As time goes by, continuing revolutions of the largest gamma-ray astronomy satellite ever flown, Fermi, allows for deeper and deeper studies of the sky. Joined by mature ground-based observational facilities at even higher gammaray frequencies, like MAGIC at TeV energies (TeV = 1e12 eV), as well as by a plethora of concurrent observations at lower energies (from radio to optical to X-rays), significant discoveries are driving theoretical research.
This year has seen the announcement of the most energetic light ever observed from a pulsar, for the first time with energies of more than trillion electron volts, about a thousand times larger than previously observed, arriving at the detector concurrently with the pulsar period. These photons are thus coming from the close proximity of the rotating neutron star, but exactly how and where they are generated is unknown. No theory to date can cope with such measurements.
Another first-timer this year has been the detection, at similarly such high energies, of a years-long recurrent variability. Every 4.2 years, the TeV emission from a gamma-ray binary has been seen to oscillate, in an effect theoretically predicted a few years before (Torres, D. F., et al. 2012, ApJ, 744, 106). Much is yet to learn from the recurrence of such oscillation and how can, perhaps, be driven by oscillations in the surrounding circumstellar disk of the companion.
Finally, the spatial connection of the archetypical, accreting millisecond pulsar with a gamma-ray source has been also noted for the first time. If this connections proves real, for instance via a detection of gamma-ray pulsations, it would imply rotationally-powered activity in quiescence mode, showing evidence of a transition to a rotationpowered radio pulsar state in X-ray quiescence, whilst it is observed as an accreting pulsar when it has a disk.
References
de Ona Wilhelmi E, Papitto A, Li J, Rea N, Torres DF, Burderi L, Di Salvo T, Iaria R, Riggio A & Sanna A 2016, ‘SAX J1808.4-3658, an accreting millisecond pulsar shining in gamma rays? [astro-ph/1511.05032]’, Monthly Notices of the Royal Astronomical Society, 456, 3, 2647 – 2653.
The MAGIC Collaboration (including Torres DF – responsible team) 2016, ‘Teraelectronvolt pulsed emission from the Crab pulsar detected by MAGIC’, Astronomy & Astrophysics, 585, A133.
The MAGIC Collaboration (including Torres DF – responsible team) 2016, ‘Super-orbital variability of LS I+61 degrees 303 at TeV energies’, Astronomy & Astrophysics, 591, A76.
The neutron star (red sphere) with its strong magnetic field (white lines) spins around itself nearly 30 times per second injecting energetic electrons in the space region around it. The green and blue shaded regions depict different particle acceleration zones from where the detected photons could originate. The green zone lies in the vicinity of the pulsar’s magnetosphere, whereas the blue zone could be as far as 100.000 km away from the pulsar. Credit: Artistic view not to scale by Patricia Marco, MAGIC collaboration. Caption Credit: from MAGIC collaboration press release.
