Isolated Neutron Stars

Group members: Sandro Mereghetti, Patrizia Caraveo, Andrea De Luca, Andrea Tiengo, Paolo Esposito, Martino Marelli, Nicola Sartore, Andrea Giuliani

The advances obtained thanks to observations in the optical, X-ray and gamma-ray energy bands have dramatically changed our vision of isolated neutron stars, that was previously based mainly on the observations of the population of radio pulsars. The new emerging picture implies a much wider variety in the birth properties of neutron stars and their evolution in time, as testified by the presence of different classes of objects: radio-quiet pulsars, gamma-ray pulsars, compact central objects in supernova remnants, dim X-ray emitting neutron stars, magnetars. High-energy observations of these neutron stars provide a unique way to study a variety of phenomena which are relevant for different branches of physics. For example, the study of the thermal X-ray emission from the star photospheres and of their cooling history gives information on the equation of state of nuclear density matter, while non-thermal processes observed at X and gamma ray energies are relevant for the physics of particle acceleration, relativistic shocks, and strong magnetic fields.

The understanding of the various classes, and the building up of a consistent evolutionary scenario is the long term goal of our research group which has been involved since more than thirty years in the identification and study of isolated neutron stars exploiting all the main satellites for X-ray and gamma-ray astronomy (e.g. Einstein Observatory, ROSAT, BeppoSAX, ASCA, XMM-Newton, Chandra, Swift, COS-B, AGILE, Fermi), complemented by optical observations with the Hubble Space Telescope and ground telescopes at ESO.

Some highlights of our researches are:

• We have discovered extended X-ray tails produced by different kinds of isolated neutron stars: the radio-quiet gamma-ray pulsar Geminga, the old radio pulsar PSR B1929+10, and the middle-aged pulsar PSR J0357+3205.

• The identification of the new class of Anomalous X-ray Pulsars which probably contain neutron stars endowed of extreme magnetic fields (up to 10¹⁵- 10¹⁶ G), the so called "Magnetars". See the recent examples of SGR 0501+4516 and SGR 1627-41.

• The discovery with INTEGRAL of the first gamma-ray afterglow after the giant flare of a magnetar.

• The discovery of X-ray dust scattering rings after the outburst of the Anomalous X-ray Pulsar 1E 1547-5408 allowed us to constrain its distance.

• Our measurement of a low external magnetic field in the Soft Gamma-ray Repater SGR 0418+5729 indicates that many 'ordinary' pulsars could be dormant magnetars with high internal magnetic fields. A second member of this subclass of neutorn stars has recently been discovered.

• The study of the peculiar Central Compact Object in the supernova remnant RCW 103.

• We have detected with XMM-Newton multiple absorption features in the spectrum of the outstanding Central Compact Object 1E 1207-5209.

• Our phase-resolved spectroscopy of PSRs with XMM-Newton provided a link between the thermally emitting neutron star surfaces and their charge-filled magnetospheres.

• We discovered the long-sought pulsations from RX J1856.7-3754, proving the source to be an INS, with the smallest known pulsed fraction observed so far (~1%).