A Beacon-Oriented Strategy for SETI

by Paul Gilster on October 31, 2008

I’ve spent so much recent time on two SETI/METI papers by James, Gregory and Dominic Benford because they contain powerful arguments for re-thinking our current SETI strategy. By analyzing how we might construct cost-optimized interstellar beacons, the authors ask what those beacons might look like if other civilizations were turning them toward us. The results are striking: A distant beacon operating for maximum effect consistent with rational expense would offer up a pulsed signal that will be short and intermittent, recurring over periods of a month or year.

It will, in other words, be unlike the kind of persistent signal that conventional SETI is optimized to search for. Searches designed to sweep past stars quickly, hoping to find long-lasting beacons whose signature would be apparent, would rarely notice oddball signals that seem to come out of nowhere and then vanish. Tracking such signals, looking for signs of regularity and repetition, calls for a different strategy.

Image: Looking toward galactic center, a line of sight that may offer our best chance to intercept an interstellar beacon. The center of the Milky Way is behind the center of the photo. As many as thirty Messier objects are visible, including all types of nebulae and star clusters. The lines through the picture were caused by airplanes, and the dark objects in the foreground are trees. Credit: NASA/Dave Palmer.

But let’s stop for a moment. Given our inability to know who might be transmitting to us, how can we make any assumptions, given that we’re relating our own optimum cost strategy for beacons to theirs? The principle of parsimony seems reasonable to us — economic demands have a shaping effect on any project, especially long-term ones like interstellar communication. But can we assume the principle is equally valid for alien civilizations?

The Benfords’ METI paper analyzes these concerns, and their SETI paper summarizes nicely:

The optimum cost strategy leads directly to a remarkable cost insensitivity to the details of economic scaling. The ratio of costs for antenna area and system power depends on only the ratio of exponents… and not on the details of the technology. That ratio varies on Earth by only a factor of two. Both these costs may well be related principally to labor cost; if so it cancels out. This means fashions in underlying technology will matter little, and our experience may robustly represent that of other technological societies.

It’s an interesting argument to chew on and one developed at much greater length than I can summarize here — a read through the relevant sections will expose you to elegant and ingenious reasoning. And if we assume that cost-optimized beacons do exist, the interesting thing is that current SETI methods would be unlikely to find them. In current practice, search dwell times (the time when the apparatus is looking at a specific place) are only a few seconds long in survey operations, while ranging between 100 and 200 seconds for targeted searches. Couple that with long integration times on the order of 100 seconds — this means that short pulses tend to be integrated out of the results.

If we’re looking for short, pulsed signals rather than persistent ones, we’re also hampered by the fact that in most SETI searches to date, the amount of time between when the observer first looks at a target and then looks back at it in an attempt to confirm it is often a matter of days. It can be years when we’re talking about data from the SETI@Home effort. What it would take to identify beacons of the kind described here are searches of the galactic plane that operate for lengthy time frames, on the scale of years. Slow and steady does the job.

Our distant beacon will, given these assumptions, show up in our detectors for a much shorter period than conventional SETI assumes. It’s intriguing that previous searches have indeed noticed at least a few pulsed, intermittent signals that resemble this description of a beacon’s activities. A survey of galactic center reported on in 1997 by W.T. Sullivan (reference below), for example, could confirm nothing in repeat observations that would indicate a persistent beacon. But it did record “…“intriguing, non-repeatable, narrowband signals, apparently not of manmade origin and with some degree of concentration toward the galactic plane…” Again, these are one-time signals that do not repeat within the search period.

A close look at GCRT J1745-3009, an unusual transient bursting radio source some 1.25° south of galactic center, first discovered in 2002 and re-observed several times since, is a useful chance for the Benfords to do a beacon analysis using their parameters. It’s an interesting source, to be sure, with explanations from masers to flare stars, double neutron star binary pulsars, white dwarf pulsars and more all failing to fit the observations. The paper’s analysis shows that GCRT J1745-3009 is unlikely to be artificial, but if it is a cost-optimized beacon, it must be targeted given the fact that the field of stars it covers is quite small.

A rational follow-up strategy might be as follows:

1) Stare at the direction of GCRT J1745-3009 at higher frequencies as both cost optimization and higher information-carrying ability argue. Another information-bearing signal could be at the optimum high frequencies, ~10 GHz. A temporal analysis should be conducted to search for structure in the bursts, since measurements to date have not looked for any message content.

2) look in the opposite direction, 180 degrees from the Center, to see if there’s another beam communicating toward GCRT J1745-3009.

The method could tell us whether we’ve by chance intercepted an interstellar communication link. This particular source seems an unlikely candidate, but it is interesting enough to trigger subsequent study and gives us a chance to put the cost-optimized strategy to work. Another interesting candidate is the so-called ‘WOW’ signal seen at Ohio State in 1977. A year-long campaign directed at galactic center might uncover a follow-up, but it also gives us an idea of the kind of effort the search for the fleeting signals of such beacons might involve.

Image: The WOW! Signal. Credit: The Ohio State University Radio Observatory and the North American AstroPhysical Observatory (NAAPO).

We will need to be patient and wait for recurring events that may arrive in intermittent bursts. Special attention should be paid to areas along the Galactic Disk where SETI searches have seen coherent signals that are non-recurring on their limited listening time intervals. Since most stars lie close to the galactic plane, as viewed from Earth, occasional pulses at small angles from that plane should have priority.

Thus the call for systematic scans of the entire galactic plane, emphasizing the importance of galactic center, with steady observing periods covering a span of years. For reasons discussed in the METI paper, the 10 GHz range is optimum as opposed to lower-frequency, more conventional SETI choices. Re-thinking our SETI strategy offers the chance to maximize the chances for detection, just as, the Benfords show, analyzing the demands of interstellar messaging (METI) can provide significant insights into the kind of signal we might find.

The papers are James Benford et al., “Cost Optimized Interstellar Beacons: METI,” available here, and Gregory Benford et al., “Cost Optimized Interstellar Beacons: SETI,” available here. As I’ve mentioned before, these papers work as a unit and should be read together, the one illuminating the other. The Sullivan paper I discuss above is Sullivan et al., “A Galactic Center Search For Extraterrestrial Intelligent Signals,” Astronomical and Biochemical Origins and the Search for Life in the Universe, IAU Colloquium 161, Publisher: Bologna, Italy, p. 653.

{ 26 comments }

tacitus October 31, 2008 at 15:36

The beacon idea has a lot of merit — but only if it’s tied in with some form of bootstrapping. But that I mean you have a “we are here” signal to attract the attention of alien civilizations, but once you have their attention, you need to do something else with it.

Given that a powerful sweeping beacon is likely to little to no bandwidth available, as the sender, you will probably have to bank on the expectation that once aliens have seen the beacon they will be focusing all their instruments and attention on the tiny patch of sky where they saw it.

Thus there should be some form of companion signal, perhaps much weaker but loaded with some level of information useful to the receiver. If we’re very lucky, that information would, say, provide us with the math necessary to find an even better way to hook into their interstellar internet. So, I would think, in the end we’ll be looking at a three stage signaling system:

1) Beacon — “we are here”
2) Instructions — “this is how you talk to us”
3) Communications — “Hello!”

tacitus October 31, 2008 at 15:52

I still wonder if it isn’t more likely that a SEGI (search for extra-galactic intelligence) would be more fruitful than a plain old SETI regarding beacons. If ETI is commonplace in our galaxy, then it is likely they are already here, on their way, or just about to get news that there is something here worth visiting. Any old interstellar civilization that is interested in seeking out new life, new civilizations is likely, at least, to have set up a network of powerful autonomous telescopes to keep an eye open for signs of civilization in the galaxy’s inventory of inhabitable planets. We have been modifying our planet’s atmospheric signature at least since the Industrial Revolution in ways that can probably be picked up from many light years away already.

But suppose that ETI is very rare, as some suggest. Perhaps not even all galaxies harbor a successful ET civilization. As an advanced civilization, once you have explored the bounds of your home galaxy and found no one else to talk to, what do you do then? My guess is that you start reaching out into the vastness of intergalactic space to make contact with other species who might live far away in other galaxies. Sure, it would take millions of years to do so, but once you have reached a certain point, then what else would there be left to discover about the universe anyway? And it’s quite likely that such an advanced race will have conquered the inconvenience of death in some way, so time would no longer be as limiting a factor as it is for us.

And so, given the millions (billions?)) of galaxies visible from Earth, I would not be at all surprised if, one day, we discover a beacon hidden amongst them. It would not be as “immediate” as finding one in our own backyard, but it would be a world-changing moment, nonetheless.

ljk October 31, 2008 at 16:08

At the present time, no professional astronomer would seriously
suggest that a celestial object like GCRT J17445-3009 might be
artificial in origin. At least not unless they want to be ostracized
by their peers.

When the first solid evidence for an alien artifact does appear,
however, there will be a mad scramble while everyone and his
or her brother claim they knew it all along!

There have been past searches in the Wow! signal area of space,
none successful of course. I believe in addition to several radio
amateur attempts, the VLA was even involved in looking for the
Wow! signal. How appropriate if so, ala Contact.

This article from 2007 says that the Allen Telescope Array will
search the Wow! signal region once it is up and running. There
is also a link to a 30th anniversary report on the signal by its
finder, Jerry Ehman:

http://cosmiclog.msnbc.msn.com/archive/2007/08/15/319127.aspx

stargazerdude22 October 31, 2008 at 16:10

We should dedicate one radar telescope and leave it pointed in the direction of the “WOW” signal indefinitely. I’ve always thought that the transient, non-repeatable signals that we’ve seen may be due to scintillation effects from ETI sources very far away. Now, the most powerful bursts that Earthers give out are for radar mapping of other solar system objects; so at what frequency is that? Is there an optimal “mapping” frequency? Perhaps we should look at that frequency vs. the watering hole. Plus, due to the fact that our atmosphere is opaque to the higher interesting frequencies… we need a big ear on the far side of the moon.

vavatch October 31, 2008 at 20:23

Is there a link to any more information about GCRT J17445-3009? Google only returns a link to this page and no other – wondering if a typo crept in somehow.

Administrator October 31, 2008 at 21:19

Odd. I just checked Google and vavatch is right — we’re the only one showing GCRT J17445-3009. I did a cut and paste from the Benford paper and went back and checked there, but can find no typo. Let me run that past Jim Benford and see what I come up with, vavatch. Until I find anything else, the best source is the Benford SETI paper, which also offers several papers on this one in the bibliography:

http://arxiv.org/abs/0810.3966

Administrator October 31, 2008 at 21:28

Whoops. I think it may be GCRT J1745-3009 (instead of GCRT J17445-3009) — I’ve written Jim for a check on this, but run GCRT J1745-3009 exactly as is through Google, vavatch, and you’ll get a bunch of entries. I don’t have time tonight to check these out, but I bet what happened is that the paper accidentally added an extra ’4′ to the term. I’m sure enough about this that I’ve corrected the original entry.

ljk October 31, 2008 at 21:40

It is a shame we could not keep Big Ear at Ohio State
University aimed at the Wow! signal region, or even just
observing the sky at all.

In 1998 the radio telescope which had conducted the longest
SETI program in history was torn down to make way for a
golf course and condos.

http://www.setileague.org/photos/bigear00.htm

Maybe this is why we haven’t found anyone else yet.

ljk October 31, 2008 at 21:44

Any new information on this unusual radio object?

http://www.centauri-dreams.org/?p=1482

Benjamin November 1, 2008 at 2:39

As for extra-galactic beacons,

We’re talking about enormous distances and an inverse square law, here. I’d be sceptical about the likelihood of any meaningful signal being received except if it was being broadcast with some exotic physics. Though, you never know…

And vavatch, would your name be from Consider Phlebas, after the orbital? That is, I think, my favourite sci-fi novel ever.

David November 1, 2008 at 2:53

Paul you are correct, standard IAU nomenclature is to have eight digits with a plus or minus separating the first four from the second four. The first four digits give the right ascension of the object in terms of just hours and minutes, the second four for declination in terms of degrees and arcminutes, with the plus and minus denoting a position north or south of the celestial equator. That extra 4 is very likely a typo. The J term by the way, refers to the position being as of epoch 2000.

vavatch November 1, 2008 at 10:10

Benjamin: heh, that is correct. A very good book – like most stuff by Iain M Banks.

I was reading the Benford’s paper, and they say that the transmission frequency of this object is 0.3GHz – which is not at all optimal in terms of reaching the centre of the waterhole.

They suggest this could be because the transmitters are deliberately trying to be easily detectable by lower technology civilisations. But this violates the first principle of the rest of their paper – the of practical low cost high benefit beacons – by saying that the transmitters are deliberately making their beacon less efficient in that feature but not in others.

Also, it seems a strange strategy to aim your signal at a civilisation within its first 50 years of possessing radio technology – even we have surpassed that level!

So it seems rather unlikely that GCRT J1745-3009 is in fact a beacon – and the paper concludes as much. Still, it is interesting, because there’s no obvious explanation for what it could be..

tacitus November 1, 2008 at 11:45

We’re talking about enormous distances and an inverse square law, here. I’d be sceptical about the likelihood of any meaningful signal being received except if it was being broadcast with some exotic physics. Though, you never know…

The thought is that a highly-advanced civilization may have reached the point where it has already explored and expanded into all reaches of their home galaxy without finding a community of intelligent species with which to interact with. They may wonder if they might be alone in the universe, and thus attempt to find out.

With the resources and energies of a whole galaxy at their disposal, a highly advanced species would likely have little trouble harnessing the energies required to produce a signal that could reach across intergalactic distances.

I’m not saying it’s likely, but then, neither is detecting a beacon in our galaxy. And if you compare the chances of both, I would propose that finding a SEGI beacon may just be as likely as a SETI beacon.

ad koppen November 2, 2008 at 8:10

Is it possible we witnessed in case of GRTJ1745-3009 the final death of a high technological civilisation. They had their super nova close by and saw and expect the incoming deathly radiation for many years, to protect themselfs as possible they can, they radiate back super emissions in bursts to neutralise, block or deflect incoming radiation.
And it didn`t worked out. It is silent now.

ljk November 2, 2008 at 12:19

That is what we should be scanning: Old Sol type stars on their way
out. The assumption is they may have had civilizations on planets
circling them and in a last-ditch effort to preserve their culture
(assuming they have not or cannot leave their system), they
broadcast everything about themselves into the galaxy. They
are going to be extinct anyway, so telling everything about
themselves won’t harm them.

See James Gunn’s classic SF novel, The Listeners, for an example
of this idea. I am sure there are others.

Perhaps one day that is what our distant descendants will do as
one way to preserve themselves.

yeti101 November 2, 2008 at 17:57

very powerfull beacons may be detectable but i heard seth shostak say the problem with looking towards the galactic center is that its very noisy.

seti will scan the galactic center for “uber” signals of collosal power but targeted searches will be on stars away from the center.

Ron S November 2, 2008 at 21:33

I believe yeti is wrong in the particular case the METI paper describes. At 10 GHz, per their suggestion, the galactic center looks pretty quiet on the charts I have here. There may be exceptions since there are many hot spots, though I don’t know how bright they are at this frequency. So their choice of a 7 K sky temperature at 10 GHz may even be a bit on the high side, on average.

There were however a few puzzling points in the paper I may comment on later if I have time.

Mr. Intense November 3, 2008 at 4:40

Here’s a thought: perhaps ETI beacons might be designed intentionally with such a sophisticated level of complexity and subtleness in their emissions that only comparatively sophisticated SETI receiving technologies of relatively equal sensitivity and machine/sensor intelligence would be able to recognize such “stealthy” beacons. After all, you wouldn’t want to unduly alert the lower level sentients in the galactic neighborhood for two reasons: one, violation of a possible “prime imperative,” or deleterious impact on a less sophisticated culture (you know, like ours), and, two, so that the “berserker” or malignant Bracewell probes and species with FTL capabilities aren’t attracted to the locale of such beacons. (Unless that’s an element of detection strategy, sort of like a moth to a flame, which presumes your beacon is nowhere near your locale, perhaps just to see what other or who else might show up.)

If Deardorff’s “leaky embargo hypothesis” has any basis, however, at least some advanced non-human sentients may already be here and/or aware of our presence. Say, could one motivation for METI be to arbitrarily increase the “signal within the noise” of our planet, in order to bluff/threaten our own exposure to malign sentients so that those benign or neutral sentients who may already be here or nearby would perhaps be thus “forced” to intervene to protect us from ourselves and our foolish, blind attempts to “shout in the dark night of the intragalactic jungle”? In other words, perhaps one motive by frustrated SETI scientists, tired on not being able to detect any confirmable ETI signal in their searches for the past 50 years would be to roll the METI dice in order to gamble that such unthinking puerile actions might force the hand, or compel the revelation of any other ETI’s who may be covert, but benign, to intervene to stop such untoward exposure of our own presence and locale? That’s a METI gamble that should not be risked, IMHO.

Last thought: what if some potential beacons are intended to attract other sentients’ attention, but not for benign purposes of pointing to or allowing such others to vector or travel to “your” locale, but are the cosmological equivalent of enticing “roach motels”–you can “walk in,” but you don’t walk out. Sort of electromagnetic spectrum “rabbit snares.” That’s a rather chilly idea. Brrrr! OK, now I’m starting to scare myself, so I better shut up…8^}

Ronald November 3, 2008 at 5:16

And isn’t the galactic center (any galactic center) rather reactive? I mean relatively large number of supernova explosions, possibly black holes?

As far as I know the galactic habitable zone, not clearly delimited as it may be for the moment, is usually defined (Lineweaver, Reid, and others) as a belt in the galactic disk, roughly between 6 and 10 kiloparsec from the center.

ljk November 3, 2008 at 9:53

I will say it again – we should be searching the cold outer less
populated regions of the galaxy far away from the center.

That’s where certain artificial intelligences might prefer the
deep cold of interstellar space so they can dump all the
excess heat they are creating.

Ron S November 6, 2008 at 21:53

The close association between SETI and METI Paul mentions is very true. There must be a close correlation between transmitting and receiving strategies. Or, failing that, the transmitting strategy should be to achieve notice whether you are searching or not! I believe Carl Sagan understood that when he wrote Contact; he made the signal ridiculously loud and in the direction of one of the brightest stars in the sky. This caused the protagonists some puzzlement: how could they not have noticed it before. Now that’s an example of effective METI!

We can’t count on that since we are hearing nothing like it. The Wow event doesn’t count since, if really from ETI, it was unique and transient, not at all like a METI strategy.

As tacitus said in the first comment in this thread, there does have to be a beacon that calls attention to itself, inciting the listener to look deeper for a richer signal. The beacon ought to be loud. The lower the amplitude, the more problematic detection becomes. I don’t mean just the S/N, it’s that the receiver detector/algorithm must match the transmitter modulation/pattern more closely; roughly speaking, amplitude and receiver/transmitter matching are inversely related. A loud signal cuts through all this nonsense since any detector except the most obtuse would register a ‘hit’.

Long integration time? Need a loud signal, either a transient or a continuous midling-amplitude signal. Narrow filter bandwidth for high S/N? Need an unmodulated sinusoidal signal (with compensation for rotational drift) or one that is loud and broadband.

The idea of selecting preferred directions, as described in the papers, reduces only somewhat the need for a loud beacon. All it does is shorten the time to detection (if the signal is really in one of these directions) but does not address the need to match transmitter/receiver strategies. Loud is still best.

The METI paper talks more about technology vs. signal bandwidth, which I would like to address in a later post.

Ron S November 9, 2008 at 17:26

Earlier I said I wanted to write something about bandwidth in regards to the METI article. I see this as more of a supplement to the papers rather than criticism. (There are a few threads now so hopefully this is as good a place as any for this comment.) Sorry for the length.

As the article says, while a 1 MHz signal has disadvantages it is at least economical, and it is more likely to fall within the bandpass of a SETI receiver detector. I think the price is quite high, which I’ll try to explain.

If we have a purely sinusoidal continuous signal (unmodulated and uninterrupted) it occupies an infinitesimal bandwidth. Mathematically we can achieve any S/N we desire with increasingly narrow (and stable!) filters. This is impossible but we can do wonders with the present state of the art. Let’s place this pure signal within the passband of a 1 MHz filter. We detect the signal and 1 MHz of noise. If we reduce the filter to 1 Hz, the signal is undiminished but the noise (if white) is attenuated 60 db. However if the signal is 1 MHz wide, as you narrow the filter both noise and signal are reduced; the reduction isn’t quite equal since the signal has a central peak, so it depends on where you center the narrower filter. Even if you know the signal is there it would be very difficult to distinguish it from noise or, at least, as having a non-natural cause. Also, the narrower and more stable the signal the more likely it is artificial.

Now let’s pulse the broadband signal as suggested in the article. The pulse rate has to be lower than the receiver’s filter if it is to be reliably detected. For example, the pulse rate should be less than 100 per second, regardless of pulse width, to be detected following a 100 Hz receiver filter. The reason is that pulsing is modulation, and it broadens the signal so that the filter in a sense ‘fills in’ the inter-pulse silences. If you go to high pulse rates, intervals down to ~1 us as stated in the paper, the original 1 MHz signal grows to become 3 MHz wide. This increases the challenge of detecting the signal. It therefore seems better to have a slow pulse rate to increase the chance of fitting within a narrow receiver filter, and with a pulse width high enough that an integrating detector will see a high relative amplitude.

Of course narrow filters make the likelihood of detecting a narrow signal less likely. The likelihood declines in proportion to the relative widths of filters being compared. Except when you get very narrow you will likely see a sharp drop off in likelihood below ~20 kHz if the signal source is on a planet. The rotational doppler of an Earth-like planet would be up to about +/-15 kHz at a frequency of 10 GHz. At these narrower bandwidths the signal stands a good chance of drifting across the filter’s edges. Earth’s own rotation can be easily compensated.

Finally, there is the matter of why the bandwidth is ~1 MHz. While the article doesn’t say so directly I believe the reason is the authors assume the klystron, magnetron or whatever is operating as a high-power oscillator. If so I can well understand their analysis of the difficulty of getting a narrower signal out of those devices; high-Q resonators are temperamental beasts, which I discovered the one time I played with cavities at frequencies below 1 GHz. To get a narrower signal we would ideally want to generate the signal at a low level where it is a perfectly tractable problem. The downside is cost – it would require a chain of amplifiers to boost the signal to the target power, which can be far more expensive.

James Benford November 10, 2008 at 11:55

tacitus: It’s true most SETI thinking is that a Beacon would draw attention to a lower-power data channel at another frequency, perhaps much higher to have higher bandwidth and therefore data rate.

For intergalactic Beacons the power and hence the cost, will be much higher. Keeping the signal-to-noise ratio constant, so to be heard, requires that power required goes up with the square of the distance. Consider that the nearest galaxy, the Andromeda galaxy, is 100 times further away than our Galactic Center, a Beacon will be 10,000 times the power of the ones we describe in or papers. The optimal cost scales with the distance (see our METI paper for the relations, combine eqs. 4 and 9), so will be 100 times greater.

We find the optimum total capital costs for a galactic-scale Beacon to be in the range of 10 $B. The Apollo project cost about 300 B$ in current dollars, and large science today, such as the Large Hadron Collider, International Linear Collider and ITER fusion reactor, are of order 10 B$. This suggests that galactic-scale Beacons are plausible luxuries.

Consequently, a rough cost for a intergalactic Beacon will be 1T$, a non-trivial amount. Whether it’s worth it to send a message depends on ones motive. See section 2.2. Beacon–builder Motives of the paper for a variety of possible motives.

Ljk: Note that, in our SETI paper, we use GCRT J1745-3009 as an example of using cost optimized Beacon analysis (from our METI paper) for SETI purposes, and conclude that “J1745-3009, probably isn’t a Beacon”. This shows how to use the analysis we’ve developed for considering ‘anomalous’ objects.

Also, you ask if there’s anything new on it. The references in our SETI paper are the latest.

Ronald: Indeed, the Center is very energetic, so the Habitable Zone doesn’t include it. However, there are more stars along a galactic radius than any other chord. A lot of the stars are in the Galactic Hub, which is in only a few percent of the sky, so we should look toward Main Street, as we sometimes call it. Go out and look at the Milky Way on a good night and you’ll see what I mean.

Ron S: I agree with your points. Note in Appendix B of the METI paper that high power is associated with larger bandwidth, so we argue that we should not be looking for very narrow bandwidth, as have most SETI searches in the past.

Dr. Alexander Zaitsev November 12, 2008 at 4:47

Jim, I think all your reasonings do for FUTURE (may be).

And now we have only three radars on the Earth:

Arecibo Radar Telescope
Goldstone Solar System Radar, and
Evpatoria Planetary Radar

which are fit for METI, and all three above radar telescopes are not pulse, but coherent continuous radio systems.

ljk November 13, 2008 at 0:17

GCRT J1742-3001: A New Radio Transient Towards the Galactic Center

Authors: Scott D. Hyman, Rudy Wijnands, T. Joseph W. Lazio, Sabyasachi Pal, Rhaana Starling, Namir E. Kassim, Paul S. Ray

(Submitted on 12 Nov 2008)

Abstract: We report the detection of a new transient radio source, GCRT J1742-3001, located ~1 degree from the Galactic center. The source was detected ten times from late 2006 to 2007 May in our 235 MHz transient monitoring program with the Giant Metrewave Radio Telescope (GMRT). The radio emission brightened in about one month, reaching a peak observed flux density of ~100 mJy on 2007 January 28, and decaying to ~50 mJy by 2007 May when our last monitoring observation was made. Two additional faint, isolated 235 MHz detections were made in mid-2006, also with the GMRT.

GCRT J1742-3001 is unresolved at each epoch, with typical resolutions of ~20 arcsec x 10 arcsec. No polarization information is available from the observations.

Based on nondetections in observations obtained simultaneously at 610 MHz, we deduce that the spectrum of GCRT J1742-3001 is very steep, with a spectral index less than about -2. Follow-up radio observations in 2007 September at 330 MHz and 1.4 GHz, and in 2008 February at 235 MHz yielded no detections. No X-ray counterpart is detected in a serendipitous observation obtained with the X-ray telescope aboard the Swift satellite during the peak of the radio emission in early 2007.

We consider the possibilities that GCRT J1742-3001 is either a new member of an existing class of radio transients, or is representative of a new class having no associated X-ray emission.

Comments: 19 pages, 3 figures, submitted to ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0811.1972v1 [astro-ph]

Submission history

From: Paul S. Ray [view email]

[v1] Wed, 12 Nov 2008 20:24:48 GMT (392kb,D)

http://arxiv.org/abs/0811.1972

ljk January 4, 2009 at 23:15

MASER Navigation in the Milky Way and Intergalatic

Authors: Jiang Dong

(Submitted on 31 Dec 2008)

Abstract: The traditional celestial navigation system (CNS) is used the moon, stars, and planets as celestial guides. Then the star tracker (i.e. track one star or planet or angle between it) and star sensor (i.e. sense many star simultaneous) be used to determine the attitude of the spacecraft. Pulsar navigation also be introduced to CNS.

Maser is another interested celestial in radio astronomy which has strong flux density as spectral line. Now I analysis the principle of maser navigation which base measure Doppler shift frequency spectra and the feasibility that use the exist instrument, and discuss the integrated navigation use maser, then give the perspective in the Milk Way and the intergalatic.

Maser navigation can give the continuous position in deep space, that means we can freedom fly successfully in the Milk Way use celestial navigation that include maser, pulsar and traditional star sensor. Maser as nature beacon in the universe will make human freely fly in the space of the Milk Way, even outer of it. That is extraordinary in the human evolution to type III of Karadashev civilizations.

Comments: submitted

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0901.0068v1 [astro-ph]

Submission history

From: Jiang Dong [view email]

[v1] Wed, 31 Dec 2008 06:53:24 GMT (46kb)

http://arxiv.org/abs/0901.0068

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