3 Magnetars: A Short Review and Some Sparse Considerations
103
well understood. A handful of magnetar has been detected also as pulsating sources
at longer wavelength, in the radio band. We give an overview of the properties of
magnetars at radio frequencies in Sect. 3.2.2.3.
3.2.2 Transient Activity
Magnetars are certainly characterised by an extremely rich observational phenomenology, but this is particularly true when speaking of their transient activity:
They display unpredictable and dramatic variations in their emission and timing
properties in all the wavelengths at which they are detected, on time scales from
milliseconds to months or years, and often with a dynamic range unparalleled by
any other embodiment of isolated neutron stars. Their transient radiative events
are usually outlined in two main categories: short-duration (ms–minutes) explosive
events (giant flares and bursts) and outbursts, in which the X-ray luminosity rises to
up to ∼10 3 times the quiescent level and then decays in weeks to months/years.
Perhaps, an outburst more than an event could be considered a ‘syndrome’, in
the sense that the flux enhancement is generally accompanied by bursts, spectral
changes and timing anomalies, including glitches.
3.2.2.1 Giant Flares
Giant flares are the rarest and most energetic events associated with magnetars.
They are also the most important, at least historically, as it was the first giant flare,
from SGR 0526–66 in the Large Magellanic Cloud on 1979 March 5 [34, 149],
that brought magnetars on the astrophysical scene, provided clear-cut evidence of
their neutron-star nature and propensity to produce multiple events (at variance with
the gamma-ray bursts discovered by the Vela satellites), and prompted—among a
multitude of different models (see e.g. Norris et al. [163], Woods and Thompson
[232])—the concept of super-magnetic neutron stars [56, 165, 208]. Moreover, they
still provide some of the most compelling clues for the presence of magnetic fields
of 10 14 G close to the neutron star surface in magnetars.
A total of three giant flares have been observed. In 1979 from SGR 0526–66,
on 1998 August 27 from SGR 1900+14 [111] and on 2004 December 27 from
SGR 1806–20 [112, 167]. It is worth noticing that the three giant flares were
emitted from the three first SGRs discovered (but only SGR 0526–66 was discovered
because of the event). Since a mere coincidence seems unlikely and a giant flare
makes a significant dent in the total magnetic energy pool of a magnetar, a natural
explanation would be that those three sources are at the magnetic-activity pinnacle
of their life and their frequent bursting activity got them noticed earlier than other
magnetars (see also Perna and Pons [169], Viganò et al. [226]).
All three giant flares started with a short (∼0.1–0.2 s) flash of hard X-rays with
peak luminosity 10 44 –10 45 erg s −1 ( 10 47 erg s −1 in the case of SGR 1806–20;
103
well understood. A handful of magnetar has been detected also as pulsating sources
at longer wavelength, in the radio band. We give an overview of the properties of
magnetars at radio frequencies in Sect. 3.2.2.3.
3.2.2 Transient Activity
Magnetars are certainly characterised by an extremely rich observational phenomenology, but this is particularly true when speaking of their transient activity:
They display unpredictable and dramatic variations in their emission and timing
properties in all the wavelengths at which they are detected, on time scales from
milliseconds to months or years, and often with a dynamic range unparalleled by
any other embodiment of isolated neutron stars. Their transient radiative events
are usually outlined in two main categories: short-duration (ms–minutes) explosive
events (giant flares and bursts) and outbursts, in which the X-ray luminosity rises to
up to ∼10 3 times the quiescent level and then decays in weeks to months/years.
Perhaps, an outburst more than an event could be considered a ‘syndrome’, in
the sense that the flux enhancement is generally accompanied by bursts, spectral
changes and timing anomalies, including glitches.
3.2.2.1 Giant Flares
Giant flares are the rarest and most energetic events associated with magnetars.
They are also the most important, at least historically, as it was the first giant flare,
from SGR 0526–66 in the Large Magellanic Cloud on 1979 March 5 [34, 149],
that brought magnetars on the astrophysical scene, provided clear-cut evidence of
their neutron-star nature and propensity to produce multiple events (at variance with
the gamma-ray bursts discovered by the Vela satellites), and prompted—among a
multitude of different models (see e.g. Norris et al. [163], Woods and Thompson
[232])—the concept of super-magnetic neutron stars [56, 165, 208]. Moreover, they
still provide some of the most compelling clues for the presence of magnetic fields
of 10 14 G close to the neutron star surface in magnetars.
A total of three giant flares have been observed. In 1979 from SGR 0526–66,
on 1998 August 27 from SGR 1900+14 [111] and on 2004 December 27 from
SGR 1806–20 [112, 167]. It is worth noticing that the three giant flares were
emitted from the three first SGRs discovered (but only SGR 0526–66 was discovered
because of the event). Since a mere coincidence seems unlikely and a giant flare
makes a significant dent in the total magnetic energy pool of a magnetar, a natural
explanation would be that those three sources are at the magnetic-activity pinnacle
of their life and their frequent bursting activity got them noticed earlier than other
magnetars (see also Perna and Pons [169], Viganò et al. [226]).
All three giant flares started with a short (∼0.1–0.2 s) flash of hard X-rays with
peak luminosity 10 44 –10 45 erg s −1 ( 10 47 erg s −1 in the case of SGR 1806–20;