A mysterious gamma-ray source powered by a Magnetar?

May 2016

Fig. 1: H.E.S.S. gamma-ray image of the VHE source HESS J1808-204 with 6, 5, and 4 sigma statistical significance contours (black solid lines) and intrinsic extension (white dashed circle). Green cross: best fit position with 1 sigma errors; red star: location of the massive stellar cluster C1*1806-20. Magenta dashed ellipse: 1 sigma location error of the Fermi-LAT GeV source 3FGL J1809.2-2016c.

In 2012, H.E.S.S. reported the discovery of a new very high energy (VHE) gamma-ray source towards the magnetar/soft-gamma-repeater SGR1806-20. Magetars are highly magnetised neutron stars, with a surface field of 10¹⁴ to 10¹⁵ Gauss – about 1000 times higher than for “ordinary” neutron stars. SGR1806-20 is famous for its giant flare of December 27, 2004 [1], which might be attributed to deformations of the neutron star surface. This magnetar is embedded in the massive stellar cluster C1*1806-20, which harbours four Wolf-Rayet (WR) blue giant stars plus one of the most massive stars in our galaxy – the luminous blue variable candidate LBV1806-20 [2]. Before they die in supernova explosions, these stars exhibit incredibly powerful stellar “winds” of particles which may heavily influence their surroundings.

In a paper recently accepted by the Astronomy and Astrophysics journal [3] as a part of a special feature devoted to H.E.S.S., the final analysis of the H.E.S.S. observations (94 hr in total) confirms the VHE emission positionally consistent with C1*1806-20 and intrinsically extended with a radius of about 0.1deg (see Figure 2). At the assumed distance to C1*1806-20 of 8.7 kpc [4], this extension amounts to about 15 parsecs, which, interestingly, matches the size of the radio nebula G10.0-0.3 [5] believed to be powered by LBV1806-20 (Figure 2). The energy spectral shape of the VHE source is quite similar to that of the GeV Fermi-LAT source nearby (see Figure 3), although the GeV source is labelled as “confused” (i.e. it is potentially compromised by the Galactic diffuse GeV emission, making its location and spectral fluxes systematically uncertain).

The extreme energetics in winds of the stellar objects and the magnetic dissipation energy in SGR1806-20 could easily account for the VHE gamma-ray luminosity (about 10³⁴ erg/s). The power source and parent particle nature (protons and/or electrons) of the VHE source, however, still remain unclear. If the source is indeed associated with SGR 1806−20, the young age of the magnetar (~650 yr [6]) could explain the measured the size of the VHE emission based on how far the accelerated particles can travel in this limited time.

Fig. 2: VLA 1.4 GHz radio image of the radio nebula G10.0−0.3 (Kulkarni et al.(1994)) with red solid VHE gamma-ray contours of the VHE source HESS J1808−204 – as per Fig. 1. Magenta dashed ellipse: 1 sigma location error of 3FGL J1809.2−2016c. Locations of the magnetar SGR 1806−20 (x) and the luminous blue variable LBV1806−20 (+) are indicated.

Fig. 3: Energy fluxes (1 sigma error bars) for the VHe source HESS J1801−204 and multiwavelength comparisons: Fermi-LAT GeV emission, VLA radio emission for G10.0−0.3, SGR 1806−20 X-ray unpulsed emission (1. Beppo-SAX, 21 Mar. 1999; 2. XMM-Newton, 3 Apr. 2002; 3. XMM-Newton, 6 Oct. 2004; 4. INTEGRAL IBIS, 2006/2007), and X-ray upper limit for LBV1806−20.

References:
[1] K. Hurley et al. An exceptionally bright flare from SGR 1806-20 and the origins of short-duration γ-ray bursts. Nature 434, 1098-1103 (2005)
[2] Wikipedia LBV 1806-20.
[3] H.E.S.S. Collaboration, Astron. Astrophys. (accepted).
[4] J.L. Bibby et al. A downward revision to the distance of the 1806-20 cluster and associated magnetar from Gemini Near-Infrared Spectroscopy, MNRAS (2008) 386, L23-L27
[5] S.R. Kulkarni et al. The radio nebula of the soft γ-ray repeater 1806-20. Nature (1994) 368, 129-131.
[6] S.P. Tendulkar et al. Proper Motions and Origins of SGR 1806-20 and SGR 1900+14. Astrophys. J. (2012) 761, 76.