Plasma-magnetic field interactions in pulsars and their environments

Introduction

The evolution of astrophysical systems of all kinds, from planets to stars, to galaxies and clusters, are almost invariably linked to the magnitude and spatial distribution of their magnetic field. In many cases, the latter is the key player in dominating energy and momentum transport, in securing pressure balances, in releasing energy in flares or outbursts, or in accelerating leptons and hadrons to relativistic energies via reconnection or diffusive processes. The plasma-magnetic field interaction regulates the state and evolution of many, if not all, astrophysical settings. And for relativistic objects, those generating extreme environments within and surrounding them, all our understanding actually comes and goes with how well are we able to comprehend the microphysics of the interaction between plasma and magnetic field. It is this microphysics that finally leads to observable, multi-frequency events, which we are able to track. We focus in linking the modelling of the microphysics in different plasma-field conditions with the observation of overall astronomical behaviors.

Obvious environments where plasma and magnetic field interaction play a determining role are pulsars and their nebulae. In reduced physical scales, even for the isolated pulsar case: from the 10 km of the star, to the sub astronomical unit of the magnetosphere, to about 0.2 pc for the wind, and a few pc for the supernova remnant (SNR) shell --for a typically young nebula- we can essentially see all physics we know at play. The complex made of pulsars (PSR) / pulsar-wind nebula (PWN) / SNR changes conditions from high (or from the highest, in the case of some magnetars) magnetic fields and low magnetospheric densities (particularly in putative accelerating gaps), to weak magnetic fields in low-density (pulsars winds and young SNRs) and high-density plasma (old SNRs, particularly those evolving into dense clouds). The physical scale and the range of densities are significantly smaller and larger, respectively, when the pulsar is located in close binaries. Pressure balance between magnetic field and accretion flows determine here a plethora of not-well-understood phenomena, which even for the same system allow them to transition from one visibly-different state to another. These transitional pulsars (the first one caught in the act of transitioning from one state to the other was discovered here at the Institute of Space Sciences), are one of the hottest topics in astrophysics today, linking problems that range from evolution of stars and binaries, to particle acceleration. The transition of millisecond pulsars between accretor and ejector states is directly connected with the recycling theory, according to which millisecond PSRs are spun up by accretion in Gyrs of evolution. How fast these transitions really are? Which are the duty cycle and the ultimate reasons why they happen?

Recent space (Fermi-LAT) and ground-based gamma-ray observations have revealed that rotation-powered pulsars emit pulsed high-energy (HE, 100MeV-20GeV) and very-high-energy (VHE, 20GeV-TeV) radiation from high altitudes (i.e., the outer part) of their rotating magnetospheres. This is a unifying aspect of all the PSR population. Another is, of course, the finding of magnetar behavior (fast flares of tremendous luminosity) in sub-critical (<1e14 G) magnetic field pulsars, or simply put, low-field magnetars. We have discovered the first three such systems (see below), and this has impacted on how we see the diversity of the PSR population, noting at once new unknowns. Concurrently, we discovered TeV pulsations from Crab, at the largest energies ever detected in pulsed photons, opening the gate to having different components in the pulsar spectra, or to an altogether-new mechanism.

Similarly to pulsars, PWN emits at all wavelengths from radio to TeV. The high energy reached in these processes and extreme environment conditions result in a high level of non-thermal radiation above a few keV and up to a few tens of TeV, which can be observed with satellites and ground-based telescopes. PWNe shine at TeV energies for up to 1e5 years, acting as a calorimeter in which the majority of the energy released by the pulsar is kept. In comparison, the associated X-ray-bright nebula has a much shorter lifetime and the emission is dominated by the magnetic field strength and distribution. PWNe constitute >25% of all known VHE sources. The number of PWNe detected at very high energies has increased from 1 (the Crab Nebula) to 30 in the last 10 years, a number similar to those characterized at other frequencies over decades of observations. The Cherenkov Telescope and the Square Kilometer Arrays (CTA and SKA, respectively) will detect several hundreds of nebulae, an essentially complete Galactic population.

Focus

The physical processes we study are ruled by the interaction between plasma and magnetic field, among them

-weak magnetic fields in low-density (pulsars winds and young SNRs), and high-density plasma (old SNRs evolving in dense clouds),
-medium magnetic fields in low-density (such the ones found in pulsar magnetospheres) and high-density plasma (X-ray binaries),
-the highest magnetic fields in low-density plasma (like the ones found in magnetars).

In particular, we aim to investigate pulsar winds and the progenitor SNRs, through a) observations of the dynamical evolution and related energetic processes in SNRs and PWNe b) the development of state-of-the-art modelling of the plasma/magnetic interaction in PWNe -including detailed description of their expansion and dynamical interaction between SNR / PWN, or magneto-thermal evolution in pulsars- and young SNRs. With respect to SNRs, we also aim to understanding acceleration of cosmic rays and hadronic interaction via deep observations of evolved SNRs interacting with molecular clouds. To further understand pulsars magnetospheres, we study their emission through a) determination of the phase-resolved spectral features of bright LAT pulsars, b) observations of the inverse Compton component at GeV/TeV energies similar to the one in the Crab pulsar and c) modelling the synchro-curvature radiation in gaps.

We investigate the link between LMXBs and binary MSPs through a) prompt multi-band follow-up of transient X-ray outbursts from new or known accreting neutron stars, b) optical monitoring of known binary MSPs in search for a state change, and c) theoretical modelling of the conditions for particle spectra and multifrequency emission along their different accretion phases.

Finally, we pursue the study of the multiband, steady and transient emission of strongly magnetized neutron stars through follow-up and modelling of their transient events, as well as the possible occurrence of such outbursts in lower magnetized pulsars.

Selected publications

-
Rotationally Powered Magnetar Nebula around Swift J1834.9–0846
D. F. Torres
The Astrophysical Journal, Volume 835, Issue 1, article id. 54, 13 pp. (2017)

-A systematic synchro-curvature modelling of pulsar gamma-ray spectra unveils hidden trends
D. Viganò, D. F. Torres, J. Martín
Monthly Notices of the Royal Astronomical Society, Volume 453, Issue 3, p.2599-2621

-Teraelectronvolt pulsed emission from the Crab Pulsar detected by MAGIC
-The MAGIC Collaboration, E. de Oña & D. F. Torres in the leading team
Astronomy & Astrophysics, Volume 585, id.A133, 6 pp.

-Swings between rotation and accretion power in a binary millisecond pulsar
A. Papitto et al.
Nature, Volume 501, Issue 7468, pp. 517-520 (2013)

-A Low-Magnetic-Field Soft Gamma Repeater
N. Rea et al.
Science, Volume 330, Issue 6006, pp. 944- (2010)

Senior Institute members involved

D. F. Torres, E. de Oña Wilhelmi, N. Rea
Institute of Space Sciences (IEEC-CSIC)

Campus UAB, Carrer de Can Magrans, s/n
08193 Barcelona.
Phone: +34 93 737 9788
Email: ice@ice.csic.es
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An institute of the Consejo Superior de Investigaciones Científicas

An institute of the Consejo Superior de Investigaciones Científicas
Affiliated with the Institut d'Estudis Espacials de Catalunya

Affiliated with the Institut d'Estudis Espacials de Catalunya