3 Magnetars: A Short Review and Some Sparse Considerations
111
(L X,q ) and the dynamical range of its outbursts; Coti Zelati et al. [37] found that
L X,peak /L X,q ∝ L
−0.7
X,q , where L X,peak is the maximum X-ray luminosity achieved
during the event (see also Pons and Rea [176]). Indeed, several magnetars that
could be studied in detail when they were undergoing outbursts, are completely
unnoticeable and hardly recognisable as magnetars while in quiescence (or even
nondetectable without deep and targeted observations). For this reason, the strategy
of the Swift mission [75] of slewing and pointing its X-ray telescope as soon as
possible towards the transient events detected and localized by its wide-field codedmask detector sensitive to hard X-rays, has proven tremendously successful in
discovering new magnetars.
Outbursts are usually accompanied also by changes in the timing properties of
the magnetar. The morphology of the pulse profiles, which generally are broad and
with one or two main peaks per cycle, can vary dramatically during an outburst,
both in shape (as a rule, becoming more complicate when the activity is high) and
in pulsed fraction (e.g. Woods et al. [234], Israel et al. [120], Rodíguez Castillo et
al. [188], Esposito et al. [63], Dib and Kaspi [52]). During outbursts and in general
in period of enhanced activity, magnetars show a more efficient spin-down torque,
with variations of a factor up to ≈10 (e.g. Mereghetti et al. [152], Olausen and Kaspi
[164]); the spin-down evolution, however, does not trace or correlate well with the
radiative or bursting behaviours [236, 237]. Among isolated pulsars, magnetars are
particularly noisy rotators, and also this aspect is amplified during outbursts [52, 64].
In young radio pulsars ( 10 5 yr), timing noise has been often suggested
to be linked to recovery from glitch events [104]. Magnetars are rather prolific
glitchers, comparable to the most frequently glitching radio pulsars, and glitches
often happen during outbursts (although in coincidence of some glitches, no Xray flux enhancements were detected); also, while the amplitude distribution of
their glitches peaks on larger Δν/ν with respect to the ‘normal’ pulsars, the values
observed are in the same range (Δν/ν ∼ 10 −9 –10 −5 ; Dib et al. [53], Dib and Kaspi
[52]). Magnetars glitching behaviour appears to be different in the recovery, which
is typically very strong, often resulting in an over-recovery, and in the fact that also
anti-glitches (that is, episodes of sudden spin down) have been reported. The most
eminent anti-glitch candidate was reported for 1E 2259+586 [6], where in less than
4 days, a spin-down of Δν/ν ∼ −10 −7 was achieved; the sudden variation was
accompanied by a simultaneous short burst and by a small (factor ∼2) but longlived (months) flux increase. A large ‘braking glitch’ could also have occurred in
SGR 1900+14 in an 80-days interval including the epoch of its giant flare [233]; the
observations were however too sparse to tell whether the abnormal increase of the
period resulted from a sudden event or from a prolonged period of enhanced spin
down.
Another magnetar activity associated to X-ray outbursts is the transient pulsed
radio emission observed in a few of them. Until the first detection of radio pulses
during the outburst of XTE J1810–197 [24], magnetars were (rather staunchly)
believed to be radio quiet. Ironically, at the time of its radio activation, XTE J1810–
197 was the brightest pulsar of the radio sky, with individual pulses reaching flux
111
(L X,q ) and the dynamical range of its outbursts; Coti Zelati et al. [37] found that
L X,peak /L X,q ∝ L
−0.7
X,q , where L X,peak is the maximum X-ray luminosity achieved
during the event (see also Pons and Rea [176]). Indeed, several magnetars that
could be studied in detail when they were undergoing outbursts, are completely
unnoticeable and hardly recognisable as magnetars while in quiescence (or even
nondetectable without deep and targeted observations). For this reason, the strategy
of the Swift mission [75] of slewing and pointing its X-ray telescope as soon as
possible towards the transient events detected and localized by its wide-field codedmask detector sensitive to hard X-rays, has proven tremendously successful in
discovering new magnetars.
Outbursts are usually accompanied also by changes in the timing properties of
the magnetar. The morphology of the pulse profiles, which generally are broad and
with one or two main peaks per cycle, can vary dramatically during an outburst,
both in shape (as a rule, becoming more complicate when the activity is high) and
in pulsed fraction (e.g. Woods et al. [234], Israel et al. [120], Rodíguez Castillo et
al. [188], Esposito et al. [63], Dib and Kaspi [52]). During outbursts and in general
in period of enhanced activity, magnetars show a more efficient spin-down torque,
with variations of a factor up to ≈10 (e.g. Mereghetti et al. [152], Olausen and Kaspi
[164]); the spin-down evolution, however, does not trace or correlate well with the
radiative or bursting behaviours [236, 237]. Among isolated pulsars, magnetars are
particularly noisy rotators, and also this aspect is amplified during outbursts [52, 64].
In young radio pulsars ( 10 5 yr), timing noise has been often suggested
to be linked to recovery from glitch events [104]. Magnetars are rather prolific
glitchers, comparable to the most frequently glitching radio pulsars, and glitches
often happen during outbursts (although in coincidence of some glitches, no Xray flux enhancements were detected); also, while the amplitude distribution of
their glitches peaks on larger Δν/ν with respect to the ‘normal’ pulsars, the values
observed are in the same range (Δν/ν ∼ 10 −9 –10 −5 ; Dib et al. [53], Dib and Kaspi
[52]). Magnetars glitching behaviour appears to be different in the recovery, which
is typically very strong, often resulting in an over-recovery, and in the fact that also
anti-glitches (that is, episodes of sudden spin down) have been reported. The most
eminent anti-glitch candidate was reported for 1E 2259+586 [6], where in less than
4 days, a spin-down of Δν/ν ∼ −10 −7 was achieved; the sudden variation was
accompanied by a simultaneous short burst and by a small (factor ∼2) but longlived (months) flux increase. A large ‘braking glitch’ could also have occurred in
SGR 1900+14 in an 80-days interval including the epoch of its giant flare [233]; the
observations were however too sparse to tell whether the abnormal increase of the
period resulted from a sudden event or from a prolonged period of enhanced spin
down.
Another magnetar activity associated to X-ray outbursts is the transient pulsed
radio emission observed in a few of them. Until the first detection of radio pulses
during the outburst of XTE J1810–197 [24], magnetars were (rather staunchly)
believed to be radio quiet. Ironically, at the time of its radio activation, XTE J1810–
197 was the brightest pulsar of the radio sky, with individual pulses reaching flux