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D. , S/N P ese costeGlx ol eur high require would Galaxy the across seen be o ml ouainmdluigplenligand nulling pulse using model modulation imple nrycnupinsaefis omltdby formulated first scale consumption energy n r s erg r s erg ≈ uln ouao ilehbta xesof excess an exhibit will modulator nulling a ee,C 42,USA 94720, CA keley, 6 ol ml rnmsinpwrta can that power transmission a imply would . − 6 − 1 × 1 esavne iiiain htwish that civilizations advanced Less . hc a nyb rdcdb civi- by produced be only can which , 10 ∼ 24 10 a,e  19 ∆ a Werthimer Dan , Hz USA f yextraterrestrial by r s erg   − S/N 1 10 ohpteia highly hypothetical to  P etme 5 2018 25, September ∝ r s erg b S/N ∆ f f − h power the , 1 GHz, 1 = as , . f across (1) appropriate naturally-occurring radio transmitters. In this a neutron star, or modulating the X-ray emission of ac- paper, we propose that an ETI that is moderately more creting neutron stars. Learned et al. (2008) has proposed advanced than humans but not yet achieving a higher Kar- that ETI may modulate the period of Cepheid variables to dashev type, may be able to use radio pulsars as sources achieve signaling, by triggering pulsations using neutrinos of power at levels otherwise unachievable, modulating the beamed to the stellar core. broad-band pulsar signal for communication. The mini- Cordes & Sullivan (1995) and Sullivan & Cordes (1995) mum requirement for such an endeavour would only be postulate that ETI would employ ‘astrophysical coding’ – the ability to build and launch a modulating satellite to a i.e., transmitting signals that can be detected using astro- nearby pulsar. physical signal analysis – in beacons. They argue that such A pulsar is a neutron star that emits coherent radio a signal is more likely to be detected because astronomers radiation from its magnetic poles (see Lorimer & Kramer, would be able to easily analyse it. The idea proposed in 2005). Pulsars are fast-rotating, and usually detected due this paper is a kind of astrophysical coding technique and to the fact that an offset exists between their magnetic enjoy the benefit of higher likelihood of detectability. and rotational axes, causing them to appear as periodic The outline of this paper is as follows: In §2, we de- signals, with an observer typically receiving one pulse per scribe our proposed modulation mechanism, and in §3, we one complete rotation of the pulsar. The radio luminosity discuss the information content of the beacon. In §4, we of a pulsar with spin period P situated at a distance d discuss potential observational signatures of artificial mod- from an observer is given in terms of the measured flux ulation, and in §5, we analyse energy considerations for density as this signalling scheme, before concluding in §6.

2 f2 4πd 2 ρ L = sin Smean(f) df, (2) δ 2 f1 2. Modulation mechanism   Z where δ = Weq/P is the pulse duty cycle (Weq is the equiv- Installing a modulator on a pulsar would require con- alent pulse width), ρ is the radius of the pulsar emission siderations of the emission geometry of the pulsar being cone, the integrand is the mean flux density of the pulsar engineered. If we assume an inclination angle α = 90◦ (i.e., as a function of frequency f, and f1 and f2 bound the the magnetic axis orthogonal to the spin axis), the mod- spectral range of the observation. Using typical values of ulating satellite could orbit synchronously with the pulsar δ and ρ, a pulsar with P = 1 s situated at a distance of spin period to allow the signal to be transmitted over the 1 kpc, with a measured 1400 MHz flux density of 1 mJy, entire area of the sky covered by the pulsar beam. In the 27 −1 would have a radio luminosity ≈ 7.4 × 10 erg s . On more typical case of non-orthogonal axes, a polar orbit the Kardashev scale, such a pulsar would therefore cor- in which the satellite intersects the pulsar beam periodi- respond to a beacon produced by a civilization between cally would result in directional transmission. A scaffold- Type-I and Type-II. We speculate that a civilization with ing shell around the pulsar in which modulating elements the minimum capability of sending a spacecraft to a nearby are placed at locations where the pulsar beam intersects pulsar to install an orbital modulator for the sweeping pul- with the scaffold would result in the ability to cover the sar beam would be able to harness the energy emission of entire beaming solid angle of the pulsar. pulsars without actually building and operating a trans- We first consider a toy model of an orbital modula- mitter so powerful (or being capable of doing so). tor that is synchronous with the pulsar rotation, assuming Previous works have considered extraterrestrial civi- that the inclination angle of the pulsar beam, α = 90◦, lizations making use of naturally-occurring phenomena to as shown in Figure 1(a). For a pulsar with mass M and announce their presence to any listeners. For example, period P , equating centripetal acceleration to the acceler- Cordes (1993) has suggested that extraterrestrial civiliza- ation due to gravity gives an orbital radius tions may make use of astrophysical masers to amplify engineered signals, thereby transmitting more power than 1/3 2/3 3 M P their position on the Kardashev scale might allow them to. r ≈ 1.7 × 10 km. (3) 1.4M⊙ s A critical drawback of using a maser-based communication     system is that masers are usually directional, and hence For a canonical 1.4M⊙ pulsar with P = 1 s, this gives require the transmitter and receiver to be serendipitously r ≈ 1700 km, with a tangential velocity component of aligned. Pulsar beams, on the other hand, albeit direc- approximately 4% the speed of light. To probe the struc- tional, are swept around due to the rotation of the star, tural integrity of the satellite at this distance, we model thereby covering a much larger area of the sky, increasing the satellite as a solid steel cylindrical bar 10 m in length the probability of detection. A system that makes use of and 1 m in radius, oriented in such a way that the long pulsars, in addition to being used as beacons, can also be axis is directed radially outwards from the pulsar. The configured for directional communication, with say, a dis- elongation of the bar due to the differential gravity on ei- tant spacecraft or planetary system. Fabian (1977) and ther of its ends is of the order of 10−5 m, and therefore, is Corbet (1997) have discussed the possibility of generat- inconsequential. ing X-ray pulses by dropping matter onto the surface of 2 (a) (b) Ω Ω

Emitted beam

Dyson Emitted shell beam α = 90° Modulated Pulsar Pulsar beam Modulating elements

Modulating Modulated satellite beam

Figure 1: (a) Schematic of a modulating satellite co-rotating with a pulsar that has an inclination angle α = 90◦; (b) Cross-section of a Dyson shell around a pulsar. Modulating elements are placed along the loci of the pulsar beams on the shell.

Instead of a satellite, a civilization capable of advanced where f is the frequency of radio emission at rem, P˙ is astronomical engineering could build an equatorial ring the period derivative, and P is the period of the pulsar. around the pulsar that covers the entire area swept by the Assuming that the lowest frequency of interest to the mod- beam. A less desirable option would be to have a satellite ulation is 10 MHz, a pulsar with P = 1 s and P˙ = 10−15 s in a non-synchronous orbit periodically intercepting the s−1 would have a maximum emission height of interest of pulsar beam, but this would severely reduce the beaming ∼ 1300 km. This gives the minimum radius of the Dyson fraction of the modulated beam, and also make message sphere. reconstruction more difficult. As in any communication system, the modulation could The typical case of non-orthogonal beams, however, is be one of many types. It could be amplitude modulation, more complicated. A modulating satellite in a polar or- frequency modulation, or phase modulation, either ana- bit that intercepts the pulsar beams periodically could be logue or digital. In this paper, we consider the simplest built, but this has the problem of low beaming fraction, case, where an amplitude modulator toggles between 0% which would not serve as a beacon, but could be used for modulation (modulator transparent to pulsar radiation) directional communication. For a beacon, the last option and 100% modulation (modulator opaque to radiation) – albeit one that would require a significant amount of to achieve preferential nulling, resulting in single-bit data astronomical engineering – would be a scaffold around a transmission. For the sake of simplicity, we also assume pulsar akin to a Dyson sphere (Dyson, 1960), with modu- that the nulling is frequency-independent, that is, during lating elements placed at the points where the pulsar beam a null, the entire radio emission of the pulsar is blocked. intersects the scaffold, as shown in Figure 1(b). Tradi- The information content of this system is discussed in §3. tional Dyson structures around main-sequence stars pro- We do not speculate on the nature of the modulator vide general-purpose energy for the consumption of an ad- as it would most likely be based on technology not yet in- vanced civilization. In the case of a Dyson sphere around vented by humans, although it would seem that the signal- a pulsar as outlined in this paper, the energy of the host modulating mechanism could be based on confined plasma, star is used only for producing a beacon. The materials or perhaps, electro-optic modulators (see Purvinis & Maldonado, that make up the scaffold, and the structural engineering 2010). In the case of nulling modulation, the modulating of the scaffold should be such that it should not interact material would need to scatter, absorb or redirect the en- with the particles and field lines within the pulsar magne- tire radiation falling on it. If the radiation is absorbed, tosphere, except at the modulating elements. The radius this would manifest as an increase in the temperature of of the shell should be large enough such that the pulsar the modulator and could show up as thermal radiation emission region lies within this shell. Kijak & Gil (2003) when the energy is re-radiated. This is discussed in more give a semi-empirical formula to calculate the heights of detail in §4. The effect of radiation-induced heating of the emission regions of pulsars, which, for a 10 km-radius pul- modulator on the lifetime of the system is discussed in §5. sar, is

− 0.07 f 0.26 P˙ P 0.30 rem ≈ 400 km, (4) GHz 10−15 s   !   3 3. Information content 30 A single-bit modulation system as mentioned in the previous section would support only low bit rates. For the 25 nulling method, the data rate, R = 1/P bits per second, where P is the spin period of the pulsar in seconds. Even 20 utilizing the fastest pulsars, the transmission rate would be less than 1 Kbps. In this model, we have assumed that 15 the nulling is independent of frequency. A more complex system could make use of frequency-dependent nulling, thereby increasing the data rate of the signal. Further in- 10 crease in data rate using amplitude modulation would re- quire more complex modulation mechanisms wherein the 5 amplitude of a pulse varies over a range of modulation 0 depths. Another possibility is using a modulating signal 1 2 3 4 5 6 7 8 whose frequency is much larger than the pulsar period (but Nulling duration (number of rotations) less than the ‘carrier’ frequency, or the radio frequency). This would manifest as narrow features in the time do- Figure 2: Example histogram of null duration in terms of number of main, within the on-pulse of the pulsar. rotations of the pulsar. This histogram is indicative of a non-natural nulling process. The artificially-modulated pulsar signal contains an- other piece of ‘information’ that is astrophysically-coded – the fact that the ETI identified the neutron star that was pulsar. In this case, a histogram for the number of rota- engineered as a pulsar indicates that their civilization may tions between nulls would look similar to Figure 2. Any be based on at least one planet that is within the beaming other information the ETI would like to transmit could solid angle of the pulsar. If the inclination angle of the additionally be imposed as amplitude modulation on the pulsar beam can be determined, this helps derive a coarse non-nulled pulses. constraint on the location of the civilization. Irrespective of whether the modulation is due to an or- biting satellite or due to a Dysonian scaffold, during a null, if the pulsar signal is absorbed, the temperature of the ab- 4. Observable effects sorbing medium (modulating element) should increase. To Pulse nulling (Backer, 1970) is observed in many pul- prevent heat build-up, the modulating element will need sars, and is usually attributed to changes in the plasma to shed this excess energy in a timescale of the duration currents in the pulsar magnetosphere (see, for example, of one complete rotation of the pulsar. Looking for excess Wang et al., 2007), although this explanation has not been emission with a thermal spectrum during the null phases of conclusively established. Statistical studies of nulling and a pulsar would indicate such a process in action. Consider- the possibly related phenomena of mode-changing and drift- ing the case of an orbiting satellite as shown in Figure 1(a), ing sub-pulses have the potential to determine any sign of assuming that the power emitted by the pulsar is given by ˙ non-natural processes in action. Redman & Rankin (2009) the spin-down luminosity E and that the modulating el- treated the pulse-null stream for a set of pulsars as a bi- ement is in thermal equilibrium, the temperature of this nary sequence and performed a statistical runs test, and secondary emission is found that nulling is not random in many pulsars that ex- 1/4 −3/4 hibit the phenomenon. But as shown by Cordes (2013), r −1/2 P˙ P T ≈ 1.1 × 106 K, (5) this just indicates a different random Markov process in km 10−15 s !   action. An artificially-nulled pulsar would also show up as   non-random in an analysis as done by Redman & Rankin where r is the orbital radius. For a pulsar with P = 1 s and (2009), but in general, it is unclear how to distinguish be- P˙ = 10−15 ss−1, and taking r = 1700 km as derived in §2, tween artificial and natural nulling. If we assume that the we get T ≈ 2.7×104 K, which corresponds to a wavelength intention of the modulation is to serve as a beacon, one of approximately 107 nm, in the ultraviolet. An excess of of the easiest ways to display an artificial nature would be thermal emission that peaks in the ultraviolet during the to have the null runs last for prime numbers of rotations. null phases of this pulsar, therefore, would indicate the An example histogram of null-run duration is shown in presence of an absorbing medium. Figure 2, which is extremely unlikely to be produced due Pulsar signals are affected by the cold plasma that to any natural process. Another way would be to null a makes up the interstellar medium (ISM) in various ways pulse once after every n complete rotations of the pulsar, (Rickett, 1990; Cordes, 2002). A challenging aspect of where n is a prime number. This system might be prefer- detecting an intelligent signal within the pulsar beam is able if nulling the pulsar is expensive in terms of energy, decoupling the modulation and the effects of the ISM, as each null run lasts for only one complete rotation of the particularly for pulsars in the strong scattering regime. 4 A simple-minded approach would be to assume that the longer, with an upper limit given by the radio lifetime of ETI-imposed nulling covers the entire band of radio emis- the pulsar, which is about 107 yr for normal pulsars or sion from the pulsar, whereas diffractive scintillation is about 109 yr for millisecond pulsars. frequency-dependent. Another factor that helps discrimi- For a Dyson shell, the factors affecting feasibility are nate between artificial modulation and scintillation is the different. The cost incurred in installation would be much difference in time scales. Diffractive scintillation timescales higher than that of a modulating satellite. The lifetime are usually of the order of minutes to hours (Coles et al., would depend on the structural properties of the scaffold, 2010), which is significantly longer than any variation due and also whether the surrounding environment of the pul- to artificial modulation, which is of the order of the pulse sar contains potentially destructive asteroids or other de- period. bris. Another factor that affects the lifetime of a satellite or a Dyson shell around a pulsar at the distances computed 5. Discussion in §2 are radiation-induced heating and induction heating The decision of using a conventional radio transmitter that would cause the satellite or Dyson shell to melt and vis-`a-vis a pulsar-based beacon depends on the number of evaporate. Cordes & Shannon (2008) show that an aster- pulsars required to cover the entire sky, the cost of in- oid with a radius of ∼ 100 m at a distance of ∼ 1000 km stalling modulators around those pulsars, and the lifetime from a pulsar will evaporate in less than a second. The of the modulating system. Assuming that the beaming materials used in the construction of such systems should solid angle of a pulsar is about 20% of the sky, it would therefore be able to overcome heating, and would require take at least five such pulsars to cover the entire sky. If the the development of technology not yet known to humans. energy requirements for sending modulating satellites to For Dyson shells, another way to reduce heating would be those pulsars (or building modulating Dyson shells around by having a larger radius. This would incur a higher cost those pulsars) is less than that of building and operating as more material would be needed for its construction. a perpetual, omnidirectional, conventional transmitter of Another way of overcoming heating would be to have a the same power, the former would be the optimal solution, satellite orbiting the pulsar at a safe distance and inject- provided the lifetime of such a system is long enough. ing material into the magnetosphere to modulate emission. A spacecraft that has to be inserted into orbit around There is one major caveat to the pulsar modulation another star has to accelerate to some maximum velocity scheme discussed in this paper, namely, that we ignore the v and then decelerate. To first order, the energy required rate of increase of energy consumption of the extraterres- for this is twice the kinetic energy of the spacecraft from trial civilization. If the time it takes to advance to a higher launch to achieving maximum velocity, E = mc2(γ − 1), developmental stage – one at which the ETI can afford to where m is the mass of the spacecraft (including fuel mass) build a beacon matching typical pulsar luminosity – is less and γ =1/ 1 − v2/c2 is the Lorentz factor. Assuming a than that for their spacecraft to reach the target pulsars, spacecraft mass of 109 kg, travelling at a constant velocity the best choice for the civilization would be to wait. It p equal to 10% of the speed of light, taking a time t to travel is hard to predict rates of development, and depending on a distance equal to the minimum Earth-pulsar distance for the availability of interstellar travel technology and nearby reported values in the ATNF Pulsar Catalogue2(160 pc; pulsars, civilizations may or may not choose to implement Manchester et al., 2005), the average energy consumption this scheme. For instance, in the event that intelligent, rate is approximately given by E/t ∼ 1019 erg s−1. Since technological life evolved on a planet orbiting a compan- this is much less than the energy output of a pulsar, the ion to a pulsar in a multiple-star system, it would not only cost involved in installing a few such satellites is negligible be energetically favourable, but quicker, to implement a compared to the transmission power achieved. The major pulsar modulation scheme. factor determining feasibility is then the lifetime of the system. 6. Conclusion For a modulating satellite with no attitude stabiliza- tion, the lifetime depends on two factors: (a) the radiation It is reasonable to assume that energy production/consumption pressure exerted by the pulsar beam and (b) the pressure goes hand-in-hand with the development of technological due to particles that follow magnetic field lines impinging civilizations, as seen on Earth. Technological civilizations on the satellite. These two effects combine to push the should, therefore, sooner or later, embark on large-scale satellite out of its orbit. For a modulating satellite with energy harvesting endeavours, such as building Dyson spheres. attitude control, the lifetime would depend on the amount Even though it is unclear how inclined a civilization would of fuel it can carry. It is conceivable that electrical energy be to announce their presence explicitly using beacons, if for attitude correction can be extracted from the pulsar we assume that they are so inclined, modulating the sig- beam itself, in which case, the lifetime will be considerably nal of a nearby pulsar would be one of the most energy- efficient ways of doing it. Building a Dysonian scaffold around a pulsar would cost much less in terms of mate- 2http://www.atnf.csiro.au/people/pulsar/psrcat/ rial than building a Dyson sphere at a habitable distance 5 around a Sun-like star, and would also be an engineering Lorimer, D. R., Kramer, M., 2005 Handbook of Pulsar Astronomy, proof-of-concept for a pre-Kardashev-Type-II civilization. Cambridge Univ. Press, Cambridge, UK Manchester, R. N., Hobbs, G. B., Teoh, A., Hobbs, M., 2005. The Statistical studies of pulsar emission, such as those of Australia Telescope National Facility pulsar catalogue. AJ 129, nulling and pulse-to-pulse and intra-pulse intensity varia- 1993–2006 tion, have the potential to discover non-natural processes Purvinis, G. M., Maldonado, T. A., 2010. Atmospheric Optics, Mod- in action, thereby indicating the presence of technologi- ulators, Fiber Optics, X-Ray and Neutron Optics, in: Bass, M., DeCusatis, C. M., Enoch, J. M., Lakshminarayanan, V., Li, G., cally advanced civilizations in the Galaxy. Single-pulse MacDonald, C., Mahajan, V. N., Stryland, E. V. (Eds.), Hand- observations of pulsars using radio telescopes with large book of Optics, third ed. Vol. V, McGraw-Hill collecting areas will provide the high-quality data required Redman, S. L., Rankin, J. M., 2009. On the randomness of pulsar for this purpose. nulls. MNRAS 395, 1529–1532 Rickett, B. J., 1990. Radio propagation through the turbulent inter- stellar plasma. Annu. Rev. A&A 28, 561–605 Acknowledgements Sullivan, W. T., III, Cordes, J. M., 1995. Astrophysical coding: A new approach to SETI signals – II. Information about the sender’s environment, in: Shostak, G. S. (Ed.), Progress in the Search for We thank the anonymous referee for valuable com- Extraterrestrial Life, ASP Conf. Ser., Vol. 74. Astron. Soc. Pac., ments and suggestions, including pointing out the possibil- San Francisco, pp. 337–342 ity of injecting material into the magnetosphere to achieve Wang, N., Manchester, R. N., Johnston, S., 2007. Pulsar nulling and modulation. We also thank Manjari Bagchi for feedback mode changing. MNRAS 377, 1383–1392 on an earlier version of the manuscript and Nikhil Mehta, Benetge Perera, Nipuni Palliyaguru, and Joanna Rankin for useful discussions. A. P. V. S. and D. W. received support from a competitive grant awarded by the John Templeton Foundation.

References

Backer, D. C., 1970. Pulsar nulling phenomena. Nature 228, 42–43 Cocconi, G., Morrison, P., 1959. Searching for interstellar communi- cations. Nature 184, 844–846 Coles, W. A., Rickett, B. J., Gao, J. J., Hobbs, G., and Verbiest, J. P. W., 2010. Scattering of pulsar radio emission by the interstellar plasma. ApJ 717, 1206–1221 Corbet, R. H. D., 1997. SETI at X-energies – parasitic searches from astrophysical observations. JBIS 50, 253–257 Cordes, J. M., 1993. Astrophysical Masers as Amplifiers of ETI Sig- nals, in: Shostak, G. S. (Ed.), Third Decennial US-USSR Con- ference on SETI, ASP Conf. Ser., Vol. 47. Astron. Soc. Pac., San Fracisco, pp. 257–266 Cordes, J. M., 2002. Pulsar observations I. – Propagation effects, searching distance estimates, scintillations and VLBI, in: Sta- nimirovic, S., Altschuler, D., Goldsmith, P., Salter, C. (Eds.), Single-Dish Radio Astronomy: Techniques and Applications, ASP Conf. Ser., Vol. 278. Astron. Soc. Pac., San Francisco, pp. 227–250 Cordes, J. M., 2013. Pulsar state switching from Markov transitions and stochastic resonance. ApJ 775:47 Cordes, J. M., Shannon, R. M., 2008. Rocking the lighthouse: Cir- cumpulsar asteroids and radio intermittency. ApJ 682, 1152–1165 Cordes, J. M., Sullivan, W. T., III, 1995. Astrophysical coding: A new approach to SETI signals – I. Signal design and wave prop- agation, in: Shostak, G. S. (Ed.), Progress in the Search for Ex- traterrestrial Life, ASP Conf. Ser., Vol. 74. Astron. Soc. Pac., San Francisco, pp. 325–334 Dyson, F. J., 1960. Search for artificial stellar sources of infrared radiation. Science 131, 1667–1668 Fabian, A. C., 1977. Signalling over stellar distances with X-rays. JBIS 30, 112–113 Horowitz, P., Sagan, C., 1993. Five years of Project META – an all-sky narrow-band radio search for extraterrestrial signals. ApJ 415, 218–235 Kardashev, N. S., 1964. Transmission of information by extraterres- trial civilizations. SvA 8, 217–221 Kijak, J., Gil, J., 2003. Radio emission altitude in pulsars. A&A 397, 969–972 Learned, J. G., Kudritzki, R.-P., Pakvasa, S., Zee, A., 2008. The Cepheid Galactic internet. arXiv:0809.0339

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