Crab Giant Pulses at Low Frequencies
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A&A 538, A7 (2012) Astronomy DOI: 10.1051/0004-6361/201117667 & c ESO 2012 Astrophysics Crab giant pulses at low frequencies R. Karuppusamy1,3, B. W. Stappers2,3,4,andK.J.Lee1 1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Bonn, Germany e-mail: [email protected] 2 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK e-mail: [email protected] 3 Sterrenkunde Instituut Anton Pannenkoek, University of Amsterdam, Kruislaan 403, Amsterdam, The Netherlands 4 Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA, Dwingeloo, The Netherlands Received 8 July 2011 / Accepted 3 December 2011 ABSTRACT We report observations of the Crab pulsar at frequencies in the range of 110–180 MHz. The combination of coherent dedispersion and the narrow synthesised beam of the Westerbork Synthesis Radio Telescope resulted in a sensitive observation. Our improved sensitivity and resolution allow us to confirm the presence of a precursor to the interpulse at these frequencies. We also detected more than 1000 giant pulses and find that the interpulse precursor component shows no giant pulse emission. Therefore, we attribute it to a similar emission source as the precursor to the mainpulse. Together these precursors might be the normal emission seen from the majority of radio pulsars. From the dispersion-free giant pulses, we find that the emission rate is ∼10–20 × 10−3 s−1 and the scatter timescale in the range of ∼1.5–5.6 ms. We further find that the radio flux of the pulsar is 6–11 Jy in this frequency range. Key words. radiation mechanisms: non-thermal – scattering 1. Introduction at a frequency of 9 GHz (Hankins & Eilek 2007). However, at low radio frequencies, the signal propagation effects in the in- The Crab nebula and pulsar are amongst the most intensively terstellar medium (ISM) blur the narrow pulses considerably. studied objects on the sky. The discovery of the Crab pulsar The first of these effects is dispersion in the ISM that results (Staelin & Reifenstein 1968) in the Crab nebula (identified as in higher radio frequencies arriving earlier than the lower at the the remains of the supernova, SN 1054) was the first observa- telescope which smears the pulse. Secondly, the intrinsically nar- tional evidence for neutron stars being formed in supernova ex- row pulse is broadened due to multipath propagation effects (e.g. plosions. The Crab pulsar is a bright source of electromagnetic Williamson 1973). Past studies at low frequencies were limited radiation and is detected at all observable wavelengths. At radio either by insufficient removal of dispersion or low sensitivity of frequencies, the pulsar emission shows a variety of features of observing systems and hence resulted in fewer giant pulses be- which the evolution of the average pulse profile with increasing ing detected at low frequencies. For example, the early studies radio frequency is readily noticed. For example, in addition to relied on fewer than 400 giant pulses which were also not coher- the main- and interpulse emission visible at most frequencies, ently dedispersed (e.g. Sutton et al. 1971, at 160 MHz; and both other emission components appear at certain frequencies; the Argyle & Gower 1972; Gower & Argyle 1972, at 146 MHz). precursor emission component is visibile only at low frequen- More recently, Popov et al. (2006b) used a sensitive telescope, cies (Rankin et al. 1970); the low-frequency component appears but incoherent dedispersion of the pulsar signal resulted in only between 600–4800 MHz and two high-frequency components in 128 detected giant pulses at 111 MHz. Similarly, Bhat et al. the 4000–8400 MHz frequency range (Moffett & Hankins 1996). (2007) found 31 pulses at 200 MHz by coherently dedispers- In the latter work, it is also seen that no interpulse emission is ing the pulsar signal, but used a less sensitive system with an 2 present at 2700 MHz, and reappears slightly earlier in phase effective area, Aeff ≈ 50 m . The need for coherent dedispersion above 4700 MHz. An enigmatic feature in the radio emission is highlighted by the ∼23 pulse periods or 766.75 ms of disper- from the pulsar is the intense radio bursts called giant pulses, sion smearing within the 2.5 MHz band at 116.75 MHz, which which actually led to the discovery of the Crab pulsar (Staelin is the lowest band in our observations. Even with a reasonable & Reifenstein 1968). The giant pulses were reported for the first 64-channel hardware filter bank, the dispersion smearing in each time by Sutton et al. (1971) as “jumbo” pulses that formed a channel is 11.9 ms, which if not removed allows the detection of long tail in the single pulse intensity histogram. Soon after this only the very bright pulses. work, the individual pulses from the Crab pulsar were exten- The Crab pulsar has a rather steep spectrum with a spectral sively characterised and the power-law nature of the pulse en- index α = −3(Rankin et al. 1970; Maron et al. 2000). This im- ergy distribution was firmly estalished (Argyle & Gower 1972; plies that a large number of giant pulses should be detectable at Gower & Argyle 1972). low radio frequencies, if their occurence rate remains the same Giant pulses are now defined as pulses with energy greater as that reported at higher frequencies (Lundgren et al. 1995). than ten times the average pulse energy and have widths very Scattering in the ISM affects normal pulses and giant pulses narrow compared to the average pulse emission (Knight 2007). alike, but the effect is more easily measured in the intense gi- As an example, narrow pulses on the order of 0.4 ns are observed ant pulses owing to their narrowness. If the dispersion is fully Article published by EDP Sciences A7, page 1 of 7 A&A 538, A7 (2012) Table 1. Observation details. Frequency Session I Session II Session III Session IV (MHz) 08:59:50a 11:59:50a 14:59:50a 17:59:50a 116.75 . X X X X 122.75 . * * * – 139.125 . X X X – 141.75 . – – X X 147.50 . – X X X 157.00 . X X X X 162.50 . X X X – 173.85 . X X X X Notes. (a) Observation start times of the 15-min sessions in UTC. (b) Successful runs are marked “X”, runs that failed with “–” and ses- sions severely affected by RFI with “*”. corrected, the giant pulse emission rates and the scattering phe- nomena can be studied in much greater detail. The observing system we use has the possibility to coherently dedisperse the pulsar signal. Thus this study marks the first coherently dedis- persed study of the emission from the Crab pulsar with a res- onably sensitive telescope, resulting in a large number of giant pulses in the 110–180 MHz frequency range. The preceeding discussion motivates the revisit of the Crab giant pulse emission at low radio frequencies. The rest of the paper is organised as follows. The observa- Fig. 1. Plot of total intensity of Crab pulsar against time in the four observing sessions shown separated by dashed horizontal lines. The left tions are described in Sect. 2. The flux calibration, average ra- panel corresponds to the band centred at 116.75 MHz, while the panel dio emission and the spectra of the Crab pulsar are discussed on the right is at 157 MHz. The elapsed time shown on the vertical axis in Sect. 3. Scattering in single pulses and across the frequency is the observation time within each session. The vertical axis is labelled range is treated in Sect. 5. The single pulse statistics are dis- with the hour angle of the Crab pulsar on the right side of the second cussed in Sect. 4, followed by discussion and conclusions. panel and corresponds to the middle of the 15-min observing sessions. 2. Observations and data reduction 3. Radio flux of the Crab pulsar The observations reported here were carried out at the 3.1. Flux calibration Westerbork Synthesis Radio Telescope (WSRT) using the Low The Crab nebula is a bright extended object with a typical size of Frequency Front Ends (LFFEs) on 13 May 2006 (for details, see Table 1). The pulsar signal was recorded and processed using 6 ×4 and therefore dominates the telescope system temperature the PuMa-II instrument (Karuppusamy et al. 2008). The LFFEs when the width of the telescope beam is comparable to the angu- lar size of the nebula. The frequency dependence of the nebular were tuned to eight frequencies each of which was 2.5 MHz − . wide in the 110–180 MHz frequency range and the signals were flux can be expressed as 955ν 0 27 Jy, where ν is the frequency recorded to disk. The recorded signal was spread over four 15- in GHz (Bietenholz et al. 1997). For these observations, the min sessions (see Table 1). This baseband data was then coher- WSRT was configured to operate in the tied array mode, where ently dedispered using the dispersion measure (DM) obtained the signals from fourteen 25-m telescopes were coherently added from the Crab pulsar ephemeris maintained by the Jodrell Bank by purpose-built hardware. The resulting synthesised beam de- Observatory1. A 32-channel coherent filterbank was formed us- pends on the source hour angle and radio frequency. In the 110– ing the open-source pulsar data processing software package 180 MHz range the beam is smaller than the nebula at higher DSPSR (van Straten & Bailes 2011) and all single pulses were frequencies and at small hour angles of the Crab pulsar.