Gamma-ray pulsars: a multi-band view

This Thesis is based mainly on the results of the timing analysis applied to the gamma-ray pulsars observed principally by the AGILE satellite and also extended, in some cases, to observations with the Fermi-LAT satellite and the MAGIC Cherenkov telescope. Aim of this extended study of pulsars at the high energies was to characterize their properties based, now, on a more statistically relevant sample, and be able to disentangle useful informations that can be key to explain the emission mechanisms in pulsars. THE SCIENTIFIC CONTEXT Pulsars are highly-magnetized, rapidly-rotating neutron stars. As explained in Chapter 1, they have been observed in the radio band over the past forty years, due to their highly anisotropic emission, which, when combined with the misalignment between the rotation and the magnetic axis, produces the pulsed emission we observed, also called the ”lighthouse effect”. Pulsars have been observed as gamma-ray emitters as well, but it is only in the past three years that their number hit the double digits and they started to yield their potential as keys to explain the neutron stars mechanisms. In Chapter 1, the basics of pulsar theory are given. While the phenomenological aspects have been widely studied thanks to extensive radio observations through the years, their electrodynamics represents an articulated field that is difficult to probe. The ”classical” models of magnetosphere predict the presence of regions of particles’ under-density (a ”gap”), inside an overall force-free magnetized area that surrounds the pulsar, where the particles can be accelerated and can produce the observed radiation, after a number of cascade processes. More ”modern” models of magnetosphere, with their roots in old predictions, discuss the hypothesis of a totally force-free magnetosphere. The discussed theories search for a confirmation in our gamma-ray observations, as the gamma-rays are the ones carrying away a good fraction of the rotational energy loss. THE NEW PERSPECTIVES OPENED BY THE MULTI-BAND OBSERVATIONS In April 2007 the Italian Space Agency launched the AGILE satellite for gamma-ray astronomy. About one year later, AGILE was joined in the observation of gammarays by the 16 times bigger Fermi-LAT satellite launched by NASA. AGILE and Fermi-LAT, with their wide field of view and large collective area, are particularly suited for the study of pulsars at high energies. Most recently, the window of veryhigh energy observations has opened up to pulsar studies and, in particular, by the MAGIC telescope, with the lowest up to now threshold for ground-based telescopes, at 25 GeV. Its observations are briefly described, together with AGILE and Fermi-LAT’s, in Chapter 2. The techniques for studying pulsars in the gamma-rays are also explained in Chapter 2, with the fundamental premise about the radio observations which were part of my analysis work, as they constitute the primary basis for the gammaray observations. The advances with respect to the observations of the previous generation gamma-ray instruments are highlighted. In particular, AGILE was able to take into account, for the first time in gamma-ray observations, the timing noise that affects young pulsars. In this way, the observations can be carried out for longer time spans without being affected by sensible light curve smearing. Thus, we could take advantage of the long time span, up to now the longest for gamma-ray observations, to increase the resolution of our light curves and see structures at the sub-millisecond level. GAMMA-RAY PULSARS AGILE and Fermi-LAT pulsar observations first concentrated on the known gamma-ray pulsars. As shown in Chapter 3, the apparently ”familiar” pulsars actually hid thriving new prospects for pulsar studies, as well as the new pulsars subsequently detected, described in Chapter 4. In these two Chapters, the properties of the gamma-ray emission ars analyzed for a number of pulsars, mainly using AGILE data, but also with Fermi-LAT and MAGIC observations. The light curves are investigated with increased resolution from previous observations and the spectral properties are addressed. The availability of a statistically significant sample of gamma-ray pulsars led us to draw some lines on the models. The classical polar cap model seems to be failing the test of gamma-ray observations for most of the present sample, and a simple explanation of which can be found in conservation laws arguments discussed in Chapter 4. At the same time it starts getting clear that a model that contemplates a single gap zone does not seem to be feasible to explain the observed pulse profiles. And, possibly, the entire gap theory should be combined with the more physical force-free models. Episodes of variability in pulsars have been observed and studied in this context. The Vela glitch of August 2007 was observed by AGILE in search for gamma-ray emission. The Crab pulsar could have a contribution to the emission from a newly observed third pulsar peak, that is less significant and much weaker than the canonical two, and could be due to giant pulses. AGILE observed the first gamma-ray millisecond pulsar but its emission only appeared in a restricted time interval, leading to the interesting possibility that pulsar emission might have some intrinsic variability. HIGH MAGNETIC FIELD PULSARS After the advent of Fermi-LAT, AGILE found its collocation in the gamma-ray astronomy in the characterization of the low-energy gamma-rays (from 30 to 100 MeV), where the collective areas of the two instruments is equivalent, but AGILE deals with much lower background. For this reason, we concentrated on those pulsars that show a low-energy cutoff, which were theorized to emit gamma-ray radiation through the exotic QED process of photon splitting. A detailed analysis of the two most significant cases is given in Chapter 5. We have found that the concurrence of a high magnetic field and an aligned geometry, could overcome the objections from Chapter 4 against inner magnetosphere emission and be, indeed, dominated by polar cap emission. Interestingly, this phenomenology, that is observed in pulsars that are similar to magnetars, may be observed in objects that are transitioning from pulsar to magnetar. THE ENVIRONMENT OF PULSARS Young pulsars are known to power a relativistic wind of particles that surrounds the pulsar and is best known as its Pulsar Wind Nebula (PWN). Important phenomena take place in the PWN and they are powered by the pulsar inside it. As discussed in Chapter 6, very high energy emission was already observed from PWN, but high energy emission was missing, in a spectral region where important constraints on the emission processes could be given. AGILE was the first satellite to detect GeV emission from a PWN apart from Crab, Vela X, and it was also the first to claim the unexpected flux variation in the Crab Nebula which underwent two intense flares in 2010 and 2011. In Chapter 6 we give a description of the events and a possible trail for an interpretation, although no clear picture can yet emerge from the observed events. CONCLUSIONS AND FUTURE PROSPECTS The multi-band approach that has been used for the observations described in this Thesis has proven valid for the exploitation of new science and the most useful approach for the comprehensive analysis of pulsar phenomena across the electromagnetic spectrum. As a completion to this work, the more comprehensive AGILE Pulsar Catalog is in preparation. It will comprise all the pulsars observed by AGILE and particularly focus on the low-energy tail of them, which present interesting properties that bridge pulsars and magnetars.