The exceptionally long follow-up of the X-ray afterglow of GRB 130427: what it means for GRB physics.

The Swift satellite, part of which was built at the Mullard Space Science Laboratory, detected the remarkable Gamma-ray Burst (GRB) 130427A about 3 years ago. This burst has the highest fluence (energy divided by surface) of the over 1000 events detected by Swift and indeed by any space observatory for 30 years.

The reason for this record is the very rare combination of very high energy emitted and of being relatively close to us. GRB 130427A produced about 1054 erg, which corresponds to 3 quadrillion times the energy our Sun emits every day, and it was comparatively close, being “only” 4 billions light years away. GRBs are the most luminous explosions in the Universe. Yet, events as energetic as GRB 130427A are rare; they are usually detected only when watching very large volumes of the Cosmos and they are thus tens of billions of light-years from us. Closer to the Earth, we typically see only relatively low luminosity bursts, which are more common.

After the very bright initial emission of gamma-rays, which usually lasts up to a few hundreds seconds, GRBs normally give out the so called “afterglow emission”. The afterglow is a long-lived, slowly decaying emission detected throughout the electromagnetic spectrum, from radio to optical to X-ray. GRB 130427A also had an afterglow and, not surprisingly, was among the brightest Swift ever observed. Studying the afterglow is of utmost importance, because it enables us to constrain the emission mechanisms in this phase, the physical properties of the GRB, such as the geometry of the explosion and the energy of the ejecta, and the characteristics of the environment where the GRB occurred. As expected, GRB 130427A and its afterglow have been the subject of a large body of scientific literature. Such studies, however, relied on observations taken within ~100 days after the event. GRB 130427A was so bright and close that we thought we might still be able to observe it over a much longer duration; and such extended observations could then be used to confirm or rule out models proposed to explain the early data.

We decided to take this once-in-a-lifetime opportunity to observe this burst in the X-ray band, requesting observations with the advanced space observatory XMM-Newton up until late 2015, i.e. ~1000 days after the GRB had occurred. This was the first time such late time X-ray observations of a GRB afterglow had been performed. In addition to XMM data, we analysed data produced by Chandra, the other leading X-ray observatory, which was built by NASA. By combining Swift, Chandra and XMM observations, we built the X-ray light-curve shown below. The X-ray flux F decays as a power-law with time t, i.e. F ~t-a, in an interrupted fashion over the whole temporal baseline.

GRB130427A_new_new_new_new

Figure 1 The X-ray afterglow light-curve of GRB 130427A extended up to 1000 days after the trigger. The top panel shows the Swift, XMM and Chandra observations; the gray line is the best-fit power-law model. The bottom panel shows how much the data points “deviate” from the best fit. (De Pasquale et al. 2016 MNRAS submitted).

We then moved to test several models put forward for GRB 130427A. These models can be broadly split into two classes.

In one class, the GRB occurred in an environment where the density of medium decreases rapidly with radius. This setting is common for massive stars that produce very strong “stellar winds”, which are analogous to the wind produced by the Sun but much more powerful. These winds carve a so-called “stellar wind bubble” in the pre-existing material around the massive star.

The stellar wind bubble cannot, however, extend indefinitely; it will have an edge, beyond which the density of the material is roughly constant. Analysis of the afterglow emission can reveal when the ejecta transition from a stellar wind region into the constant density region. However, our extended X-ray afterglow light-curve shows no signature of such a transition for a very long time. We derived that, if GRB 130427A happened in a stellar wind region, its radius must exceed ~100 parsecs. According to theory, such a large size would be possible only if the density of the pre-existing medium were as low as as ~10-4 atoms/cm3, and the wind encountered almost no resistance. However, the density needed would be orders of magnitude below the typical density of matter around massive stars. Observationally, we do not find stellar wind bubbles of ~300 light years around single massive stars.

The other class of models proposed for GRB 130427A assume that the medium around the GRB explosion had constant density. These models, however, require that the ejecta produced by the explosion have a lot of energy, to add to the already vast amount of energy radiated in gamma-ray by this event. To reduce the problem, these models assume that GRB 130427A explosion was not spherical but “beamed” into two jets, and the observer was looking along the axis of one jet. If the emission is jet-like, then the energy associated to the event can be much lower than that calculated assuming a spherical emission.

However, jets cause observable changes in the afterglow of GRBs. Since the emitting surface of a jet is smaller than in the case of a sphere, the emission will have to be lower. It is possible to derive that the flux decay must become much steeper from a certain time, called “jet break”. We know that the wider the opening angle of the jet, the later the jet break; we can thus compute the opening angle of the jets by measuring when the jet break occurs.

Our observations show that the X-ray light-curve decay does not get steeper; thus we can derive the minimum opening angle of any jet and, as consequence, the lower limit on the energy produced by GRB 130427A. We find that we cannot reduce the energy produced by this GRB to less than ~1053 erg. This is a very high value, which would suggest that a black hole must be involved. Several variants of the constant density model have been proposed, but they all require a similar amount of energy. This requirement leads to another problem with this class of models: to achieve such a high energy, the ejecta need to move faster than 99.99997% the speed of light. Even for extreme events like GRBs, this may difficult to attain.

The exceptional GRB 130427A has enabled us to follow up the X-ray afterglow of a GRB for an unprecedented amount of time. Our study has shown that several models presented so far have difficulty in explaining observations, requiring far-fetched values for the physical parameters involved. More theoretical work is required to address the problems highlighted and devise more satisfying explanations for the long-term behaviour of GRBs.

The results of this analysis have been incorporated in a paper, of which I am first author, that is currently under review by Monthly Notices of the Astronomical Society. The same paper can be accessed free a http://arxiv.org/abs/1602.04158.

You can find further information on GRB 130427A, Swift and GRBs in two previous posts on this blog.

Dr. Massimiliano De Pasquale, Research Assistant.

 

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