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Analyzing Transients: Pulsars or Beacons?

Recently we looked at James and Gregory Benford’s thoughts on interstellar beacons, noting that using cost as a likely constraint allowed the authors to discuss how cost would affect design, and therefore the parameters of any beacon we would be likely to observe. But what is it about interstellar beacons that sets them apart from transient phenomena? After all, it was no longer ago than 1963 that Nikolai Kardashev proposed that the radio source CTA 102 could be evidence of a Type II or III extraterrestrial civilization (i.e., one that is able to use the entire energy output of its star, or in the most extreme case, of its entire galaxy).

When Gennady Sholomitskii announced his observation that CTA 102’s radio emission was varying, something of a sensation ensued. Those of us of a certain age can recall Roger McGuinn’s song ‘CTA 102,’ written and performed by McGuinn’s group The Byrds. It was on their Younger Than Yesterday LP, released in 1967. A sample:

CTA 102
Year over year receiving you
Signals tell us that you’re there
We can hear them loud and clear

and so on. We soon learned, of course, that the source of these emissions was a quasar, one that has since been observed by a huge range of instruments. But the question that lingers is how we would separate out an atypical pulsar that might be producing odd transients from a genuine interstellar beacon. It’s a problem James Benford attacks in a new paper.

A Puzzling Transient Analyzed

Take the case of PSR J1928+15, a transient bursting source observed in 2005 near the galactic disk at 1.44 GHz. A two-minute observation by the Arecibo dish noted the signal but did not find it again despite 48 minutes of revisits. Three pulses were received, according to Benford’s paper, the first and third down a factor of ten from the 0.180 Jy central pulse. The source is roughly 24,000 light years away, putting it close to galactic center.

A pulsar? Pulsars are marked by radiation from a rotating neutron star’s magnetosphere. One explanation for this event is an asteroid falling into the neutron star from a circumpulsar disk, perturbing its magnetosphere. But because we’re trying to learn how to distinguish a beacon from a natural source, Benford looks at how we might analyze the observational data in ways that would allow us to deduce a beacon’s parameters. It’s a fascinating exercise:

We make two working assumptions:

1) The Beacon is a ‘lighthouse’ scanning the galactic plane. The source is a scanning beacon and, as it swept past, Arecibo caught the central pulse, the true beam. The first and third pulses are at the edges of the antenna’s acceptance angle, which is 3.5 arcmin=1 mrad.

2) The beam bandwidth covers all channels of the 100 MHz span of the detector array. (The channel BWs are 0.39 MHz, with total BW 100 MHz.) This assumption drives the Beacon power estimate.

If this is the case, then we can start to plug in values to make sense of the signal. Benford tries out a beacon antenna diameter of 100 kilometers, working out a total power of 190,000 TW. Think of this as a beacon and you are dealing with a civilization much more advanced and powerful than our own. It’s one that ranks above Kardashev Type I but falls far short of Kardashev Type II. Benford would rank it at Kardashev 1.13 (Earth is 0.73 on this scale).

Moreover, if the beacon is scanning the disk at a thickness of 1300 light years (roughly what the disk thickness is at our distance), then the signal cycle can be estimated. Benford works out a cycle around the galactic circumference of roughly fifteen hours, noting “It’s understandable that 48 minutes of revisits hasn’t seen it again, as that is only 5% of the revisit time. Of course, it could be scanning a smaller area, so that the revisit time would be sooner.”

Playing with Beacon Assumptions

Assume an antenna diameter of 1 kilometer and interesting changes to the conclusions occur:

The beamwidth is reduced by a factor of 10 to 5 x 10-4 rad. Spot size diameter falls to 12.2 ly, As~117 ly2. Power in the spot falls to 1900 TW, Kardashev scale falls to K=0.93. This is a civilization intermediate between ours and the planetary scale civilization of the previous example.

The spot moves at the same rate, 30 ly/sec. But since the spot is smaller, the number of strips in the scan increases to 1350ly/12.2ly = 110. So the Beacon will return in 110 x 5 x 103 sec = 5.5 x 105 sec = 150 hours. Observers have revisited the site for 48 minutes, only 0.5% of the revisit time, and haven’t seen it again.

So a civilization lower on the Kardashev scale, i.e., K= 0.93 will have a narrower beam, revisit less frequently, be harder to observe…

To reach a civilization at this level given the energy growth rate we see on Earth during the 20th Century would require about 2000 years, a small time on the cosmic scale. This corresponds to a civilization of Kardashev 0.93, not yet a full Type I.

The broader principles: We can begin to distinguish beacons from pulsars by bandwidth, for pulsars have large bandwidths. But bandwidth by itself is not definitive, because advanced methods of microwave generation might allow very broadband emissions with huge data transmission rates. We can also add in pulse length (“[c]ost optimized Beacons will likely be pulsed to lower cost, with a preference for shorter pulses due to source physics”) and frequency. Benford notes about the latter that pulsar searches cluster in the lower end of the microwave, but beacons are more likely to appear at higher frequencies “due to the favorable scaling of cost with frequency.”

Learning how we would set about observing a candidate beacon signal is not only an ingenious exercise in itself, but a necessary warm-up in case of future detections of even more puzzling transients than PSR J1928+15. The fact that natural phenomena can produce some of the same observables as an interstellar signal behooves us to sharpen our tools for analyzing and differentiating between such signals. The paper is James Benford, “How can we distinguish transient pulsars from SETI beacons?” (preprint available).

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  • John Hunt April 6, 2010, 12:23

    If an intelligent civilization is trying to broadcast its presence then I would figure that they would do a decent job of it. If they are able to control shades around their star then I would presume that they would control those shades in a manner which was truly unambiguous.

  • NS April 6, 2010, 14:54

    A beacon may be the artificial source we’re most likely to see (and may be the only thing we have a reasonable chance of seeing) but it’s possible we might detect other transients. Radar scans for asteroids or transmissions to/from probes would not of course be aimed at us and would be unlikely to repeat at all. The ‘Wow’ signal comes to mind. As our detection methods improve we might expect to detect more of these if they’re out there to be found.

  • Daniel April 6, 2010, 15:07

    by the way about a nearby brown dwarf that it’s been found….

    Discovery of a very cool brown dwarf amongst the ten nearest stars to the Solar System

    http://arxiv.org/pdf/1004.0317

    Here we report the discovery in the UKIDSS Galactic Plane Survey of a brown dwarf, UGPSJ0722-05, that is not only far less luminous and significantly cooler than previously known objects but also the nearest to the Solar System. The measured distance is 2.9±0.4 pc, from which we deduce an effective temperature in the range 400-500 K. The Gemini/NIRI near infrared spectrum displays deeper water vapour and methane absorption bands than the coolest known T dwarfs, and an unidentified absorption feature at 1.275 μm. Time will tell whether this object is regarded as a T10 dwarf or the first example of a new spectral type.

    The SM2008 evolutionary models indicate that the mass is between 5 and 30 MJup

  • Daniel April 6, 2010, 15:08

    then imagine what WISE can do.

  • Daniel April 6, 2010, 15:11

    Discovery of a Planetary-mass Companion to a Brown Dwarf in Taurus

    http://arxiv.org/abs/1004.0539

    We have performed a survey for substellar companions to young brown dwarfs in the Taurus star-forming region using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. In these data, we have discovered a candidate companion at a projected separation of 0.105″ from one of the brown dwarfs, corresponding to 15 AU at the distance of Taurus. To determine if this object is a companion, we have obtained images of the pair at a second epoch with the adaptive optics system at Gemini Observatory. The astrometry from the Hubble and Gemini data indicates that the two objects share similar proper motions and thus are likely companions. We estimate a mass of 5-10 Mjup for the secondary based on a comparison of its bolometric luminosity to the predictions of theoretical evolutionary models. This object demonstrates that planetary-mass companions to brown dwarfs can form on a timescale of <=1 Myr. Companion formation on such a rapid timescale is more likely to occur via gravitational instability in a disk or fragmentation of a cloud core than through core accretion. The Gemini images also reveal a possible substellar companion (rho=0.23") to a young low-mass star that is 12.4" from the brown dwarf targeted by Hubble. If these four objects comprise a quadruple system, then its hierarchical configuration would suggest that the fragmentation of molecular cloud cores can produce companions below 10 Mjup.

  • Duncan Ivry April 6, 2010, 15:42

    Interesting and fascinating, what the Benfords do. It is to be hoped that their results will be used as soon as possible.

    Mentioning the Kardashev classification of civilizations, and John Hunt’s assumption “they would control” — let me say: certain appliances — “in a manner which was truly unambiguous”, provokes an idea, how non-scientists could support the scientific efforts of the Benfords and others. Citizen science projects like Zooniverse.org use the massive amount of data being already there for scanning celestial objects with respect to certain features. A lot of data of our own galaxy must be available, especially images, which can be scanned by laypeople, if they are told, e.g. by the Benfords according to their beacon assumptions, what to look for.

    Remarkably until now, nobody has come up with “hey, this galaxy out there looks like engineered”, but this may be the case because nobody scanned appropriately, or even knows how to do it.

  • Lionel W April 6, 2010, 19:45

    Wow – the brown dwarf is so young, and is close to us!

    By finding a 1 million years old BD so close to home, can we not infer that there must be heaps more of these objects in our vicinity? (meaning even closer than the star(planet?) in question)

    Please can somebody say if I am right in thinking that Brown Dwarfs would stay around for a very long time. I just tried googling to find what the life of a brown dwarf is and found some figure saying that they’d fuse hydrogen for just 10 million years. But after this hydrogen burning process ends, wouldn’t a huge, dead planet remain? For billions and billions of years?

    If brown dwarves have an abundance similar to, say main sequence stars, and if they exist for billions of years, then it would be highly highly improbable to find such a young brown dwarf this close.

    So is it logical to infer that there are highly likely to be loads more brown dwarfs than main sequence stars?

  • Adam April 6, 2010, 21:38

    Hi All

    John, just how fast do you think Sun-embracing shades can be moved in position to create a signal? The bit-rate is ridiculously slow, so why bother? Sure, the power is huge, but for any reasonably sized Sun-shade the lag-time is insanely long. Plus the shadow would deprive a good fraction of the circum-stellar space of light – who would agree to that?

  • T_U_T April 7, 2010, 16:53

    I think that trying to find sophisticated ways how to discriminate pulsars from beacons is a giant waste of time.
    Anyone who can build beacons powerful enough to be mistaken for pulsars will surely know how, and will go to great lengths to make sure that no sane person mistakes them for pulsars or other astronomical phenomena.

  • Terraformer (a.k.a Tobias Holbrook) April 7, 2010, 18:02

    “Please can somebody say if I am right in thinking that Brown Dwarfs would stay around for a very long time. I just tried googling to find what the life of a brown dwarf is and found some figure saying that they’d fuse hydrogen for just 10 million years. But after this hydrogen burning process ends, wouldn’t a huge, dead planet remain? For billions and billions of years?”
    I think the main source of energy for a brown dwarf would be gravitational contraction rather than fusion burning, which can last a lot longer. Also, it takes a long time to cool down to the same temperature as the background, so it should be able to last quite a long time. Even after all that has been exhausted, there’ll still be the massive radiation belts providing energy.

    I do hope there’s lot’s of brown dwarfs about… a story I wrote used an assumption of a brown dwarf or rogue superjovian every 2 light months. :)

  • Duncan Ivry April 7, 2010, 20:54

    @ T_U_T
    Being able to recognize signals doesn’t come just like that. It’s especially difficult, because we don’t know much about the sender, and the sender doesn’t know much about us.

    You may consider the efforts of the Benfords and others as just reaching this: knowing how to build those powerful beacons and making sure that no sane person mistakes them for natural phenomena. Some time in the future we may want to build such a beacon too.

  • Eniac April 7, 2010, 22:04

    John, just how fast do you think Sun-embracing shades can be moved in position to create a signal? The bit-rate is ridiculously slow, so why bother? Sure, the power is huge, but for any reasonably sized Sun-shade the lag-time is insanely long. Plus the shadow would deprive a good fraction of the circum-stellar space of light – who would agree to that?

    Such shades could be made quite fast, in the manner of venetian blinds. They could hover over the star, held up by radiation pressure (not sure if that is possible with known materials, though). And they could be positioned off-ecliptic, so nobody would miss the light.

  • tesh April 8, 2010, 7:57

    Though it would be a dark place, life could evelove on potential moons around brown drawfs.

  • lionel w April 9, 2010, 8:36

    @Terraformer (a.k.a Tobias Holbrook): a frequency anything like one rogue brown dwarf/superjovian per 2 light months that would make things extremely interesting

    i just did a back of the envelope calculation on this. the ratios of total population in space would go as the cube of the frequency in one dimension. according to this page http://www.solstation.com/stars/s10ly.htm , within a 10 year radius of the sun, there are 12 stars

    wheras if brown dwarfs and rogue superjovians could be found on average every 2 light months in each direction, there would be 900,000 of them within a 10 light year radius of us

    so in our vicinity, such bodies would outnumber stars by about 80,000 to one

  • Ronald April 11, 2010, 9:05

    @lionel w;
    I know this has been postulated many times before, but once again a question: if brown dwarfs and stray super-Jovians are indeed so numerous, could that possibly explain (a significant part of) missing dark matter?

    Also, because dark matter seems mostly present *within* galaxies.

  • Terraformer (given name = Tobias Holbrook, getting my digital paper trail up) April 11, 2010, 14:37

    Well, there’s two types of dark matter problem – one related to gravitational lensing, the other galactic rotation.

  • John Hunt April 12, 2010, 13:16

    Eniac explains exactly what I mean by a sun shade. I’m not talking a single large shade which would take forever to move into place. Rather, a swarm of much smaller shades which would synchronously open and close at whatever frequency and pattern as one would desire. And yes, the shades could be programed so as to never block the sun from striking the inhabited parts of their solar system.

    This is what I’m talking about:
    http://ducksmahal.files.wordpress.com/2009/07/465px-seaman_send_morse_code_signals2.jpg

    Advanced intelligent civilizations will have mastered nanotechnology and artificial superintelligence which means that, without a great amount of time, they should be able to harness their solar system’s material and energy. Likewise, with nanotechnology and tremendous power they should be able to send very small, very sophisticated probes across the galaxy at a significant fraction of the speed of light and should be able to therefore establish a presence across the galaxy.

    It doesn’t take too much imagination to figure out how such a powerful civilization could make themselves blatantly visible across intergalactic distances if:
    1) they existed / survived and
    2) they had the desire to communicate with others.
    It’s not a questions of possibility. If they mastered nanotech, then given a modest amount of time then they could.

    So, if they don’t exist or universally didn’t survive then that should surprise or scare us respectively. But if they exist and are choosing to not make themselves known then we can probably rest easy but it would still be good to try and figure out why.

  • Lionel W April 12, 2010, 21:45

    @Ronald
    As a mere undergraduate student, and in a subject little to do with astrophysics, I really don’ t know much about the problems of dark matter.

    But certainly, 80,000 Jupiters to every Sun in the universe would more than make up for all the dark matter, I think. The Sun is about 1000 Jupiter masses – dark matter outnumbers normal matter about 5 to one, so perhaps 5000 currently unobservable Jupiters for every sun would account for the dark matter?

    There are entirely amateurish postulations on my part. I think that by trying to explain dark matter with normal matter, we might be being contrained by the limits of our knowledge. Nonetheless, your question has brought up some questions of my own:

    1) To put your question in other words -is the stuff in stars the only matter which we can see? Could there be abundant matter in sub-stellar sized objects?

    2) Would brown dwarfs and sub-stellar objects be like a matter trap? By this I mean, the matter stars is recycled, the larger the star, the faster is goes supernova and the remnants go to form new stars. But brown dwarfs and planets of any size would last for tens or hundreds of billions, even trillions of years, wouldn’t they? Hence once matter gets into planets and brown dwarfs it is ‘stuck’ in that form unless the planet is consumed by a star. Hence the percentage of matter in the universe constituting planets and brown dwarfs continually increases.
    I feel this makes sense, opinion anyone?

    3) A tangent into nuclear physics, but it’s related: at the end of the fusion cycle is Iron, isn’t it? This iron, as well as other heavy elements, isn’t going to do anything to make itself visible to us anymore, I believe. Hence isn’t there a great uncertainly about how much of these fused elements are in space, left over by all the stars that have come before?
    And so isn’t it a possibility that this is some of the missing matter?

    Thanks in advance to the informed person who can shed light on my uninformed musings ;)

  • Administrator April 13, 2010, 8:11

    Lionel W writes:

    To put your question in other words -is the stuff in stars the only matter which we can see? Could there be abundant matter in sub-stellar sized objects?

    One of the constraints on this are the studies that have been done looking for lensing events caused by possible sub-stellar objects. Evalyn Gates covers these in Einstein’s Telescope: The Hunt for Dark Matter and Dark Energy in the Universe. If massive objects unrelated to stars are indeed out there in such profusion, we should see their signature as we study, for example, the Magellanics, but the work that has been done to date puts firm limits on their population. Much more needs to be done, but the Gates book is an excellent intro to current dark matter research.

  • Lionel W April 13, 2010, 18:09

    Hi Paul,

    The Evalyn Gates book looks like a good read, thanks.

    I’m definitely interested to read more about that work which looked for lensing events from sub-stellar objects. I’ve read that in our solar system 99.8% of the matter is found in the Sun, and perhaps throughout the universe such a high percentage of matter is found in stars and black holes.

    Still, I just don’t get it – why would matter have such a disposition against to almost always form stars, and not sub-stellar objects?

    Changing the subject, I do like it when astrophysicysts give funny names that are also memorable and descriptive . I’ve just been doing a bit of online reading about dark matter, and discovered that WIMPs and MACHOs are the monikers for baryonic and non-baryonic matter.