INTEGRAL observes intense activity of a neutron star

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On January 22, 2009, the anti-coincidence system (ACS), a massive ring of gamma-ray detectors on-board ESA's INTEGRAL satellite, detected about 200 intense flashes of gamma-ray light, lasting from 0.1 to 8 seconds in duration. The events were among the brightest ever seen in ACS since INTEGRAL's launch in October 2002, much brighter than the usual gamma-ray burst. On a normal day, INTEGRAL detects between none and a few faint events in ACS. As also NASA's Swift satellite and the Fermi satellite had seen some of the bright flares (GCN 8833, GCN 8835), it became clear that these outbursts are indeed due to the astrophysical source 1E 1547.0-5408, a so-called anomalous X-ray pulsar in the Norma spiral arm of our galaxy. These stars, which are about 25 kilometers in diameter but are more massive than the sun, are believed to have extremely strong magnetic fields, about 10¹⁵ times stronger than the magnetic field of the Earth. The outbursts are thought to be connected to these magnetic fields - perhaps reconnection of the magnetic field lines due to a breaking of the neutron star's outer crust.

A short observation of the Swift satellite shows rings around the star in the X-rays (see image on the left). These are caused by dust in between the star and us, as pointed out by A. Tiengo (GCN 8848). The dust scatters the X-ray light - a similar effect as one sometimes sees on Earth as halos around the Sun or Moon.

The ISDC has asked ESA to point the INTEGRAL satellite now towards this enigmatic object, and INTEGRAL started with a dedicated observation on Saturday, January 24. The data of this observation are immediately publicly available for analysis. Through the analysis of the data we hope to learn more about the physical processes which are responsible for these elusive events. A quick analysis of the data shows that the magnetar is indeed still in an active phase, with several bright bursts caught by the main instrument IBIS/ISGRI (see a short report of Baldovin et al. 2009, Astronomer's Telegram 1908). Apart from bursts, which occurred during the observation and were again seen by several satellites, it was also possible to determine the X-ray spectrum of this AXP. The spectrum extends up to energies of at least 150 keV and has the signature of processes usually associated with emission of non-thermal origin, rather than showing a "hot spot" on the neutron star's surface. In addition we have informed the astrophysical community about the events seen by INTEGRAL in two GCN notes by Volodymyr Savchenko of the ISDC (GCN 8837) and by Sandro Mereghetti (IASF/Milano, GCN 8841).

The image on top of this page is an artist's impressions of an anomalous X-ray pulsar (Credit: John Rowe Animations). The light curve on the bottom shows 15 of the 200 outbursts seen from 1E 1547.0-5408 by the ACS. All the bright ACS events can be found here

In case of questions and comments: contact me at the APC. Last update: January 27, 2009