98
P. Esposito et al.
focuses were published on magnetars in the last few years: among others, Israel
and Dall’Osso, Bursts and flares from highly magnetic pulsars, in Proceedings of
the First Session of the Sant Cugat Forum on Astrophysics High-Energy Emission
from Pulsars and Their Systems, ed. by D.F. Torres, N. Rea. Astrophysics and
Space Science Proceedings (Springer, Heidelberg, 2011), pp. 279–298, Rea and
Esposito, Magnetar outbursts: an observational review, in High-Energy Emission
from Pulsars and Their Systems. Proceedings of the First Session of the Sant Cugat
Forum on Astrophysics, ed. by D.F. Torres, N. Rea. Astrophysics and Space Science
Proceedings (Springer, Heidelberg, 2011), pp. 247–273, Turolla and Esposito Int J
Mod Phys D 22:1330024-163, 2013, Mereghetti, et al. Space Sci Rev 191:315–338,
2015, Turolla et al., 78:116901, 2015; Kaspi and Beloborodov Annu Rev Astron
Astrophys 55:261–301, 2017. Here, we quickly recall the history of these sources
and travel through the main observational facts, trying to touch some recent and
sometimes little-discussed ramifications of magnetars.
3.1 Historical Overview
The event at the birth of magnetar studies was the landmark giant flare observed in
1979 from SGR 0526–66 [34, 149]. Nothing alike had been observed before from
a pulsar, and the flare was also different from and much brighter than any other
gamma-ray burst. The extreme properties of the event forced the astronomer to
conceive unusual scenarios. The most appealing one was the existence of a class
of neutron stars with super-strong magnetic fields of 10 14 –10 15 G [56, 165]. In
particular, the word magnetar was introduced to designate these objects by Duncan
and Thompson [56]. The giant flare and the more common emission of short
fainter bursts of these sources were explained by impulsive releases of magnetic
energy stored in the neutron star, which could be triggered by fractures in the
magnetically stressed crust, perhaps associated to sudden magnetic reconnections
in the star’s magnetosphere [56, 208, 209]. The handfuls of sources associated to
these recurrent hard X-/gamma-ray transients were dubbed soft gamma repeaters
(SGRs), to distinguish them from the gamma-ray bursts.
In the same years, another class of X-ray pulsars defying any easy pigeonholing
was emerging: the anomalous X-ray pulsars (AXPs; Mereghetti and Stella [150],
van Paradijs et al. [221]). They were characterised as persistent X-ray pulsars with
periods of a few seconds and owed the adjective ‘anomalous’ to the fact that their
X-ray luminosity exceeded that available from spin-down energy loss (while the
accretion was ruled out by the lack of any trace of a stellar companion). Thompson
and Duncan [209] noted that except for the emission of short bursts, which had never
been observed in AXPs, these sources were very similar to the recently-discovered
persistent soft X-rays counterparts of SGRs [159, 190, 223]. It was suggested that
AXPs could be evolved SGRs, which ended or drastically reduced their explosive
activity after having largely depleted their reservoir of magnetic energy, and the
P. Esposito et al.
focuses were published on magnetars in the last few years: among others, Israel
and Dall’Osso, Bursts and flares from highly magnetic pulsars, in Proceedings of
the First Session of the Sant Cugat Forum on Astrophysics High-Energy Emission
from Pulsars and Their Systems, ed. by D.F. Torres, N. Rea. Astrophysics and
Space Science Proceedings (Springer, Heidelberg, 2011), pp. 279–298, Rea and
Esposito, Magnetar outbursts: an observational review, in High-Energy Emission
from Pulsars and Their Systems. Proceedings of the First Session of the Sant Cugat
Forum on Astrophysics, ed. by D.F. Torres, N. Rea. Astrophysics and Space Science
Proceedings (Springer, Heidelberg, 2011), pp. 247–273, Turolla and Esposito Int J
Mod Phys D 22:1330024-163, 2013, Mereghetti, et al. Space Sci Rev 191:315–338,
2015, Turolla et al., 78:116901, 2015; Kaspi and Beloborodov Annu Rev Astron
Astrophys 55:261–301, 2017. Here, we quickly recall the history of these sources
and travel through the main observational facts, trying to touch some recent and
sometimes little-discussed ramifications of magnetars.
3.1 Historical Overview
The event at the birth of magnetar studies was the landmark giant flare observed in
1979 from SGR 0526–66 [34, 149]. Nothing alike had been observed before from
a pulsar, and the flare was also different from and much brighter than any other
gamma-ray burst. The extreme properties of the event forced the astronomer to
conceive unusual scenarios. The most appealing one was the existence of a class
of neutron stars with super-strong magnetic fields of 10 14 –10 15 G [56, 165]. In
particular, the word magnetar was introduced to designate these objects by Duncan
and Thompson [56]. The giant flare and the more common emission of short
fainter bursts of these sources were explained by impulsive releases of magnetic
energy stored in the neutron star, which could be triggered by fractures in the
magnetically stressed crust, perhaps associated to sudden magnetic reconnections
in the star’s magnetosphere [56, 208, 209]. The handfuls of sources associated to
these recurrent hard X-/gamma-ray transients were dubbed soft gamma repeaters
(SGRs), to distinguish them from the gamma-ray bursts.
In the same years, another class of X-ray pulsars defying any easy pigeonholing
was emerging: the anomalous X-ray pulsars (AXPs; Mereghetti and Stella [150],
van Paradijs et al. [221]). They were characterised as persistent X-ray pulsars with
periods of a few seconds and owed the adjective ‘anomalous’ to the fact that their
X-ray luminosity exceeded that available from spin-down energy loss (while the
accretion was ruled out by the lack of any trace of a stellar companion). Thompson
and Duncan [209] noted that except for the emission of short bursts, which had never
been observed in AXPs, these sources were very similar to the recently-discovered
persistent soft X-rays counterparts of SGRs [159, 190, 223]. It was suggested that
AXPs could be evolved SGRs, which ended or drastically reduced their explosive
activity after having largely depleted their reservoir of magnetic energy, and the