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The Active Phase Of Anomalous X-Ray Pulsar & Magnetar 4U 0142+61 (1RXS J014621+614509)

16/08/2011

Anomalous X-ray pulsars, with their huge magnetic fields (~1014G) and slow rotation periods (~1-15s) are generally considered to be a class of relatively young, unbounded and highly magnetised neutron star. A magnetar in isolation (Thompson & Duncan 1995; Duncan & Thompson 1996). Such AXPs have X-ray luminosities of the order of 1033 – 1035 erg s-1, which cannot be accounted for by their available spin-down energy.

The magnetar model was first proposed to explain the dramatic behavior exhibited by an apparently different object class: the Soft Gamma-Ray Repeaters (SGRs). SGRs show persistent properties similar to AXPs, but they were first discovered by their enormous bursts of soft gamma rays and their much more frequent, shorter and thus less energetic bursts of hard X-rays. To date, SGR-like X-ray bursts have been observed from six AXPs (Gavriil et al. 2002; Woods et al. 2005; Mereghetti et al. 2009; Kumar & Safi-Harb 2011), thus solidifying the connection between the two source classes.

Thus far, only the magnetar model proposed by Thompson & Duncan (1995) is adequate enough to explain the bursts observed from both SGRs and AXPs. Both objects display signs of giant flares, which are due to rearrangements of the magnetar’s external magnetic field caused by magnetic field stresses exerted on the crust. If this stress becomes much more concentrated at some locality then the crust can be fractured. This fracturing displaces the magnetic field footprints which results in the release of short X-ray bursts.

Due to matter frozen magnetic fields inside the magnetar and the assumed intense movement of this matter, the internal magnetic field becomes highly twisted. In turn, slowly twisting up the external field; thus, magnetospheres of magnetars may be globally twisted (Thompson et al. 2002). Reconnection of this twisted field is the most likely explanation for the short X-ray bursts associated to SGRs and AXPs (Lyutikov 2002). Such reconnection events are also believed to be a causes of X-ray emissions from solar flares (Nakariakov et al. 2010).

In addition to bursts, AXPs and SGRs exhibit pulsed and persistent flux variations on multiple different timescales. An increase in the pulsed flux over several hours has been seen to follow a burst in AXP 1E 1048.1–5937 (Gavriil et al. 2006). On longer timescales, AXPs can exhibit abrupt increases in flux which decay over several weeks. These bursts take place in unison with X-ray bursts and are believed to be due to thermal radiation from the magnetar’s surface after the deposition of heat from the bursts. Such flux enhancements have been observed in SGRs (see Woods et al. 2001).

In March 2006, after at least six years of quiescence, the anomalous X-ray pulsar (AXP) 4U 0142+61 (a.k.a 1RXS J014621+614509) entered into an intense active phase. This active phase lasted several months and included six X-ray bursts as well as many changes in the persistent X-ray emission. 4U 0142+61 is now only the seventh such AXP which exhibits this flaring behavior. The active phase consisted of a timing anomaly that can be described as a net ‘anti-glitch’, that is, a net spin-down following an initial spin-up that decayed on a time scale of 17 days. Following the glitch, Gavriil, Dib & Kaspi (2011) detected six bursts from the pulsar, the first such bursts ever observed from this source. So what has caused 4U 0142+61 to start these outbursts?

N.B. 4U 0142+61 is classified as an 8.7s AXP with a spin derivative of , implying a surface dipole magnetic field of 1.3×1014G (Eichler & Shaisultanov 2010).

As mentioned previously, it is usually believed that it is due to a sudden magnetospheric twist. However, Gavriil, Dib & Kaspi (2011) showed that most aspects of 4U 0142+61’s emission change during the active phase. This argues against a sudden magnetospheric twist, as has been invoked for other AXP activity. Instead, Gavriil, Dib & Kaspi (2011) suggest instead that it is due to movements and changes in the star’s crust driven by its large internal magnetic field.

Journal References:

  • Dib, R.; Kaspi, V.M.; Gavriil, Fotis P. (2007) 10 Years Of RXTE Monitoring Of Anomalous X-Ray Pulsar 4U 0142+61: Long-Term VariabilityAstrophysics & Space Science, 308  (1-4): pp. 487-491.
  • Trümper, J. E.; Kylafis, N. D.; Ertan, Ü.; Zezas, A. (2010) The Spectral & Beaming Characteristics Of The Anomalous X-Ray Pulsar 4U 0142+61. DOI: 2010arXiv1011.1678T.
  • Enoto, T.; et al. (2011) Soft & Hard X-Ray Emissions From The Anomalous X-Ray Pulsar 4U 0142+61Japan Astronomical Society Publications63 (2): pp.387-396.
  • Gavriil, F.P.; Dib, R.; Kaspi, V.M. (2011) The 2006–2007 Active Phase Of Anomalous X-ray Pulsar 4U 0142+61: Radiative & Timing Changes, Bursts & Burst Spectral Features. The Astrophysical Journal, 736 (2): Article I.D.: 138.

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