3 Magnetars: A Short Review and Some Sparse Considerations
115
SGR J1745–2900: The Galactic Centre Magnetar
When on 24 April 2013 Swift detected with its X-ray telescope a large flare
from the region of Sgr A* [45], many minds went to the much anticipated
pericenter passage of the object G2, which at the time was expected around
mid-2013, and its possible tidal disruption by the Milky Way’s 4 × 10 6 -M
black hole [79, 80].
Two days later, however, a magnetar-like short burst was detected by the
Swift’s coded mask instrument and also a typical magnetar period of 3.8 s
was detected, using NuSTAR [128, 157]. The situation was definitely settled
when an observation carried out with Chandra (the only X-ray telescope
with sufficient angular resolution) showed that a 3.8-s magnetar in outburst,
SGR J1745–2900, very close to the position of Sgr A* (angular separation
of (2.4 ± 0.3) arcsec) was responsible for all the X-ray luminosity increase
measured by Swift (≈ 2 × 10 35 erg s −1 at 8.3 kpc, while Sgr A* was not
detected in the same exposure; Rea et al. [184]). (The closest approach of
G2 to Sgr A* actually took place in early 2014; G2 survived and no flaring
activity clearly associated to the event was observed; Phifer et al. [172],
Pfuhl et al. [171], Ponti et al. [179], Plewa et al. [174].) On 28 April 2013,
the new magnetar was also detected as a radio pulsar, the one with the
highest dispersion measure and rotation measure, suggesting that the source
is embedded in the dense and magnetized plasma of the Galactic center
[19, 58, 145, 168, 184, 214].
The angular separation between the magnetar and Sgr A* corresponds to
a projected distance of only 0.1 pc, and Rea et al. [184] estimated that if
SGR J1745–2900 was born within 1 pc of Sgr A*, its probability of being in
a bound orbit around the black hole is of ∼90%. Bower et al. [19] measured
a transverse velocity of the source relative to Sgr A* of (236 ± 11) km s −1
and provided further support to the possibility that the magnetar is bound to
Sgr A*. Rea et al. [184] also noted that the high-energy emission produced
by the past activity of the magnetar, passing through the molecular clouds
surrounding the Galactic center region, might be responsible for a substantial
fraction of the light echoes observed in the Fe fluorescence features.
SGR J1745–2900 is proving to be an important probe for the compact
object population and the interstellar medium in the Galactic center, but is also
exhibiting an interesting behaviour as a magnetar. About 3.5 years after the
outset of the outburst, it has not reached the quiescent/pre-outburst luminosity
level yet [36]. Its spectral evolution is difficult to reconcile with crustal
cooling models, while a continuous particle bombardment from returning
currents of the neutron star surface better explain the data. In this hypothesis,
both temperature and size of the region at the footprint point of the bundle
(continued)
115
SGR J1745–2900: The Galactic Centre Magnetar
When on 24 April 2013 Swift detected with its X-ray telescope a large flare
from the region of Sgr A* [45], many minds went to the much anticipated
pericenter passage of the object G2, which at the time was expected around
mid-2013, and its possible tidal disruption by the Milky Way’s 4 × 10 6 -M
black hole [79, 80].
Two days later, however, a magnetar-like short burst was detected by the
Swift’s coded mask instrument and also a typical magnetar period of 3.8 s
was detected, using NuSTAR [128, 157]. The situation was definitely settled
when an observation carried out with Chandra (the only X-ray telescope
with sufficient angular resolution) showed that a 3.8-s magnetar in outburst,
SGR J1745–2900, very close to the position of Sgr A* (angular separation
of (2.4 ± 0.3) arcsec) was responsible for all the X-ray luminosity increase
measured by Swift (≈ 2 × 10 35 erg s −1 at 8.3 kpc, while Sgr A* was not
detected in the same exposure; Rea et al. [184]). (The closest approach of
G2 to Sgr A* actually took place in early 2014; G2 survived and no flaring
activity clearly associated to the event was observed; Phifer et al. [172],
Pfuhl et al. [171], Ponti et al. [179], Plewa et al. [174].) On 28 April 2013,
the new magnetar was also detected as a radio pulsar, the one with the
highest dispersion measure and rotation measure, suggesting that the source
is embedded in the dense and magnetized plasma of the Galactic center
[19, 58, 145, 168, 184, 214].
The angular separation between the magnetar and Sgr A* corresponds to
a projected distance of only 0.1 pc, and Rea et al. [184] estimated that if
SGR J1745–2900 was born within 1 pc of Sgr A*, its probability of being in
a bound orbit around the black hole is of ∼90%. Bower et al. [19] measured
a transverse velocity of the source relative to Sgr A* of (236 ± 11) km s −1
and provided further support to the possibility that the magnetar is bound to
Sgr A*. Rea et al. [184] also noted that the high-energy emission produced
by the past activity of the magnetar, passing through the molecular clouds
surrounding the Galactic center region, might be responsible for a substantial
fraction of the light echoes observed in the Fe fluorescence features.
SGR J1745–2900 is proving to be an important probe for the compact
object population and the interstellar medium in the Galactic center, but is also
exhibiting an interesting behaviour as a magnetar. About 3.5 years after the
outset of the outburst, it has not reached the quiescent/pre-outburst luminosity
level yet [36]. Its spectral evolution is difficult to reconcile with crustal
cooling models, while a continuous particle bombardment from returning
currents of the neutron star surface better explain the data. In this hypothesis,
both temperature and size of the region at the footprint point of the bundle
(continued)