3 Magnetars: A Short Review and Some Sparse Considerations
113
spin periods between 0.1 and 0.5 s and their derivative have been measured
in three of them. Interesting, for these sources the inferred dipolar magnetic
fields are rather low: ∼10 10 –10 11 G. This prompted for the CCOs an unifying
scenario in which they are either born with weak magnetic fields or with a
normal field that has been ‘buried’ beneath the neutron-star surface by a postsupernova stage of hypercritical accretion of fallback matter [85, 102, 225]. In
the latter case, CCOs could in principle have magnetic field in the magnetar
range [225].
1E 161348–5055 in the 2-kyr-old supernova remnant RCW 103 was one of
the CCO prototypes. However, observations of large flux variations (about
2 orders of magnitude) and an unusual spin period of 6.7 h set it apart
from CCOs or any other class of isolated pulsars [44]. No information
or meaningful limit on its magnetic field are available from its rotational
parameters [65], but it is interesting to notice that De Luca et al. [44]
discussed as a possible mechanism to slow-down in ∼2-kyr a pulsar born
with a normal spin period to the rotation rate of 1E 161348–5055 the propeller
interaction between an ultra-magnetised neutron star (B ∼ 10 14 –10 15 G) and
a surrounding supernova fallback debris disk.
A major breakthrough was when, on 2016 June 22, the Swift’s Burst
Alert Telescope detected an X-ray burst (see Fig. 3.7) resembling in all
respects those of magnetars from the direction of 1E 161348–5055 [38, 187].
Its duration was ∼10 ms, its luminosity ∼2 × 10 39 erg s −1 (15–150 keV),
and the spectrum was well described by a blackbody with kT ∼ 9 keV.
Subsequent follow-up observations with Swift, Chandra and NuSTAR showed
that 1E 161348–5055 was undergoing a magnetar-like outburst: Its luminosity
was ∼100 times higher than the level the source had maintained for several
years and up at least to the last observation carried out before the burst (about
1 month earlier); The pulse profile from single- became double-peaked; A
hard power-law component was observed up to ∼30 keV for the first time
in its energy spectrum, superimposed to its usual thermal emission [187]. In
the first year from the onset of the outburst, the overall energy emitted was
∼ 3×10 42 erg [37]. Moreover, Hubble observations carried out in the summer
of 2016 unveiled at the position of 1E 161348–5055 a faint infrared source that
was not detected in older observations (implying a minimum brightening of
1.3 mag) and therefore can be assumed with a high degree of confidence to be
the counterpart of the CCO [205].
While the features exhibited by 1E 161348–5055 during its outburst match
precisely the distinguishing features of magnetars, its 6.7-h period remains
puzzling. The infrared observations definitely ruled out any doubt about a
binary system but could not confirm of exclude the presence of a fallback
disk. Recent modelling of neutron star–debris disk interaction by Ho and
Andersson ([103], see also Tong et al. [213]) has shown that a disk with mass
(continued)
113
spin periods between 0.1 and 0.5 s and their derivative have been measured
in three of them. Interesting, for these sources the inferred dipolar magnetic
fields are rather low: ∼10 10 –10 11 G. This prompted for the CCOs an unifying
scenario in which they are either born with weak magnetic fields or with a
normal field that has been ‘buried’ beneath the neutron-star surface by a postsupernova stage of hypercritical accretion of fallback matter [85, 102, 225]. In
the latter case, CCOs could in principle have magnetic field in the magnetar
range [225].
1E 161348–5055 in the 2-kyr-old supernova remnant RCW 103 was one of
the CCO prototypes. However, observations of large flux variations (about
2 orders of magnitude) and an unusual spin period of 6.7 h set it apart
from CCOs or any other class of isolated pulsars [44]. No information
or meaningful limit on its magnetic field are available from its rotational
parameters [65], but it is interesting to notice that De Luca et al. [44]
discussed as a possible mechanism to slow-down in ∼2-kyr a pulsar born
with a normal spin period to the rotation rate of 1E 161348–5055 the propeller
interaction between an ultra-magnetised neutron star (B ∼ 10 14 –10 15 G) and
a surrounding supernova fallback debris disk.
A major breakthrough was when, on 2016 June 22, the Swift’s Burst
Alert Telescope detected an X-ray burst (see Fig. 3.7) resembling in all
respects those of magnetars from the direction of 1E 161348–5055 [38, 187].
Its duration was ∼10 ms, its luminosity ∼2 × 10 39 erg s −1 (15–150 keV),
and the spectrum was well described by a blackbody with kT ∼ 9 keV.
Subsequent follow-up observations with Swift, Chandra and NuSTAR showed
that 1E 161348–5055 was undergoing a magnetar-like outburst: Its luminosity
was ∼100 times higher than the level the source had maintained for several
years and up at least to the last observation carried out before the burst (about
1 month earlier); The pulse profile from single- became double-peaked; A
hard power-law component was observed up to ∼30 keV for the first time
in its energy spectrum, superimposed to its usual thermal emission [187]. In
the first year from the onset of the outburst, the overall energy emitted was
∼ 3×10 42 erg [37]. Moreover, Hubble observations carried out in the summer
of 2016 unveiled at the position of 1E 161348–5055 a faint infrared source that
was not detected in older observations (implying a minimum brightening of
1.3 mag) and therefore can be assumed with a high degree of confidence to be
the counterpart of the CCO [205].
While the features exhibited by 1E 161348–5055 during its outburst match
precisely the distinguishing features of magnetars, its 6.7-h period remains
puzzling. The infrared observations definitely ruled out any doubt about a
binary system but could not confirm of exclude the presence of a fallback
disk. Recent modelling of neutron star–debris disk interaction by Ho and
Andersson ([103], see also Tong et al. [213]) has shown that a disk with mass
(continued)