Sampling the Radio Transient Universe: Studies of Pulsars and the Search for Extraterrestrial Intelligence

The transient radio universe is a relatively unexplored area of astronomy, offering a variety of phenomena, from solar and Jovian bursts, to flare stars, pulsars, and bursts of Galactic and potentially even cosmological origin. Among these, perhaps the most widely studied radio transients, pulsars are fast-spinning neutron stars that emit radio beams from their magnetic poles. In spite of over 40 years of research on pulsars, we have more questions than answers on these exotic compact objects, chief among them the nature of their emission mechanism. Nevertheless, the wealth of phenomena exhibited by pulsars make them one of the most useful astrophysical tools. With their high densities, pulsars are probes of the nature of ultra-dense matter. Characterized by their high timing stability, pulsars can be used to verify the predictions of general relativity, discover planets around them, study bodies in the solar system, and even serve as an interplanetary (and possibly some day, interstellar) navigation aid. Pulsars are also used to study the nature of the interstellar medium, much like a flashlight illuminating airborne dust in a dark room. Studies of pulsars in the Galactic center can help answer questions about the massive black hole in the region and the star formation history in its vicinity. Millisecond pulsars in globular clusters are long-lived tracers of their progenitors, low-mass X-ray binaries, and can be used to study the dynamical history of those clusters. Another source of interest in radio transient astronomy is the hitherto undetected engineered signal from extraterrestrial intelligence. The Search for Extraterrestrial Intelligence (SETI) is an ongoing attempt at discovering the presence of technological life elsewhere in the Galaxy. In this work, I present my forays into two aspects of the study of the radio transient universe---pulsars and SETI.;Firstly, I describe my work on the luminosity function and population size of pulsars in the globular clusters Terzan 5, 47 Tucanae, and M 28. Applying Bayesian statistics to our data set consisting of the number of detected pulsars, their flux densities, and the amount of diffuse radio emission from the direction of these clusters, we show that the number of potentially observable pulsars in Terzan 5 should be within a 95 per cent credible interval of 147+112-65 For 47 Tucanae and M 28, our results are 83+54-35 and 100+91-52 , spectively. We also constrain the luminosity function parameters for the pulsars in these clusters.;The Galactic center pulsar population has been an interesting target for various studies, especially given that only one pulsar has been detected in the region, when we expect hundreds of pulsars to be present. In this work, we use the scattering measurements from recent observations of PSR J1745--2900, the Galactic center pulsar, and show that the size of the potentially observable pulsar population has a conservative upper limit of ~200. We show that the observational results so far are consistent with this number and make predictions for future radio pulsar surveys of the region.;The Versatile GBT Astronomical Spectrometer (VEGAS) is a heterogeneous instrument used mainly for pulsar studies with the Green Bank Telescope. I describe our work on the GPU spectrometer that we developed as part of VEGAS. The GPU code supports a dual-polarization bandwidth of up to 600 MHz.;In the field of SETI, I discuss two works. SERENDIP VI is a heterogeneous SETI spectrometer to be installed both at the Green Bank Telescope and at the Arecibo Observatory. In this work, we describe the design of the GPU spectrometer that forms part of SERENDIP VI. In the second work, we speculate on a novel search strategy for SETI, based on the idea that technological civilizations lacking the advancement required to build high-powered beacons may choose to build a modulator situated around a nearby pulsar, depending on whether it is energetically favorable. We discuss observational signatures to search for, using a model of artificially-nulled pulsars

The Galactic centre pulsar population

The recent discovery of a magnetar in the Galactic centre region has allowed Spitler et al. to characterize the interstellar scattering in that direction. They find that the temporal broadening of the pulse profile of the magnetar is substantially less than that predicted by models of the electron density of that region. This raises the question of what the plausible limits for the number of potentially observable pulsars - i.e., the number of pulsars beaming towards the Earth - in the Galactic centre are. In this paper, using reasonable assumptions - namely, (i) the luminosity function of pulsars in the Galactic centre region is the same as that in the field, (ii) the region has had a constant pulsar formation rate, (iii) the spin and luminosity evolution of magnetars and pulsars are similar, and (iv) the scattering in the direction of the Galactic centre magnetar is representative of the entire inner parsec - we show that the potentially observable population of pulsars in the inner parsec has a conservative upper limit of $\sim$ 200, and that it is premature to conclude that the number of pulsars in this region is small. We also show that the observational results so far are consistent with this number and make predictions for future radio pulsar surveys of the Galactic centre.Comment: 5 pages, 3 figures, Accepted for publication in MNRAS Letter

The Galactic Centre pulsar population

The recent discovery of a magnetar in the Galactic Centre region has allowed Spitler et al. to characterize the interstellar scattering in that direction. They find that the temporal broadening of the pulse profile of the magnetar is substantially less than that predicted by models of the electron density of that region. This raises the question of what the plausible limits for the number of potentially observable pulsars – i.e. the number of pulsars beaming towards the Earth – in the Galactic Centre are. In this Letter, using reasonable assumptions – namely (i) the luminosity function of pulsars in the Galactic Centre region is the same as that in the field, (ii) the region has had a constant pulsar formation rate, (iii) the spin and luminosity evolution of magnetars and pulsars are similar and (iv) the scattering in the direction of the Galactic Centre magnetar is representative of the entire inner parsec – we show that the potentially observable population of pulsars in the inner parsec has a conservative upper limit of ∼200 and that it is premature to conclude that the number of pulsars in this region is small. We also show that the observational results so far are consistent with this number and make predictions for future radio pulsar surveys of the Galactic Centre

Setiburst: A Robotic, Commensal, Realtime Multi-Science Backend For The Arecibo Telescope

Radio astronomy has traditionally depended on observatories allocating time to observers for exclusive use of their telescopes. The disadvantage of this scheme is that the data thus collected is rarely used for other astronomy applications, and in many cases, is unsuitable. For example, properly calibrated pulsar search data can, with some reduction, be used for spectral line surveys. A backend that supports plugging in multiple applications to a telescope to perform commensal data analysis will vastly increase the science throughput of the facility. In this paper, we presen

Constraining the luminosity function parameters and population size of radio pulsars in globular clusters

Studies of the Galactic population of radio pulsars have shown that their luminosity distribution appears to be log-normal in form. We investigate some of the consequences that occur when one applies this functional form to populations of pulsars in globular clusters. We use Bayesian methods to explore constraints on the mean and standard deviation of the luminosity function, as well as the total number of pulsars, given an observed sample of pulsars down to some limiting flux density, accounting for measurements of flux densities of individual pulsars as well as diffuse emission from the direction of the cluster. We apply our analysis to Terzan 5, 47 Tucanae and M 28, and demonstrate, under reasonable assumptions, that the number of potentially observable pulsars should be within 95% credible intervals of $147^{+112}_{-65}$, $83^{+54}_{-35}$ and $100^{+91}_{-52}$, respectively. Beaming considerations would increase the true population size by approximately a factor of two. Using non-informative priors, however, the constraints are not tight due to the paucity and quality of flux density measurements. Future cluster pulsar discoveries and improved flux density measurements would allow this method to be used to more accurately constrain the luminosity function, and to compare the luminosity function between different clusters.Comment: 9 pages, 4 figures, Accepted for publication in MNRA