Gamma Ray Bursts and Magnetars

Gamma Ray Bursts (GRBs) are the most violent events in the Universe. Originally discovered as bright flashes in Gamma-Rays, their emission has been later detected also at other wavelengths. They can be broadly divided into two populations: Long ones, where the prompt emission last more that about 5 seconds, and Short ones where the prompt emission last less that about 1-2 seconds. We know today that the Long GRBs are associated to the death of massive stars, while the Short ones are more likely due to merger events of old compact objects (Neutron Stars and/or Black Holes) [Ref 1].
There is a generals consensus today that LongGRBs are just the high tail end in the distribution of explosive events associated to the death of stars. An high-energy counterpart of regular Core-Collapse Supernovae. The violence and brief duration of such events immediately suggests that their engine is likely a stellar mass compact object, and this lead to the so called Collapsar Model: the core of the star collapses to a Black Hole and the following mass accretion powers a relativistic collimated outflows. A similar scenario can apply also to Short ones where the Black Hole originated from the merger of two Neutron Stars.  In the same years an alternative model known as “millisecond magnetar model” based on a rapidly rotating Neutron Star was also proposed, but it received much less attention [Ref 2]. The recent detection of “late time activity” in the form of flares of plateaus in the light curve of GRBs several hundreds or thousands of second after the prompt phase has however changed this situation [Ref 3]. A Neutron Star is a natural candidate for a long living engine, while once mass accretion stops a BH is virtually dead. Moreover we know that the formation of a proto-NS during core collapse is a key ingredient for a successful Supernova explosion. A NS engine, fits better into a unifying picture of stellar death, with less magnetized and slowly rotating NS formed during regular core-collapse SNe and more magnetized and rapidly rotating objects associated to GRBs. Finally there seems to be an upper limit to he energy of a LongGRBs, less than 10^53 erg [Ref 4]. This agree with the maximum rotational energy that can be stored in a NS, while for BHs there is no reason for such threshold.
Immediately following the collapse of the Iron core of a massive star, a proto-neutron star is formed. Initially very hot, it will cool down via neutrino emission on a timescale of few tens of seconds, driving a baryon loaded wind from its surface. This wind will blow a bubble inside the supernova shock as the latter propagates through the outer layer of the star. It might happen that the core is initially rapidly rotating, and endowed with a strong magnetic field. As a consequence, the proto-neutron star might be born as a fast spinning object, with rotation periods around a millisecond, and strongly magnetized (B ~ 10^15 G). The combination of fast rotation and a strong magnetic field, can increase hundredfolds the power of the neutrino driven wind, to values capable of blowing the entire stat. Under these conditions a relativistic outflow can be produced, capable of explaining typical GRBs [Ref 5, Ref6].  One the NS has cooled and the mass loss rate due to neutrinos has become negligible, the outflow becomes radiative inefficient. Further deposition of energy in the surrounding circumstellar medium is revealed in the from of plateaus in the late light-curve.
While it is still strongly debated if the NSs can be a viable engine for GRBs, and what are the possible signatures that distinguish them for BH jet, if indeed GRB are powered by rapidly rotating strongly magnetized NSs, this have important consequences for the properties of the original NS. Its EoS must be such to allow rapid rotations, and dynamo processes are likely to be active in the first few hundreds milliseconds (a seed field to strong could brake the core before collapse). An analysis of the late phases can also help us shed light on the properties of these NSs just after birth. Given the cosmological origin of GRB, a comparison with Galactic NS population could help us to understand of there is a cosmological dependence of the properties of stellar deaths, which might be reasonably associated with analogous properties of the parent stellar population (metallicity?).

Ref 1 – S.E. Woosley and J.S. Bloomb “The Supernova–Gamma-Ray Burst Connection” Annual Review of Astronomy and Astrophysics, 2006, Vol. 44: 507-556
Ref 2 – J.C. Wheeler et al. 2000, “Asymmetric Supernovae, Pulsars,Magnetars, and Gamma-Ray Bursts”, ApJ, 2000, 537, 810
Ref 3 – N. Lyons et al. “Can X-ray emission powered by a spinning-downmagnetar explain some gamma-ray burst light-curve features?”, MNRAS, 2010,402, 705
Ref 4 – P. Mazzali et al. “An upper limit to the energy of gamma-ray bursts indicates that GRBs/SNe are powered by magnetars”, MNRAS, 2014, 443, 67
Ref 5 – B.D. Metzger et al. “The protomagnetar model for gamma-ray bursts”, MNRAS, 2011, 413, 2031
Ref 6 – N. Bucciantini et al. “Short gamma-ray bursts with extended emission from magnetar birth: jet formation and collimation”, MNRAS, 2013, 419, 1537

 Spin-down Luminosity (dotted line - right axis), and wind magnetization (solid line - left axis) for a proto-NS with spin period of 1.5 ms and magnetic field 2x10^15 G. The various phases of the wind are indicated as a function of time after bounce, together with the GRB window.

Spin-down Luminosity (dotted line – right axis), and wind magnetization (solid line – left axis)
for a proto-NS with spin period of 1.5 ms and magnetic field 2E15 G. The various phases of the
wind are indicated as a function of time after bounce, together with the GRB window.

 Examples of late-activity in the afterglow of GRBs (blue line). From Ref 3

Examples of late-activity in the afterglow of GRBs (blue line). From Ref 3.

Artist’s impression of a supernova and associated gamma-ray burst driven by rapidly spinning magnetar, a neutron star with a very strong magnetic field. Credit: ESO - http://www.eso.org/public/images/eso1527a/

Artist’s impression of a supernova and associated gamma-ray burst driven by rapidly spinning magnetar, a neutron star with a very strong magnetic field.
Credit: ESO – http://www.eso.org/public/images/eso1527a/


Author: Niccolo’ Bucciantini (INAF-Arcetri Observatory)

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