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Swift/BAT 1644+57: A Flash Upon That Inward Eye

21/06/2011

It could be a secure job, a solitary wondering star. (Lonely work, if you can get it.) But sometimes, this solitary complacency can lead to all sorts of trouble. Staying clear of black holes would be my first line of advise. But, as would be expected of huge balls of plasma, these loss sentient dwellers have no way to heed this warning. To pass across these inward eyes, would be as deadly as it is foolish. But an amazing spectacle to witness nonetheless.

It therefore fills me with some degree of pride that a research team led by my old cosmology lecturer, Dr. Andrew Levan (of the University of Warwick), has confirmed one of the brightest cosmic flashes ever recorded, believed to be the result of a massive star wondering too close to a massive black hole, itself lurking at the centre of a distant galaxy.

Fig.1: What University of Warwick researchers think the aftermath of a large star being consumed by a black hole may have looked like. The event blasted jets of energy from the black hole, one of which pointed directly at our own galaxy, allowing us to be aware of this event "SW1644+57". (Credit: University of Warwick / Mark A. Garlick).

These chasms to which I refer to are, of course, black holes. Infamous cosmic objects to even the uninitiated amongst us. So what caused this intense and rather dramatic beam of energy? A massive black hole. One which has appeared to have ripped apart a star that wandered just a little bit too close for comfort.

The physics of such energy “beams” is taken straight from the accretion of gases onto very massive and very dense compact stellar objects. Accretion discs are a ubiquitous phenomenon in astrophysics; active galactic nuclei, proto-planetary discs and gamma ray bursts all involve accretion discs. These discs very often give rise to jets coming from the vicinity of the central object. Jets are an efficient way for the star-disc system to shed angular momentum without losing too much mass. The most spectacular accretion discs found in nature are those of active galactic nuclei and of quasars, which are believed to be massive black holes at the center of galaxies. As matter spirals into a black hole, the intense gravitational gradient gives rise to intense frictional heating; the accretion disc of a black hole is hot enough to emit X-rays just outside of the event horizon.

While gas accretion onto some massive black holes (MBHs) at the centres of galaxies actively powers luminous emission, the vast majority of MBHs are considered dormant. Sporadically, a star passing too near a MBH is torn to shreds by the colossal gravitational forces near a MBH, leading to what is known as a bright panchromatic tidal disruption flare (a bright multi-wavelength energy burst caused by the wondering star being accreted onto a compact object over a very short period of time.)

The team at Warwick have concluded that the only plausible explanation that fits the size and intensity of the observed GRB event, is that of the aforementioned model: a massive black hole at the very centre of its host galaxy  that has pulled in a large star and ripped it limb from limb.

The high energy X-rays and gamma-rays associated with this jet persisted at an extremely bright level for weeks after the event, with bright flares arising when further chunks of the star fell into the black hole. The extreme brightness of this event comes from the fact that it illuminated only a small fraction of the sky, pointing a jet of light towards the Milky Way, which was detected at Earth 3.8 billion years after the star was ripped apart.

Despite the power of this the cataclysmic event, we still only happen to see this event because our solar system happened to be looking right down the barrel of this jet of energy. The new research paper clearly establishes that the source of this event – known as SW1644+57 – is right at the heart of far away galaxy, 3.8 billion light years away, at a spot which would be in the constellation Draco.

A flash upon that inward eye. From the bliss of solitude; and then my heart with pleasure fills, to dance amongst the daffodils.

Journal References:

  • Levan, A.J. et al.  (2011) An Extremely Luminous Panchromatic Outburst From The Nucleus Of A Distant Galaxy. DOI: arXiv:1104.3257v1.
  • Burrows, D.N.; Page, K.L. et al. (2011) Discovery Of The Onset Of Rapid Accretion By A Dormant Black Hole. DOI: 2011arXiv1104.4787B.
Original Telegram:
  • The Astronomer’s Telegram, #3242
At 12:57 UT on 2011-Mar-28 Swift/BAT triggered on a newly discovered transient source, initially thought to be a new GRB, GRB110328A (Cummings et al., GCN #11823). A second BAT trigger on this source at 13:40UT on 2011-Mar-28, cast doubt on the GRB nature of this source (Barthelmy et al., GCN #11824), and it is likely that the triggering source is a hard galactic X-ray transient, which was named Swift J164449.3+573451.
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