On the Pulse Intensity Modulation of PSR B0823+26

We investigate the radio emission behaviour of PSR B0823+26, a pulsar which is known to undergo pulse nulling, using an 153-d intensive sequence of observations. The pulsar is found to exhibit both short (~min) and unusually long-term (~hours or more) nulls, which not only suggest that the source possesses a distribution of nulling timescales, but that it may also provide a link between conventional nulling pulsars and longer-term intermittent pulsars. Despite seeing evidence for periodicities in the pulsar radio emission, we are uncertain whether they are intrinsic to the source, due to the influence of observation sampling on the periodicity analysis performed. Remarkably, we find evidence to suggest that the pulsar may undergo pre-ignition periods of 'emission flickering', that is rapid changes between radio-on (active) and -off (null) emission states, before transitioning to a steady radio-emitting phase. We find no direct evidence to indicate that the object exhibits any change in spin-down rate between its radio-on and -off emission modes. We do, however, place an upper limit on this variation to be <= 6 % from simulations. This indicates that emission cessation in pulsars does not necessarily lead to large changes in spin-down rate. Moreover, we show that such changes in spin-down rate will not be discernible in the majority of objects which exhibit short-term (<= 1 d) emission cessation. In light of this, we predict that many pulsars could exhibit similar magnetospheric and emission properties to PSR B0823+26, but which have not yet been observed.Comment: 13 pages, 11 figures, accepted for publication in MNRAS; 1 reference correcte

Massive stars as thermonuclear reactors and their explosions following core collapse

Nuclear reactions transform atomic nuclei inside stars. This is the process of stellar nucleosynthesis. The basic concepts of determining nuclear reaction rates inside stars are reviewed. How stars manage to burn their fuel so slowly most of the time are also considered. Stellar thermonuclear reactions involving protons in hydrostatic burning are discussed first. Then I discuss triple alpha reactions in the helium burning stage. Carbon and oxygen survive in red giant stars because of the nuclear structure of oxygen and neon. Further nuclear burning of carbon, neon, oxygen and silicon in quiescent conditions are discussed next. In the subsequent core-collapse phase, neutronization due to electron capture from the top of the Fermi sea in a degenerate core takes place. The expected signal of neutrinos from a nearby supernova is calculated. The supernova often explodes inside a dense circumstellar medium, which is established due to the progenitor star losing its outermost envelope in a stellar wind or mass transfer in a binary system. The nature of the circumstellar medium and the ejecta of the supernova and their dynamics are revealed by observations in the optical, IR, radio, and X-ray bands, and I discuss some of these observations and their interpretations.Comment: To be published in " Principles and Perspectives in Cosmochemistry" Lecture Notes on Kodai School on Synthesis of Elements in Stars; ed. by Aruna Goswami & Eswar Reddy, Springer Verlag, 2009. Contains 21 figure