Quasi periodic oscillations in magnetars

Antonella Colaiuda, Kostas D. Kokkotas

In the last few years, a great attention was devoted to the study of magnetars, neutron stars with a strong magnetic field. In particular two types of sources have been classified as magnetars: Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs). Those two objects are observationally very similar: they are slowly rotating X-ray pulsars (with a period around 2-12 sec) and their magnetic field, as inferred from the classical magnetic dipole spin-down formula, is around 10¹⁴-10¹⁵ Gauss.

SGRs undergo periods of activity during which recurrent bursts with sub-second duration and peak luminosities of 10³⁸-10⁴¹ erg/s are observed. SGRs also show, on rare occasions, extreme events known as giant flares. These are characterized by an initial spike of duration comparable to that of recurrent bursts, but many orders of magnitude higher luminosity. Only three giant flares have so far been observed in over 30 yrs of monitoring and only in two of them, fast Quasi Periodic Oscillations (QPOs) have been discovered in the X-ray flux of the decaying tail. QPOs with different frequencies were detected in SGR 1806-20, some of which were active simultaneously and displayed highly significant QPO signals. Also the 1998 giant flare of another magnetar, SGR 1900+14, revealed a few QPOs in the tail.

Using our results, we can put some very preliminary results on the equation of state of magnetars.

Reference / Figure

Colaiuda, A., Kokkotas, K.D. (2011). Magnetars osciallations in the presence of a crust A. M.N.R.A.S., 414, 3014. arXiv:1012.3103v1 [gr-qc]