Soft gamma-ray repeaters and anomalous X-ray pulsars are a small (but growing) group of X-ray sources characterized by the emission of short bursts and by a large variability in their persistent flux. They are believed to be magnetars, i.e. neutron stars powered by extreme magnetic fields 1E14-1E15 G). We found evidence for a magnetar with a low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of soft gamma-ray repeaters. New X-ray observations show that its dipolar magnetic field cannot be greater than 8E12 G, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of magnetic-field strengths, ages and evolutionary stages than observed so far.
The surface dipole field strength of a non-accreting pulsar can be estimated equating the rate of rotational kinetic energy loss with the power of the magnetic-dipole radiation. At the neutron-star (magnetic) equator:
where P (in s) is the pulsar spin period and Ṗ (in s s -1 ) its spin-down rate, and we assumed R NS = 10 6 cm and I = 10 45 g cm 2 for the neutron-star radius and moment of inertia.The surface dipolar magnetic field strengths inferred in this way are in the range ∼10 11 -10 13 G for most non-recycled pulsars, but they can reach ∼10 14 -10 15 G for a handful of sources which are generally referred to as magnetars. 1 To date only a dozen and a half 2 of these ultra-magnetized neutron stars are known and, even if the distinction is becoming increasingly blurred, they are generally classified as either soft gammaray repeaters (SGRs) or anomalous X-ray pulsars (AXPs). They are all X-ray pulsars with spin periods of 2-12 s, period derivatives of ∼10 -13 -10 -10 s s -1 (corresponding to characteristic ages, τ c = P/2 Ṗ, from about 0.2 kyr to 0.2 Myr), and luminosities of L X ∼ 10 32 -10 36 erg s -1 , usually much higher than the rate at which the star loses its rotational energy through spin-down. Magnetars are also characterized by unpredictable outbursts, lasting from days to years, during which they emit characteristic short bursts of X/γ-ray photons. Their high luminosities, together with the lack of evidence for accretion from a stellar companion, led to the conclusion that the energy powering the SGR/AXP activity must be stored in their exceptional magnetic fields [2,4].
In addition to SGRs and AXPs, two other sources are known to show magnetarlike activity: PSR J1846-0258 and PSR J1622-4950. The former is a 0.3-s, allegedly rotation-powered, X-ray pulsar, with a magnetic field in the lower end of the magnetar range (B ∼ 5 × 10 13 G), from which a typical magnetar outburst and short X-ray bursts were detected [5]. In the latter, flaring radio emission with a flat spectrum (similar to those observed in the two transient radio magnetars; [6,7]) was observed from a 4.3-s radio pulsar with a magnetic field in the magnetar range (B ∼ 3 × 10 14 G) [8].
In all sources with magnetar-like activity, the dipolar field spans 5 × 10 13 G < B < 2 × 10 15 G, which is ∼10-1000 times the average value in radio pulsars and higher than the quantum electron field, B Q = m 2 e c 3 /eh ≃ 4.4 × 10 13 G (at which the electron cyclotron energy equals the rest mass), which is the traditional divide between magnetars and ordinary pulsars. The existence of radio pulsars with B > B Q and showing only normal behavior is an indication that a magnetic field larger than the quantum electron field alone may not be a sufficient condition for the onset of magnetar-like activity [9,10]. In contrast, so far the opposite always held: magnetar-like activity was observed only in sources with dipolar magnetic fields stronger than B Q .
SGR 0418+5729 was discovered on 5 June 2009 when the Fermi Gamma-ray Burst Monitor observed two magnetar-like bursts [11]. Follow-up observations with several X-ray satellites showed that SGR 0418+5729 has X-ray pulsations at ∼9.1 s [12], well within the range of periods of magnetar sources, and exhibits all the other peculiarities of magnetars: emission of short X-ray bursts, persistent X-ray luminosity larger than that the rotational energy loss, a spectrum characterized by a thermal plus non-thermal component which softened during the outburst decay, and variable pulse profile [13].
Several X-ray instruments repeatedly observed SGR 0418+5729 since its discovery and for about 160 days (then, due to solar constraints, the source came out of visibility for the satellites). This campaign allowed an accurate phase-coherent study of the pulsar rotation but no sign of a period derivative was detected [13]. The upper limit on the spin-down rate was 10 -13 s s -1 (90% confidence level), which, according to Eq. (1), translates into a limit on the surface dipolar magnetic field of B < 3 × 10 13 G [13]. This limit is quite low for a magnetar source, but not abnormally so, it is in fact comparable with the values inferred for the AXP 1E 1048.0-5937 (B ∼ 6×10 13 G) and the magnetarlike pulsar PSR J1846-0258. ). Black squares represent normal radio pulsars, light-blue squares normal radio pulsars with a magnetic field larger than our limit for SGR 0418+5729 (7.5 × 10 12 G), red stars are the magnetars, orange triangles are the magnetar-like pulsars PSR J1846-0258 and PSR J1622-4950, and the green circles are the X-ray dim isolated neutron stars (XDINSs; [16]). The solid line marks the 90% upper limit for the dipolar magnetic field of SGR 0418+5729. The value of the electron quantum magnetic field is also reported (dashed line).
In July 2010, when SGR 0418+5729 became visible again, we resumed our monitoring campaign by observing the source with Swift, Chandra and XMM-Newton (see [14] for more details). Since p
This content is AI-processed based on open access ArXiv data.
