ap-6-11.dvi Acta Polytechnica Vol. 51 No. 6/2011 Search for a Correlation Between Radio Giant Pulses and VHE Photons of the Crab Pulsar N. Lewandowska, D. Elsäesser, K. Mannheim Abstract 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. Keywords: neutron star, pulsar, Crab pulsar, regular pulses, giant pulses. 1 Introduction Embedded in the supernova remnant SN 1054, the Crab pulsar (PSR B0531+21) is currently the only pulsar known with a pulsed emission structure seen over the entire electromagnetic spectrum. It consists of amainpulse (P1) andan interpulse (P2)occurring at the rotational phases of 70 and ≈ 110 degrees, re- spectively ( [1]). Apart fromthese two regularpulses, further pulsed structures are visible at various fre- quency ranges. The precursor, for example, is an implementation which has only been observed from about300 to600MHz. Athigher frequencies, twoad- ditional high frequency components known as HFC 1 and HFC 2 are visible from ≈ 4000 to 8000 MHz, simultaneouslywith aphase shift of the interpulse by about 10 degrees [1]. While the origin of P1 and P2 can be partly described by current pulsar theories, the origin of the precursor andHFC components still remains amystery. In addition to this regular pulsed structure, the Crab pulsar is also a known source of radio GPs. Due to their properties, GPs are an un- usual and exotic form of pulsar radio emission. They have been observed in a wide frequency range from 23MHz [5] to 15.1GHz [6,7] at phases of P1 andP2, and distinguish themselves by flux densities that are higher by a factor of 5 × 105 and pulse widths from 1–2microseconds [8]. Several studies also confirmthe occurrence of GPs at both HFC components [9,6]. Nevertheless none have been verified at the phase of the precursor. The emission of GPs therefore seems to be phase bounded. Differences between GPs occurring at P1 and P2 were discovered by obervationswith the Arecibo sin- gle dish telescope1 [11]. These observations indicate that the time and frequency patterns of GPs at P1 are different from those at P2 at frequencies above 4 GHz. While Giant main pulses (GMPs) consist of narrow-band pulses of nanosecond duration, Gi- ant interpulses (GIPs) reveal narrow emission bands with durations in the microsecond range. These dif- ferences in GPs at P1 and P2 possibly arise from different emission mechanisms underlying their de- velopment. Additionally, they contradict all previous emission theories of the Crab pulsar that assume a similar development process of both pulses. Theoretical aspects of GPs have been broadly in- vestigated [15,16,4,17,18]. Nevertheless, in spite of over 40 years since the detection of the Crab pul- sar by its GPs [2], their possible origin and emission mechanism is still not understood. The only current model based on observational data refers to GIPs above 4 GHz. The Lyutikov model [12] reconstructs the emission bands of GIPs under the assumption of higher particle density on closed magnetic field lines in contrast with the stan- dard Goldreich-Julianmodel [13]. According to [14], this density is highest near the last closed magnetic field line at which a Lorentz beam develops due to magnetic reconnection events. While it moves along the 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 for Crab radio GPs, since the Lyutikov model is only applicable to GIPs above 4 GHz, where the P2 com- ponent changes in its position by 10 degrees [1]. With apparently sporadic, short pulses of this kind, the Crab pulsar, together with its twin pul- sar PSR B0540-69 in the Large Magellanic (LMC) 1http://www.naic.edu/ 34 Acta Polytechnica Vol. 51 No. 6/2011 cloud, belongs to a small group of 11 pulsars which are known to emit radio GPs. This group also con- sists of ordinary andmillisecondpulsars (MSPs) [19], and it was thought that a common feature of them could be a high magnetic field at the light cylinder. However, no uniform accordance in all 11 pulsars could be found, and this makes the GP phenomenon a still enigmatic feature of pulsar radio emission. 2 Multiwavelength observations The incentive of multiwavelength (MWL) observa- tions is to deduce the central emission mechanism underlying the GP phenomenon. Currently it is as- sumed that radio GPs could be caused by coherent emission, by pair production processes or by changes in the beaming direction. To shed more light on this topic, severalMWLobservation campaignswere car- ried out. Radio GPs and γ-ray photons observed simultaneously with the Green Bank 43 m telescope and OSSE were examined in [10], but no correlation could be verified. Aweakcorrelationbetween radioGPsandoptical photons was verified by Shearer et al. [20], who ob- served theCrabpulsar simultaneouslywith theWest- erbork Synthesis Radio Telescope (WSRT) and with the TRIFFID optical photometer. They observed an increase in theopticalfluxby≈ 3%during theoccur- rence of radio GPs, which proves that an additional non-coherent emission process accounts for the GP emission. To examine the possible MWL occurrence ofGPs in awider extent, furtherMWLstudies at ra- dio, optical and also γ-wavelengths are necessary in order to see if pair productionprocesses, for example, are involved in the GP emission. 2.1 Fermi LAT Arguing that the observations in [10] were based on insufficient sensitivity, several observationcampaigns of the Crab pulsar were carried out with the GBT and Fermi LAT2 to examine the assumptions of the Lyutikov model [21–23]. With a collection area of 0.8 m2, a total of 77 Fermi photons were detected in more than 10 hours of observations in the energy range between 100 MeV and 5 GeV, simultaneously withover210000radioGPsata frequencyof8.9GHz (Figure1) [22,23]. Ineachcase, a searchwasmade for a correlation between the GP rate and single Fermi photons in addition to a change in the γ-ray flux around single GPs. However, Bilous et al. conclude thatwith 95%probability the energy flux in a 30ms time window is not higher than 6 times the average flux, which suggests that coherent emission is the re- sponsiblemechanism forGIPs (Giant interpulses, see Introduction). One of their conclusions refers to the possible 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 in a reasonable time span by Fermi LAT [21]. Fig. 1: Distribution of GP peak flux density and energy of γ-ray photons detected by Fermi over the pulsar rota- tional phase [Bilous et al.(2010)] 2.2 Cherenkov telescopes One key question resulting from the observations with Fermi LAT is whether the correlation does not exist at higher energies. At this point, Imaging Air Cherenkov Telescopes (IACTs) with a general sensi- tivity > 60GeVare essential. Telescopes of this kind observe γ-rays indirectly through the detection of ex- tensive air showers. When a γ-ray reaches the atmo- sphere of theEarth, it strikes one of itsmolecules and produces a cascade of secondaryparticles. These sec- ondary particles produce Cherenkov radiation mov- ing nearly at the speed of light at a height of about 10–20 km in the atmosphere. The Cherenkov light is emitted in the form of a cone around the direction of the primary particle (Figure 2). These brief flashes of Cherenkov radiation are imaged by IACTs. With a primary mirror 17 m in diameter in each case, Major Atmospheric Gamma Imaging Cherenkov (MAGIC)3 telescopes are currently the biggest IACTs worldwide. Among other IACTs, for example CANGAROO III, H.E.S.S. and VERITAS, the MAGIC telescopes were the first to detect the pulsed emission of the Crab pulsar at an energy thresholdof 25GeVprovidedbya special trigger sys- tem [24]. Their large mirrors enable the detection of single VHE photons on short time scales. Due to the shortwidths of radioGPs, an accurate timing system 2http://www-glast.stanford.edu/ 3http://magic.mppmu.mpg.de/ 35 Acta Polytechnica Vol. 51 No. 6/2011 down to at least microseconds is needed for a corre- lation analysis with VHE photons. Employing the Global Positioning System, MAGIC provides a time stampwith anaccuracyof 200ns for each singleVHE photon, and permits the user to decide whether it arose simultaneouslywith a radioGP. Thus MAGIC affords a unique opportunity to search for VHE pho- tons coinciding with GPs at a sensitivity exceeding previous studies using e.g. Fermi LAT, and to test various models dealing with the possible generation of Crab GPs [15,16,4,17,18,12]. Fig. 2: Illustration of the air shower technique http://icc.ub.edu/gp oa.php References [1] Moffett, D. A., Hankins, T. H.: Astrophysical Journal, 1996, Vol. 468, p. 779. [2] Staelin, D. H., Reifenstein, E. C.: III, Science, 1968, Vol. 162, (3861), p. 1481–1483. [3] Soglasnov, V.: Proceedings of the 363rd WE- Heraeus Seminar on Neutron Stars and Pulsars 40 years after the discovery, 2007,MPE-Report 291, p. 68. [4] Hankins, T. H.: Nature, 2003, Vol. 422, (6928), 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 the Present and Future of Pulsar Astronomy, 26th meeting 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. 1166–1171. [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, 6928, 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 Royal Astronomical Society, 2007, Vol. 381, 3, p. 1190–1196. [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 363rdWE-Heraeus Seminar on Neutron Stars and Pulsars 40 years after 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, 5632, 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, eConf Proceedings C091122. 36 Acta 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, 5905, p. 1221. Natalia Lewandowska Dominik Elsäesser Karl Mannheim University of Würzburg 37