degrees from the synchronizer the concept of a "natural" beam width was previously suggested (Corbet 1999). This beam width is chosen so that at a specified distance, D, (assumed to be the maximum useful transmission distance) the maximum time delay experienced by an observer at the edge of the beam will be equal to the average repetition time of the astrophysical event. This leads, independent of the repetition rate of the astrophysical event, to a time required to (non-uniformly) illuminate 4 pi steradians = 2D/c. This long time scale for any significant distance, together with the requirement to use relatively broad beams, makes this technique less useful.
In contrast, transmitting to specified targets close to the anti-event direction does not suffer from this problem. There will be a time delay, but it is possible in principle for the receiver to calculate this, and also beam widths as narrow as possible may be used. The remainder of this paper therefore assumes that targeted signalling alone would be used.
3. Requirements for an Astrophysical Synchronizer
There are a number of requirements for an astrophysical event to be an ideal communication synchronizer:
4. Gamma-ray Bursts as Synchronizers
Gamma-ray Bursts (GRBs) appear to posses all of the requirements listed above. For more details on GRBs see e.g. Corbet (1999) and references therein. In brief, GRBs, which for many years had been extremely poorly understood have now had many of their properties elucidated through the discovery of optical and X-ray afterglow emission. While the physical cause of GRBs is still under debate, the exact GRB mechanism is irrelevant to their use as synchronizers as long as their phenomenological properties meet the requirements listed above.
5. Other Candidate Astrophysical Synchronizers
Supernovae were among the objects first proposed as synchronizers (Tang 1976) and are also energetic phenomena releasing ~ 10^{51} ergs. However supernovae lack the beaming that is assumed to make GRBs so apparently luminous. There are also several other drawbacks to using supernovae as synchronizers compared to GRBs. The primary one is that optical light curves of supernovae are relatively slowly varying, taking days to reach their peak brightness and so are significantly less sharp time markers. If the initial neutrino pulse from a supernova (e.g. Bionata 1987) could be used that would give a much sharper time marker. However, even if an alien transmitter could detect neutrinos easily we cannot yet except for very nearby supernovae. Galactic supernova occur only infrequently while the prompt detection of all extra-galactic supernovae down to a specified flux level is also more difficult than in the case of GRBs. Extragalactic supernovae can now be found in relatively large numbers but only fairly small areas of the sky are surveyed (Perlmutter et al. 1995, Schmidt et al. 1998).
Novae and other variable stars have also been proposed as possible synchronizers. Novae are significantly less energetic than supernovae, do not define a time very precisely, and, in the case of Galactic novae, it may also be necessary to accurately know the distance to the nova as well as the transmitter to be able to calculate time delays. Variable stars form a broad diverse "class" of objects. Their much lower luminosities and varied nature makes them less obvious as the synchronizers that would be universally used.
6. Practicalities
6.1. Constructing a Transmission Program
The transmitting civilization would draw up a list of "interesting" targets. For a contemporary terrestrial signaler (e.g. Dutil & Dumas 1998) a target list would likely be similar to those used for SETI programs.
The smaller the target list can be made, the more energy efficient the transmission program will be. An alien civilization may perhaps be much more restrictive. It could conceivably only broadcast to stars known to contain terrestrial mass planets within their habitable zones or even only to those planets known to be home to industrial civilizations (via e.g. the detection of radio emission or changes in atmospheric gases such as an increase in CO2 concentration).
The other decisions that need to be made by the transmitter are: (i) how large the maximum angular distances will be between transmission direction and anti-event direction and (ii) how frequently to transmit based on an intensity threshold for the astrophysical synchronizers that will be used. Even if an event exceeds the intensity threshold no transmission will be made if there is no target sufficiently close to 180 degrees from the event. Note that it is only necessary for the transmitter to immediately know the location of the GRB to somewhat better precision than the angular constraints. However, transmission should be made as rapidly as possible after the GRB occurs.
6.2. Constructing an Observing Program
To use GRBs for coordinating a SETI program two things are needed: (i) a steady stream of GRB detections with precise locations and (ii) very accurate distance measurements for target stars.
The current best set of stellar distances comes from Hipparcos parallax measurements (Perryman et al. 1997) which gave values accurate to ~ 1 milli-arcsecond. Several new astrometric satellites are now being planned. The most ambitious of these are NASA's Space Interferometry Mission (SIM, Unwin et al. 1998) and ESA's GAIA (Gilmore et al. 1998) which may yield large numbers of parallaxes with precisions better than ~ 10 micro-arcseconds. SIM is scheduled to operate from 2006 to 2011 while GAIA, if accepted by ESA, could launch in 2009 with a 5 year lifetime. SIM would provide astrometric measurements of 10,000 stars and GAIA would measure ~ 10^9 positions. The next scheduled astrometry mission, however, is FAME (Full-sky Astrometric Mapping Explorer; Horner et al. 2000). While FAME's astrometry goals are somewhat less ambitious it will still yield parallaxes accurate to 50 micro-arcseconds for stars brighter than mV = 9 and it is scheduled to be launched in 2004.
With the re-entry of CGRO, BATSE has ceased operations. The SAX mission continues to function but provides less frequent (although precise) GRB positions as events must occur within the field of view of its wide-field X-ray detectors. The HETE-II mission (Ricker 1997) should also provide about 30 GRB locations per year and is of interest, especially if precise GRB positions can be obtained from ground-based optical follow-up observations. However, the most ambitious mission for localizing GRBs so far is Swift (Gehrels et al. 1999) which is expected to detect about one burst per day. Swift will carry a gamma-ray detector, an imaging X-ray telescope and a small UV/optical telescope. Swift is designed to produce positions accurate to better than an arcsecond if optical emission is also detected and the gamma-ray detector will view an area of 2 steradians. The communication of crude GRB locations to Swift from other satellites with larger fields of view would enable even larger numbers of GRBs to be precisely localized. Swift is currently scheduled for launch in 2003.
So, as FAME astrometry starts to become available several years after its launch, and with Swift still operating at that time, can a realistic synchronized SETI observing plan be constructed using the results from these two satellites?
One "back of the envelope" approach is to set a time limit for completing observations to a certain depth. For example, consider a a program limited to a length of N days. Then, if accurate GRB locations are determined once per day the total sky area that would potentially be available for observations would be ~ pi theta^2 N. If we wish this area to be (a non-uniformly observed) 41,000 degrees^2 within a five year program then theta ~ 2.7 degrees. Now assume that the program will use the ~ 1000 stars in the Project Phoenix G dwarf sample as targets. These are old single solar-type stars within 50 pc of the Sun (Henry 1995). Of course, as GRB locations are randomly distributed on the sky some targets would not be observed during the specified time period whereas other targets would potentially be observable more than once if desired. At distances of up to 50 pc and angular separations of up to 2.7 degrees an artificial transmission would be delayed by up to about 66 days. The time delay uncertainty for a 50 micro-arcsecond parallax uncertainty would be up to ~ +0.16 days. While an observation lasting ~ 0.3 days would be considerably longer than the few hundred seconds spent per target in a program such as Project Phoenix this would still represent only ~18% of the observing time since only about half of the GRB anti-directions will contain a target within 2.7 degrees. Additionally, since observations would have to be done by a set of ground-based observatories if Earth occultation of the target is to be avoided the percentage of the time each observatory spent on these time critical observations could be reduced by the total number of observatories.
Next consider observations at twice this distance at 100 pc with about 8000 targets and the same five year program. In this case the time delay at a 2.7 degree separation would be ~130 days and the uncertainty would be ~+0.7 days and about 4 stars would have to be observed per day. For this program observations would be greatly facilitated if GAIA/SIM type 10 micro-arcsecond accuracy parallaxes were available which would yield better than +0.13 day errors on the time delay. Note that at a distance of 100 pc and an angular separation of 2.7 degrees an uncertainty in the position of the GRB of 0.3", the size of the point spread function of the Swift UV/optical telescope, would itself give a time delay uncertainty of only ~ +12 minutes.
Naturally, if a longer time was allowed to undertake the program, or more frequent GRB positions were available, then smaller angular separations could be used which would yield more accurate time delay predictions.
7. Searching with Many Small Telescopes
If a SETI observatory consists of an array of telescopes such as the 1hT then perhaps a smaller sub-array could be devoted to the longer synchronized observations while the major portion of the array made shorter unconstrained observations.
The SETI League is undertaking a project that would utilize a large number of rather small radio telescopes to search for extraterrestrial signals (e.g. Shuch 1997). In the search for a continuous beacon these small telescopes are considerably inferior to, for example, the Arecibo telescope. However, for brief pulsed emissions a large array of small telescopes with flexible pointing schedules can have a big advantage, especially if the components are well dispersed in longitude and cover both the northern and southern hemispheres. Regardless of how large a telescope is, nothing will be detected if a transmitter is not operating while the telescope is pointed at it.
8. Conclusion
Gamma-ray bursts posses a number of properties that make them very good candidates for synchronizers that could aid in the search for brief beamed extraterrestrial signals. GRB positions are now starting to be obtained with sufficient precision to make them useful for targeted searches due to the detection of optical afterglows. This is augmented by precise measurements of stellar parallaxes from satellite borne instruments that should be obtained by forthcoming astrometry satellites.
The use of astrophysical events for synchronizing SETI is a so-far almost unexplored "search space". This technique may used for searches in any waveband (e.g. radio, optical, or other) and provides the possibility of small telescopes being able to beat much larger telescopes to a discovery.
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