Tomographic Studies of Pulsar Radio Emission Cones and Searches for Radio Counterparts of Gamma-Ray Pulsars
Radio emission from pulsars is believed to originate from charged particles streaming along the open magnetic field lines, radiating within a narrow cone at each of the two magnetic poles. In each rotation of the star, the emission beam sweeping across the observer’s line of sight, is seen as a pulse of radio emission. Average pulse profiles integrated over several hundreds of individual pulses, along with polarization information, reveal the viewing geometry and various emission properties(e.g., emission in multiple cones, frequency dependence of the emission altitude, notches in the average profiles, etc.), and provide some clues about the possible emission mechanisms. The sequence of individual pulses generally exhibit richer details, e.g., pulse-nulling, variety of subpulse drifting, polarization mode-changing, micro-structure and giant pulse emission, etc., and seem to be more crucial and promising in probing the underlying physical processes. The physical understanding of many of the above properties and phenomena is still far from complete. In ﬁrst two parts of this thesis, we address a few of these aspects, and probe related details by mapping the pulsar polar emission patterns, while in the last part, we present our searches for dispersed signals(periodic as well as transient) at very low frequencies. More specifically, Part-I makes use of the present understanding of drifting subpulses phenomenon to reconstruct the emission patterns in nearly complete polar cap region of the pulsar B1237+25, and addresses the origin of emission in multiple cones using these reconstructed emission maps. In Part-II, we discuss a need for new instrumentation primarily motivated by the need for tomographic studies of pulsar polar emission regions. We report the consequent design and development of a novel, self-contained multi-band receiver (MBR)system, intended for use with a single large aperture to facilitate sensitive and high time-resolution observations simultaneously in 10 discrete frequency bands sampling a wide spectral span(100–1500MHz) in a nearly log-periodic fashion. Part-III presents our deep searches designed to detect radio transient as well as periodic signals from the (so far) “radio-quiet” gamma-ray pulsars — a population of radio silent pulsars recently discovered using the Large Area Telescope on the Fermi-satellite. Brief descriptions of the issues addressed in the three parts of the thesis, along with a summary of respective results, is as follows. 1. Origin of Radio Emission in Multiple Cones Many pulsars exhibit systematic variations in position and intensity of their subpulses, a phenomenon now well known as “subpulse drifting”. Ruderman & Sutherland(1975) suggested this regular modulation to be a manifestation of a carousel of “spark” discharges in the acceleration zone of the star, circulating around the magnetic axis because of the E×B drift. In the qualitative framework of the above carousel model, the coherent modulation in a subpulse sequence can be mapped back to the underlying pattern of sub-beams/emission-columns (see, for example, Deshpande & Rankin, 1999). However, the completeness with which the underlying configuration of sub-beams can be sampled depends on how close our line of sight approaches the magnetic axis. The bright pulsar B1237+25 has a special viewing geometry where the sightline traverses almost through the magnetic axis, thus providing an excellent opportunity to map and study the underlying patterns across the full transverse slice of its polar emission region. However, the rich variety in pulse-to-pulse fluctuations in this pulsar makes this task challenging. In Chapter 2, we present our analysis of a number of pulse-sequences from this star observed with the Arecibo telescope, wherein we search for, and use, coherent modulation in sub-sequences, to map the underlying emission patterns. The reconstructed maps provide a convenient way to study the details in multiple emission cones, and any inter-relationship between them. More specifically, we have utilized these maps to explore whether the multiple cones of this pulsar originate from a common seed pattern or not. A summary of results The results obtained from our study of B1237+25 are summarized below: 1 The underlying carousel of sparks for this pulsar appears to lack stability over long durations. The circulation period, deduced using smaller length sub-sequences, appears to vary over a large range(about18 to34 times the rotation period). 2. The emission patterns corresponding to the outer and the inner cones are found to be significantly correlated with each other, implying that the emission in the two cones share a common seed pattern of sparks. This main result is consistent with the same radio frequency emission in the two cones, originating from a common seed pattern of sparks at two different altitudes. 3 The emission patterns corresponding to the outer and the inner cones are found to be offset from each other, consistently across various sub-sequences, by about 10◦ in magnetic azimuth. This large offset indicates certainly a twist in the emission columns, and most likely in the magnetic field geometry, between the two different emission altitudes. 4. The core component also seems to share its origin with the conal counterparts. Presence of a compact, diffuse and further-in carousel of sub-beams is consistent with the observed modulation in the core component of this pulsar. The featureless spectrum observed for many core-single pulsars can be explained readily when the diffuse pattern approaches uniformity. 2.Tomography of the Pulsar Magnetosphere: Development of a Multi-band Receiver Although drifting subpulses are now routinely interpreted in the qualitative framework of the carousel model, estimation of circulation time associated with the system of emission columns has been possible so far in only a handful of pulsars, and the important details determining such configurations, their evolution across the magnetosphere, and the pattern circulation are yet to be understood. Radius-to-frequency mapping in pulsars suggests that the lower frequency emission originates farther away from the surface of the star than the higher frequency emission. Hence, the sub-beam configuration mapped at a particular frequency provides a view of a single slice of the polar emission region at the corresponding emission altitude. Mapping of the underlying emission patterns simultaneously at a number of frequencies would amount to viewing a “tomograph” of the pulsar magnetosphere. Such tomographic studies would reveal not only the evolution of sub-beams across the magnetosphere but can also provide much needed clues about the generation of the sub-beam patterns, and their possible connection with the profile/polarization mode changes observed in various pulsars. Simultaneous multi-frequency observations, which are required for many other interesting astronomical studies as well, are usually carried out by using several telescope, each observing at different frequency. Such an approach has inherent complexity in coordinating various telescopes, in addition to numerous other difficulties which limit the desired advantages of such observations. Some of these difficulties, which we faced in our attempt of carrying out simultaneous multi-frequency observations using five different telescopes, are discussed in Chapter 3. We suggest an optimum approach to carry out simultaneous multi-frequency observations, using a single large aperture. In Chapter 4, we present the design of a novel, “self-contained” multi-band receiver(MBR) system developed for this purpose. The MBR system includes a suitable feed, broadband front-end, parallel analog and digital receiver pipelines, along with appropriate monitoring, synchronization and data recording systems. When used with a large aperture, the MBR facilitates high time-resolution observations simultaneouslyin10discretefrequencybandssampling a wide spectral span(100–1500MHz) in a nearly log-periodic fashion. The raw voltage time sequences corresponding to each of the two linear polarization channels for each of the 10 spectral bands are simultaneously recorded, each sampling a bandwidth of 16 MHz at the Nyquist rate. The dual-polarization multi-band feed, a key component of the MBR, is designed to have good responses only overthe10discretebandspre-selected as relatively RFI-free, and hence provides preliminary immunity against RFI. The MBR also offers significant tunability of the center frequencies of each of the 16-MHz sub-bands separately, within the spectral spans of respective bands. Similarity of the 10 sub-band receiver chains provides desired compatibility, in addition to an easy inter-changeability of these units, if required, and an overall modularity to the system. The MBR was used with the 110 meter Green Bank Telescope to conduct test observations on a few bright continuum sources, and about 20 hours of observations on a number of bright pulsars. Using these observations, we have constructed a preliminary tomograph of the polar emission region of B0809+74, and studied the spectral evolution of emission altitudes and flux density ofB0329+54(Chapter5). Although the MBR system design is optimized for tomographic studies of pulsar polar emission regions, the simultaneous multi-frequency observations with such a system offer particular advantages in fast transient searches. The MBR is also suitable for several other astronomical investigations, e.g., studying the spectral evolution of average properties of pulsars and propagation effects, single-dish continuum studies and surveys/studies of recombination lines. 3. Searches for Decameter-wavelength Counterparts of Radio-quiet Gamma-ray Pulsars Before the launch of the Fermi gamma-ray space telescope, the “radio-quiet” gamma-ray pulsar population consisted of only one pulsar ,i.e., Geminga (for example, see Bignami& Caraveo,1996; Abdo etal.,2009). High sensitivity of the Large Area Telescope(LAT) on the Fermi-satellite made it possible, for the ﬁrst time, to perform blind searches for pulsars in γ-rays. Since the Fermi-operation started, the number of pulsars known to emit in γ-rays has seen an extraordinary increase — from less than 10 to 117 pulsars. About one-third of these pulsars have been discovered in blind searches of the LAT data. Despite deep radio searches, only 4 of these LAT-discovered pulsars could be detected, suggesting the rest of these to be “radio-quiet” gamma-ray pulsars. One of the possible explanations for the apparent absence of radio emission from these pulsars is that their narrow radio beams miss the line of sight towards earth (Brazier & Johnston, 1999), and hence appear as “radio-quiet”. The radius-to-frequency mapping in radio pulsars suggests that the emission beam becomes wider at low frequencies, increasing the probability of our line of sight passing through the beam. However, all of the deep searches mentioned above were carried out at higher radio frequencies(∼1GHz and above, and some at300MHz,Ray etal.,2011;Pletsch etal.,2012),and the lower frequency domain(<≈100 MHz) has remained relatively unexplored. Given the expected widening of emission beam, follow-up searches of the radio-quiet pulsars at low radiofrequencies could also be revealing. With this view, we searched the archival data of the pulsar/transient survey at 34.5 MHz, carried out using the Gauribidanur telescope during 2002-2006,for any periodic or transient dispersed signal along the direction of many of the LAT-discovered pulsars. Motivated by an intriguing possible detection of the pulsar J1732−3131 from the above search, we carried out further extensive follow-up observations and deep searches for pulsed(periodic as well as transient) radio emission from a selected sample of radio-quiet pulsars. Chapters 6 and 7 present details of our observations, detection strategies and methodologies, and interesting results obtained in a few of the target directions. The results obtained from these searches include: 1 A possible detection of periodic radio pulses from J1732−3131 was made, using the archival data, at a dispersion measure(DM) of15.44 ±0.32 pc/cc. We also detected 10 individual bright pulses in the same observing session, although marginally above the detection threshold, at a DM consistent with that associated with the periodic signal. The apparent brightness of these single pulses, and similarity of their apparent distribution in pulse-longitude with that of giant pulses in J0218+4232, suggest that these might be giant pulses. Our DM-based distance estimate, using Cordes & Lazio electron density model(2002),matches well with earlier estimates based on gamma-ray emission efficiency. 2 In our follow-up deep searches, we could not detect any readily apparent pulsed radio signal(neither periodic nor single pulses) from J1732−3131, i.e., above a detection threshold of 8σ. However, when we time-aligned and co-added data from observing sessions at 21diﬀerent epochs, and dedispersed using the DM estimated from the candidate detection, the average profile shape is found to be completely consistent with that from the candidate detection. Finding the same profile shape after 10 years of the original detection suggests that the signal is unlikely to be due to RFI or a mere manifestation of random noise. 3.In a couple of the observing sessions towards the telescope pointing direction of RA=06:34:30, DEC=10◦ , we detected a few ultra-bright pulses at two different DMs of about2pc/cc and3.3 pc/cc, respectively. However, when dedispersed at the DMs suggested by the bright single pulses, no significant signal was found at the expected periodicities of our targetpulsarsJ0633+0632 andJ0633+1746,which would be in the telescope beam centered at above coordinates. Energies of these strong pulses in the two observing sessions are comparable to typical energies of giant pulses from the Crab pulsar at decameter wavelengths. 4. No significant pulsed signal(periodic or transient), above a detection threshold of 8σ,was found towards the directions of other selected radio-quiet gamma-ray pulsars. Time-aligning and combining of observations at different epochs allowed us to carry out deep searches for signals at the expected periodicities of these pulsars. Despite the large background sky-temperature at decameter wavelengths, the minimum detectable flux density in our deep searches are comparable with those from previous searches at higher frequencies, when scaled using a spectral index of −2.0 and assuming no turn-over in the spectrum.
- Physics (PHY) 
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