Magnetars: Neutron Stars with Magnetic Field above the quantum limit

Neutron stars may host enormous magnetic fields, impossible to recreate in the laboratory. In particular, two peculiar classes of pulsars, Soft Gamma-ray  Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are believed to contain "magnetars", which are neutron stars with field larger than the quantum critical value, Bq~4x10^13G. Above this threshold magnetic confinement is so strong that the space available to an electron is comparable to its Compton wavelength, the smallest spatial region where  a particle can be localized according to quantum theory.

Magnetars offer  a unique environment to test our understanding of plasma physics in ultra-magnetized regimes: the interpretation of the spectra observed from these sources relies on understanding the atomic physics and radiative proocesses in regimes above the quantum critical limit. Magnetars are an elusive population: they have been predicted theoretically more than 15 years ago by Thompson and Duncan, but to probe their existence is challenging.  Traditionally,  the way to measure the magnetic field in radio-pulsars is via the detection of an electron cyclotron line. This technique is not suitable for highly magnetized objects: the nergy of the lines scales linearly with the magnetic field strenghts, so the feature falls at > 100 keV for B > 10^13 G! This band is spectroscopically unaccessible.

For Magnetars, the most important spectral feature than can be observed in the X-ray band is instead a proton cyclotron resonance, expected at E~0.63(B/10^14G) keV. This is because protons are more massive than electrons and the energy of the cyclotron line scales as the inverse of the mass of the particle. We have carried out radiative transfer computations in the magneto-active regime and predicted the observable properties of the proton line, such as central energy and equivalent width (Zane, et al., 2000, Zane et al., 2001).  The feature has been discovered in 2002 for the first time, in archive RXTE data of the soft gamma repeater SGR1806-20, together with two possible further multiple proton harmonics (Ibrahim et al., 2002). The spectrum is shown in Fig.1


                                                                     


                                                      Fig.1 :   The photon spectrum of SGR1806-20 registered
                                                                    with RXTE/PCA in 1996, during a burst of the
                                                                    soft gamma repeater (Ibrahim et al., 2002)

                                                                              
                                      
The properties of the fundamental harmonic that  has been detected agree with our earlier predictions, while the field strenght we infer from it,  ~10^15 G, is in axcellent agreement with that implied by the spin-down rate of the source.
A discovery of this kind is crucial: it represents the first ever detection of a proton cyclotron line in a cosmic source, probe the Magnetar nature of SGRs and gives the first direct measure of an ultra-strong magnetic field. Evidence for further cyclotron lines have been then found in the spectrum of an Anomalous X-ray pulsar (Rea et al., 2003) and, at lower energy, in that of a few dim Isolated Neutron Stars (see links here)

Anomalous X-ray pulsars are dubbed "anomalous"  because the nature of their X-ray emission is still mysterious and represents one of the most challenging unsolved problems in Galactic high-energy astrophysics.  The loss of rotational energy inferred from the measured period and period derivative is too small to power the detected luminosities (L ~ 10^ 34 - 10^36 erg/s). The lack of an observed main sequence or giant donor seems to exclude a binary system in favor of a scenario involving an isolated neutron
star.  The magnetar model has been originally proposed to explain the bursting behavior that hallmarks the Soft Gamma-ray Repeaters (e.g. Hurley 2000 for a review). Only later it has been extended to AXPs, mainly on the basis of the similarities between the timing properties of the two classes of sources (large spin-down rate and large spin period). Were this interpretation correct, AXPs and SGRs would represent just two different manifestations of the same physical phaenomenon. Until recently, however, SGRs and AXPs had little more in common and, in particular,  bursts have never been recorded from AXPs. Therefore, their relationship remained debatable.

For this reason, the recent discovery of a bursting activity in two AXPs with RXTE has been unexpected and extremely exciting  (1E 2259+58, Kaspi et al. 2003; 1E 1048-59, Gavriil, Kaspi & Woods 2002). 
Almost immediately, also long lasting variations, on a timescale of ~ months,  have been discovered  in the persistent flux of 1E 2259+58 (Woods et al. 2004) and,  more recently, of 1E 1048-59 (Mereghetti et al., 2004).  Puzzlingly, these two sources have a remarkably different behavior during the ``outburst'' state. The flux increase (by about a factor 4) in 1E 2259+58 followed the emission of a series of short bursts and was accompanied by substantial changes in practically every aspect of the X-ray emission: pulse profile, spectrum and pulsed fraction. Furthermore, there was evidence for the occurrence of a glitch and for the enhancement of the infrared flux immediately after the outburst.  On the contrary, the activity of 1E 1048-59 does not appear to be correlated with any burst and, despite the flux raised by a factor ~ 4 , no significant changes were observed in the source spectral and timing properties !

The importance of the discovery that AXPs too can emit bursts  is not in the mere addition of another entry in the list of flaring X-ray sources. It gives for the first time ever a direct evidence of the AXPs-SGRs link and strengthen a common interpretation in terms of the Magnetar model. These episodes did not remain isolated: during the last year it has become increasingly evident that AXPs, again similarly to SGRs, undergo periods of activity intersperse with quiescent stages.

If you wish to learn more about Magnetars and our publications on the field, just follow these links (some of them are in different languages too!) :

1) NASA Press Release (and beautiful links!!):  Scientists measure the most powerful magnet known
2) Physics World, Physics in Action.  Strongest  magnet in the cosmos
3) Published in the Italian Journal "L'Astronomia", January 2003 Issue: Il piu' forte magnete del cosmo (in Italian!)
4) Published in a Soviet scientific Journal: The strongest  magnet in the cosmos (in Russian!)
5) And here in another Soviet scientific Journal: The strongest  magnet in the cosmos (in Russian too!)
6) 19th Texas Symposium on Relativistic Astrophysics and Cosmology:  Spectra From Magnetized, Accreting Neutron Stars: the Proton Cyclotron Feature
7) Zane, et al., 2000 Magnetized Atmospheres around Accreting Neutron Stars
8) The Second National Conference on Astrophysics of Compact Objects, Bologna, 2001:  Radiative Transfer in Magnetar Atmospheres
9) Two Years of Science with Chandra, Washington, 2001:  Proton Cyclotron Feature in Thermal Spectra of Ultra-Magnetized Neutron Stars
10) Zane et al., 2001 Proton Cyclotron Features in Thermal Spectra of Ultra-magnetized Neutron Stars
11) The Ninth Marcel Grossmann Meeting, Rome, 2000: Radiative transfer in highly magnetized regimes
12) Ibrahim et al., 2002 : Discovery of Cyclotron Resonance Features in the Soft Gamma Repeater SGR 1806-20
13) Mereghetti et al., 2004: Pronounced Long Term Flux Variability of the Anomalous X-Ray Pulsar 1E 1048.1-5937