112
P. Esposito et al.
density of 10 Jy or more. A few other magnetars were subsequently detected in radio
as pulsars: 1E 1547.0–5408 [26], PSR J1622–4950 (the only magnetar discovered at
radio wavelengths so far; Levin et al. [141]), and SGR J1745–2900 [58, 184]. All
the detections happened during an X-ray outburst but, interestingly, at least in some
cases the radio emission outlived the X-ray flux enhancement [4, 28, 193]. Also,
even in periods in which they are overall active in radio, magnetars seem to switch
suddenly on and off [21]. 1
Apart from its temporary nature, the pulsed radio emission from magnetars shows
in all four sources some clear differences with respect from that of ordinary rotationpowered pulsars [27, 132, 142, 196]. The radio emission has a very hard spectrum:
S ∝ ν −0.5 or flatter (where S is the flux density and ν is the frequency), while
the typical spectral index of radio pulsars is approximately −1.8 (e.g Seiradakis
and Wielebinski [194]). Another peculiar characteristic of magnetar’s pulsed radio
emission is the instability of the pulse shape. Most radio pulsars show some pulseby-pulse variability, but the addition of a few hundred pulses is generally sufficient
to attain a stable pulse profile; more rarely, radio pulsars switches on time scales
from minutes to hours between a small number (usually two) of different pulse
profiles [131, 146]. The individual pulses of magnetars have a spiky appearance
and stable pulse profiles were never observed (e.g. Kramer et al. [132], Camilo et
al. [28]); it is therefore unclear whether the profile stabilization would require for
magnetars an unprecedented number of pulses or they simply do not have a stable
characteristic pulse profile. Finally, high degrees of linear polarization have been
measured in magnetars, up to very high frequencies [27, 215].
1E 161348–5055 in RCW 103: The CCO That Was Not
Central compact objects in supernova remnants (CCOs) are a small set
of isolated neutron stars observed close to centres of young non-plerionic
supernova remnants (see De Luca [43] for a review). CCOs are fairly steady
X-ray sources with thermal-like spectra and no counterparts detected at other
wavelengths. They owe their redundant designation to the fact that in the
years the class emerged, there was not absolute certainty of their neutronstar nature, since no periodic modulations were find in their emission and also
searches at radio frequencies failed to detect a pulsar. After numerous deep
observations, there is now little doubt that CCOs are indeed neutron stars, and
(continued)
1 In this respect, it is interesting to notice that when the radio pulsar PSR J1119–6127 emitted
a number of SGR-like bursts and entered an outburst, initially it disappeared as a radio pulsar
[22]. Furthermore, after the radio reactivation but still during the outburst, Archibald et al. [8]
using simultaneous radio and X-ray observations observed that the radio emission shut down in
coincidence with the X-ray bursts, with a recovery time of ∼70 s.
P. Esposito et al.
density of 10 Jy or more. A few other magnetars were subsequently detected in radio
as pulsars: 1E 1547.0–5408 [26], PSR J1622–4950 (the only magnetar discovered at
radio wavelengths so far; Levin et al. [141]), and SGR J1745–2900 [58, 184]. All
the detections happened during an X-ray outburst but, interestingly, at least in some
cases the radio emission outlived the X-ray flux enhancement [4, 28, 193]. Also,
even in periods in which they are overall active in radio, magnetars seem to switch
suddenly on and off [21]. 1
Apart from its temporary nature, the pulsed radio emission from magnetars shows
in all four sources some clear differences with respect from that of ordinary rotationpowered pulsars [27, 132, 142, 196]. The radio emission has a very hard spectrum:
S ∝ ν −0.5 or flatter (where S is the flux density and ν is the frequency), while
the typical spectral index of radio pulsars is approximately −1.8 (e.g Seiradakis
and Wielebinski [194]). Another peculiar characteristic of magnetar’s pulsed radio
emission is the instability of the pulse shape. Most radio pulsars show some pulseby-pulse variability, but the addition of a few hundred pulses is generally sufficient
to attain a stable pulse profile; more rarely, radio pulsars switches on time scales
from minutes to hours between a small number (usually two) of different pulse
profiles [131, 146]. The individual pulses of magnetars have a spiky appearance
and stable pulse profiles were never observed (e.g. Kramer et al. [132], Camilo et
al. [28]); it is therefore unclear whether the profile stabilization would require for
magnetars an unprecedented number of pulses or they simply do not have a stable
characteristic pulse profile. Finally, high degrees of linear polarization have been
measured in magnetars, up to very high frequencies [27, 215].
1E 161348–5055 in RCW 103: The CCO That Was Not
Central compact objects in supernova remnants (CCOs) are a small set
of isolated neutron stars observed close to centres of young non-plerionic
supernova remnants (see De Luca [43] for a review). CCOs are fairly steady
X-ray sources with thermal-like spectra and no counterparts detected at other
wavelengths. They owe their redundant designation to the fact that in the
years the class emerged, there was not absolute certainty of their neutronstar nature, since no periodic modulations were find in their emission and also
searches at radio frequencies failed to detect a pulsar. After numerous deep
observations, there is now little doubt that CCOs are indeed neutron stars, and
(continued)
1 In this respect, it is interesting to notice that when the radio pulsar PSR J1119–6127 emitted
a number of SGR-like bursts and entered an outburst, initially it disappeared as a radio pulsar
[22]. Furthermore, after the radio reactivation but still during the outburst, Archibald et al. [8]
using simultaneous radio and X-ray observations observed that the radio emission shut down in
coincidence with the X-ray bursts, with a recovery time of ∼70 s.