104
P. Esposito et al.
Hurley et al. [112]). The spectrum of this impulsive blaze extends at least to the MeV
range and can be described by a blackbody, with initial kT from ≈30 to 200 keV
(for SGR 0526–66 and SGR 1806–20, respectively). These sudden releases of an
immense amount of energy affected in a measurable way (at least for the two most
recent events) the Earth’s magnetic field [147] and ionosphere [113, 114]. Hurley et
al. [112] argued that an extragalactic giant flare as bright as that of SGR 1806–20
could appear at Earth as a short gamma-ray burst up to a distance of several tens of
Mpc an therefore magnetar flares might represent a non-negligible fraction of the
population of these transients.
After the initial spikes, followed afterglows that were clearly modulated at the
rotational period of the neutron stars. The afterglows were much softer than the
flash and over a few minutes gradually further softened and faded (Fig. 3.3). It is
extremely interesting that while the luminosity of the three peaks spans 2–3 orders
of magnitude, the total energy of the oscillating tail was similar (≈10 44 erg) in
all the events. Since the afterglow is believed to arise from a cloud of photon–
pair plasma confined by the star’s magnetic field that cools as the radiation
gradually leaks out, this clearly indicates a similar value of the magnetic field
in the three magnetars. To trap the fireball, the magnetic pressure must exceed
that of the radiation and pairs at the external boundary of the cloud: B dip >
2 × 10 14 (E fireball /(10 44 erg)) 1/2 (ΔR/(10 km)) −3/2 ((1 + ΔR/R)/2) 3 G, where R
is the stellar radius and ΔR the characteristic size of the fireball [208].
In the SGR 1900+14 and SGR 1806–1920s giant flares, observations of transient
nebular radio emissions provided evidence for outflows [71, 72, 76]. For the beststudied case of SGR 1806–20, the minimum energy in the extended radio emission
was estimated at ≈ 10 43 erg, which seems too much to be consistent with pair
plasma leaked form the fireball. Indeed, the structure, which was observable for
more than a year, is better explained in terms of an baryon-rich mildly-relativistic
ejection interacting with matter surrounding the star [76, 96].
The detection of quasi-periodic oscillations (QPOs) in the tails of the giant flares
from SGR 0526–66 (with detectors aboard the Prognoz 7 satellite and the Venera
11 and 12 space probes; Barat et al. [12]), SGR 1900+14 [199] and SGR 1806–
20 (with RossiXTE and RHESSI; Israel et al. [117], Strohmayer and Watts [200],
Watts and Strohmayer [231]), likely associated to seismic vibrations excited by the
powerful explosion, started the field of asteroseismology for neutron stars (see gray
box QPOs) and offered a new clue of the presence of a magnetic field 10 14 G
in magnetars. Vietri et al. [224] observed that for any source there is a maximum
rate of variation of the luminosity (ΔL) on a certain timescale (Δt): ΔL/Δt <
η(2.8 × 10 18 )/σ T erg s −2 , where σ T is the Thomson cross section and η the energy
extraction efficiency [29, 66]. The 1840-Hz QPO detected in SGR 1806–20 implies a
ΔL/Δt exceeding this limit by a factor larger than 10/η. However, a strong magnetic
field suppresses the electron-scattering cross section (for one photon polarization
mode) below the Thomson’s value by a factor ∝ B 2 ([101] see also van Putten
et al. [222]). The presence of a magnetic field B 2 × 10 15 G at the surface of
SGR 1806–20 would reconcile the QPOs with the luminosity variability limit [224].
P. Esposito et al.
Hurley et al. [112]). The spectrum of this impulsive blaze extends at least to the MeV
range and can be described by a blackbody, with initial kT from ≈30 to 200 keV
(for SGR 0526–66 and SGR 1806–20, respectively). These sudden releases of an
immense amount of energy affected in a measurable way (at least for the two most
recent events) the Earth’s magnetic field [147] and ionosphere [113, 114]. Hurley et
al. [112] argued that an extragalactic giant flare as bright as that of SGR 1806–20
could appear at Earth as a short gamma-ray burst up to a distance of several tens of
Mpc an therefore magnetar flares might represent a non-negligible fraction of the
population of these transients.
After the initial spikes, followed afterglows that were clearly modulated at the
rotational period of the neutron stars. The afterglows were much softer than the
flash and over a few minutes gradually further softened and faded (Fig. 3.3). It is
extremely interesting that while the luminosity of the three peaks spans 2–3 orders
of magnitude, the total energy of the oscillating tail was similar (≈10 44 erg) in
all the events. Since the afterglow is believed to arise from a cloud of photon–
pair plasma confined by the star’s magnetic field that cools as the radiation
gradually leaks out, this clearly indicates a similar value of the magnetic field
in the three magnetars. To trap the fireball, the magnetic pressure must exceed
that of the radiation and pairs at the external boundary of the cloud: B dip >
2 × 10 14 (E fireball /(10 44 erg)) 1/2 (ΔR/(10 km)) −3/2 ((1 + ΔR/R)/2) 3 G, where R
is the stellar radius and ΔR the characteristic size of the fireball [208].
In the SGR 1900+14 and SGR 1806–1920s giant flares, observations of transient
nebular radio emissions provided evidence for outflows [71, 72, 76]. For the beststudied case of SGR 1806–20, the minimum energy in the extended radio emission
was estimated at ≈ 10 43 erg, which seems too much to be consistent with pair
plasma leaked form the fireball. Indeed, the structure, which was observable for
more than a year, is better explained in terms of an baryon-rich mildly-relativistic
ejection interacting with matter surrounding the star [76, 96].
The detection of quasi-periodic oscillations (QPOs) in the tails of the giant flares
from SGR 0526–66 (with detectors aboard the Prognoz 7 satellite and the Venera
11 and 12 space probes; Barat et al. [12]), SGR 1900+14 [199] and SGR 1806–
20 (with RossiXTE and RHESSI; Israel et al. [117], Strohmayer and Watts [200],
Watts and Strohmayer [231]), likely associated to seismic vibrations excited by the
powerful explosion, started the field of asteroseismology for neutron stars (see gray
box QPOs) and offered a new clue of the presence of a magnetic field 10 14 G
in magnetars. Vietri et al. [224] observed that for any source there is a maximum
rate of variation of the luminosity (ΔL) on a certain timescale (Δt): ΔL/Δt <
η(2.8 × 10 18 )/σ T erg s −2 , where σ T is the Thomson cross section and η the energy
extraction efficiency [29, 66]. The 1840-Hz QPO detected in SGR 1806–20 implies a
ΔL/Δt exceeding this limit by a factor larger than 10/η. However, a strong magnetic
field suppresses the electron-scattering cross section (for one photon polarization
mode) below the Thomson’s value by a factor ∝ B 2 ([101] see also van Putten
et al. [222]). The presence of a magnetic field B 2 × 10 15 G at the surface of
SGR 1806–20 would reconcile the QPOs with the luminosity variability limit [224].