The Crab pulsar is a unique source of pulsar radio emission. Its regular pulse structure is visible over the entire electromagnetic spectrum from radio to GeV ranges. Among the regular pulses, radio giant pulses (GPs) are known as a special form of pulsar radio emission. Although the Crab pulsar was discovered by its GPs, their origin and emission mechanisms are currently not understood. Within the framework of this report we give a review on radio GPs and present a new idea on how to examine the characteristics of this as yet not understood kind of pulsar emission

Elsäesser, D.

Lewandowska, N.

Mannheim, K.

English

N. Lewandowska

D. Elsäesser

K. Mannheim

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Acta Polytechnica

Search for a Correlation Between Radio Giant Pulses and VHE Photons of the Crab Pulsar

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Acta Polytechnica Vol. 51 No. 6/2011Search for a Correlation Between Radio Giant Pulses andVHE Photons of the Crab PulsarN. Lewandowska, D. Elsa¨esser, K. MannheimAbstractThe Crab pulsar is a unique source of pulsar radio emission. Its regular pulse structure is visible over the entireelectromagnetic spectrum from radio to GeV ranges. Among the regular pulses, radio giant pulses (GPs) are known asa special form of pulsar radio emission. Although the Crab pulsar was discovered by its GPs, their origin and emissionmechanisms are currently not understood. Within the framework of this report we give a review on radio GPs andpresent a new idea on how to examine the characteristics of this as yet not understood kind of pulsar emission.Keywords: neutron star, pulsar, Crab pulsar, regular pulses, giant pulses.1 IntroductionEmbedded in the supernova remnant SN 1054, theCrab pulsar (PSR B0531+21) is currently the onlypulsar known with a pulsed emission structure seenover the entire electromagnetic spectrum. It consistsof a main pulse (P1) and an interpulse (P2) occurringat the rotational phases of 70 and ≈ 110 degrees, re-spectively ( [1]). Apart from these two regular pulses,further pulsed structures are visible at various fre-quency ranges. The precursor, for example, is animplementation which has only been observed fromabout 300 to 600 MHz. At higher frequencies, two ad-ditional high frequency components known as HFC1 and HFC 2 are visible from ≈ 4 000 to 8 000 MHz,simultaneously with a phase shift of the interpulse byabout 10 degrees [1]. While the origin of P1 and P2can be partly described by current pulsar theories,the origin of the precursor and HFC components stillremains a mystery. In addition to this regular pulsedstructure, the Crab pulsar is also a known source ofradio GPs. Due to their properties, GPs are an un-usual and exotic form of pulsar radio emission. Theyhave been observed in a wide frequency range from23 MHz [5] to 15.1 GHz [6,7] at phases of P1 and P2,and distinguish themselves by ﬂux densities that arehigher by a factor of 5 × 105 and pulse widths from1–2 microseconds [8]. Several studies also conﬁrm theoccurrence of GPs at both HFC components [9, 6].Nevertheless none have been veriﬁed at the phase ofthe precursor. The emission of GPs therefore seemsto be phase bounded.Diﬀerences between GPs occurring at P1 and P2were discovered by obervations with the Arecibo sin-gle dish telescope1 [11]. These observations indicatethat the time and frequency patterns of GPs at P1are diﬀerent from those at P2 at frequencies above4 GHz. While Giant main pulses (GMPs) consistof narrow-band pulses of nanosecond duration, Gi-ant interpulses (GIPs) reveal narrow emission bandswith durations in the microsecond range. These dif-ferences in GPs at P1 and P2 possibly arise fromdiﬀerent emission mechanisms underlying their de-velopment. Additionally, they contradict all previousemission theories of the Crab pulsar that assume asimilar development process of both pulses.Theoretical aspects of GPs have been broadly in-vestigated [15, 16, 4, 17, 18]. Nevertheless, in spite ofover 40 years since the detection of the Crab pul-sar by its GPs [2], their possible origin and emissionmechanism is still not understood.The only current model based on observationaldata refers to GIPs above 4 GHz. The Lyutikovmodel [12] reconstructs the emission bands of GIPsunder the assumption of higher particle density onclosed magnetic ﬁeld lines in contrast with the stan-dard Goldreich-Julian model [13]. According to [14],this density is highest near the last closed magneticﬁeld line at which a Lorentz beam develops due tomagnetic reconnection events. While it moves alongthe closed ﬁeld line, it dissipates by curvature radi-ation. Furthermore, the Lyutikov model also pre-dicts the occurrence of γ-radiation together with ra-dio GPs, and provides the motivation for simultane-ous observations at γ-wavelengths.Currently, no universal model is available forCrab radio GPs, since the Lyutikov model is onlyapplicable to GIPs above 4 GHz, where the P2 com-ponent changes in its position by 10 degrees [1].With apparently sporadic, short pulses of thiskind, the Crab pulsar, together with its twin pul-sar PSR B0540-69 in the Large Magellanic (LMC)1http://www.naic.edu/34Acta Polytechnica Vol. 51 No. 6/2011cloud, belongs to a small group of 11 pulsars whichare known to emit radio GPs. This group also con-sists of ordinary and millisecond pulsars (MSPs) [19],and it was thought that a common feature of themcould be a high magnetic ﬁeld at the light cylinder.However, no uniform accordance in all 11 pulsarscould be found, and this makes the GP phenomenona still enigmatic feature of pulsar radio emission.2 MultiwavelengthobservationsThe incentive of multiwavelength (MWL) observa-tions is to deduce the central emission mechanismunderlying the GP phenomenon. Currently it is as-sumed that radio GPs could be caused by coherentemission, by pair production processes or by changesin the beaming direction. To shed more light on thistopic, several MWL observation campaigns were car-ried out. Radio GPs and γ-ray photons observedsimultaneously with the Green Bank 43 m telescopeand OSSE were examined in [10], but no correlationcould be veriﬁed.A weak correlation between radio GPs and opticalphotons was veriﬁed by Shearer et al. [20], who ob-served the Crab pulsar simultaneously with the West-erbork Synthesis Radio Telescope (WSRT) and withthe TRIFFID optical photometer. They observed anincrease in the optical ﬂux by≈ 3 % during the occur-rence of radio GPs, which proves that an additionalnon-coherent emission process accounts for the GPemission. To examine the possible MWL occurrenceof GPs in a wider extent, further MWL studies at ra-dio, optical and also γ-wavelengths are necessary inorder to see if pair production processes, for example,are involved in the GP emission.2.1 Fermi LATArguing that the observations in [10] were based oninsuﬃcient sensitivity, several observation campaignsof the Crab pulsar were carried out with the GBTand Fermi LAT2 to examine the assumptions of theLyutikov model [21–23]. With a collection area of0.8 m2, a total of 77 Fermi photons were detectedin more than 10 hours of observations in the energyrange between 100 MeV and 5 GeV, simultaneouslywith over 210 000 radio GPs at a frequency of 8.9 GHz(Figure 1) [22,23]. In each case, a search was made fora correlation between the GP rate and single Fermiphotons in addition to a change in the γ-ray ﬂuxaround single GPs. However, Bilous et al. concludethat with 95 % probability the energy ﬂux in a 30 mstime window is not higher than 6 times the averageﬂux, which suggests that coherent emission is the re-sponsible mechanism for GIPs (Giant interpulses, seeIntroduction). One of their conclusions refers to thepossible existence of a correlation at very high ener-gies ≥ 100 GeV, at which a decisive number of pho-tons for a correlation analysis cannot be provided ina reasonable time span by Fermi LAT [21].Fig. 1: Distribution of GP peak ﬂux density and energyof γ-ray photons detected by Fermi over the pulsar rota-tional phase [Bilous et al.(2010)]2.2 Cherenkov telescopesOne key question resulting from the observationswith Fermi LAT is whether the correlation does notexist at higher energies. At this point, Imaging AirCherenkov Telescopes (IACTs) with a general sensi-tivity > 60 GeV are essential. Telescopes of this kindobserve γ-rays indirectly through the detection of ex-tensive air showers. When a γ-ray reaches the atmo-sphere of the Earth, it strikes one of its molecules andproduces a cascade of secondary particles. These sec-ondary particles produce Cherenkov radiation mov-ing nearly at the speed of light at a height of about10–20 km in the atmosphere. The Cherenkov light isemitted in the form of a cone around the direction ofthe primary particle (Figure 2). These brief ﬂashesof Cherenkov radiation are imaged by IACTs.With a primary mirror 17 m in diameter ineach case, Major Atmospheric Gamma ImagingCherenkov (MAGIC)3 telescopes are currently thebiggest IACTs worldwide. Among other IACTs, forexample CANGAROO III, H.E.S.S. and VERITAS,the MAGIC telescopes were the ﬁrst to detect thepulsed emission of the Crab pulsar at an energythreshold of 25 GeV provided by a special trigger sys-tem [24]. Their large mirrors enable the detection ofsingle VHE photons on short time scales. Due to theshort widths of radio GPs, an accurate timing system2http://www-glast.stanford.edu/3http://magic.mppmu.mpg.de/35Acta Polytechnica Vol. 51 No. 6/2011down to at least microseconds is needed for a corre-lation analysis with VHE photons. Employing theGlobal Positioning System, MAGIC provides a timestamp with an accuracy of 200 ns for each single VHEphoton, and permits the user to decide whether itarose simultaneously with a radio GP. Thus MAGICaﬀords a unique opportunity to search for VHE pho-tons coinciding with GPs at a sensitivity exceedingprevious studies using e.g. Fermi LAT, and to testvarious models dealing with the possible generationof Crab GPs [15, 16, 4, 17, 18, 12].Fig. 2: Illustration of the air shower techniquehttp://icc.ub.edu/gp oa.phpReferences[1] Moﬀett, D. A., Hankins, T. H.: AstrophysicalJournal, 1996, Vol. 468, p. 779.[2] Staelin, D. H., Reifenstein, E. C.: III, Science,1968, Vol. 162, (3 861), p. 1 481–1483.[3] Soglasnov, V.: Proceedings of the 363rd WE-Heraeus Seminar on Neutron Stars and Pulsars40 years after the discovery, 2007, MPE-Report291, p. 68.[4] Hankins, T. H.: Nature, 2003, Vol. 422, (6 928),p. 141–143.[5] Popov, M. V., Kuzmin, A. D., Ulyanov, O. M.,Deshpande, A. A., Ershov, A. A., Kon-dratiev, V. I., Kostyuk, S. V., Losovsky, B. Ya.,Soglasnov, V. A., Zakharenko, V. V.: On thePresent and Future of Pulsar Astronomy, 26thmeeting of the IAU, Joint Discussion 2, 2006.[6] Hankins, T. H.: Proceedings of the 177th Collo-quium of the IAU held in Bonn, 2000, Vol. 202,p. 165.[7] Jessner, A., Popov, M. V., Kondratiev, V. I.,Kovalev, Y. Y., Graham, D., Zensus, A., So-glasnov, V. A., Bilous, A. V., Moshkina, O. A.:Astronomy and Astrophysics, 2010, Vol. 524,(id.A60).[8] Kuzmin, A. D.: Astrophysics and Space Science,2007, Vol. 308, 1–4, p. 563–56.[9] Jessner, A., Slowikowska, A., Klein, B.,Lesch, H., Jaroschek, C. H., Kanbach, G., Han-kins, T. H.: Advances in Space Research, 2005,Vol. 35, 6, p. 1 166–1 171.[10] Lundgren, S. C., Cordes, J. M., Ulmer, M.,Matz, S. M., Lomatch, S., Foster, R. S., Han-kins, T.: Astrophysical Journal, 1995, Vol. 435,6 928, p. 433.[11] Hankins, T. H., Eilek, J. A.: Astrophysical Jour-nal, 2007, Vol. 670, 1, p. 693–701.[12] Lyutikov, M.: Monthly Notices of the RoyalAstronomical Society, 2007, Vol. 381, 3,p. 1 190–1 196.[13] Goldreich, J., Julian, W. H.: Astrophysical Jour-nal, 1969, Vol. 157, p. 86.[14] Gruzinov, A.: Physical Review Letters, 2005,Vol. 94, 2, id. 021101.[15] Mikhailovskii, A. B., Onishchenko, O. G.,Smolyakov, A. I.: Soviet Astr. Lett. (Tr:Pisma), 1985, Vol. 11, No. 2/MAR/APR, p. 78.[16] Weatherall, J. C.: Astrophysical Journal, 2001,Vol. 559, 1, p. 196–200.[17] Petrova, S. A.: Astronomy and Astrophysics,2004, Vol. 424, p. 227–236.[18] Istomin, Y. N.: Astronomical Society of the Pa-ciﬁc, 2004, p. 369.[19] Slowikowska, A., Jessner, A., Kanbach, G.,Klein, B.: Proceedings of the 363rd WE-HeraeusSeminar on Neutron Stars and Pulsars 40 yearsafter the discovery, 2007, MPE-Report 291,p. 64.[20] Shearer, A., Stappers, B., O’Connor, P.,Golden, A., Strom, R., Redfern, M., Ryan, O.:Science, 2003, Vol. 301, 5 632, p. 493–495.[21] Bilous, A. V., Kondratiev, V. I., McLaugh-lin, M. A., Mickaliger, M., Lorimer, D. R.,Ransom, S. M., Lyutikov, M., Stappers, B.,Langston, G. I.: 2009 Fermi Symposium, eConfProceedings C091122.36Acta Polytechnica Vol. 51 No. 6/2011[22] Bilous, A. V., Kondratiev, V. I., McLaugh-lin, M. A., Ransom, S. M., Lyutikov, M., Mick-aliger, M., Stappers, B., Langston, G. I.: Pro-ceedings of the ISKAF2010 Science Meeting.June 10–14, 2010.[23] Bilous, A. V., Kondratiev, V. I., McLaugh-lin, M. A., Ransom, S. M., Lyutikov, M., Mick-aliger, M., Langston, G. I.: Astrophysical Jour-nal, 2011, Vol. 728, 110.[24] Aliu, E., et al.: Science, 2008, Vol. 322, 5 905,p. 1 221.Natalia LewandowskaDominik Elsa¨esserKarl MannheimUniversity of Wu¨rzburg37

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Search for a Correlation Between Radio Giant Pulses and VHE Photons of the Crab Pulsar

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