The Swift UVOT team at MSSL find surprisingly bright UV emission from the first ever visible counterpart to a gravitational wave event. Here is their story… from Paul Kuin
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.
One of the main challenges in modern cosmology is to understand how the very low-density matter between galaxies (known as the inter-galactic medium, or IGM) came to be hot and ionized today, reaching temperatures of up to 10 million degrees. It hasn’t always been this way – after the Big Bang the Universe expanded and cooled, eventually reaching temperatures low enough for much of the Hydrogen and Helium plasma within it to combine and form a neutral atoms in a process known as recombination around 378,000 years after the Big Bang. After this, the expansion and cooling of the Universe continued for hundreds of millions of years, leaving it in a dark and increasingly cold state – an era cosmologists refer to as the ‘Dark Ages’.
In our first science blog post, we discussed Gamma-Ray Bursts (GRBs) and the research carried out on these extreme cosmic explosions here at MSSL, thanks to our involvement in the Swift satellite mission. I am Dr Massimiliano De Pasquale, a Research Associate at the Mullard Space Science Laboratory. I would like to talk to you about one of these spectacular cosmic events recently studied by Swift. On April 27th, 2013, Swift detected an exceptionally bright burst, ”GRB130427A”, named after the date it occurred. The Burst Alert Telescope (BAT) instrument onboard Swift detected a fluence (energy units divided by surface units) of 6.5e-4 erg cm-2 for this source. Of the 700 GRBs discovered by Swift so far, this is the highest fluence detected.
Swift is not the only facility which detected GRB130427. Other space borne satellites, such as INTEGRAL, Konus-Wind, Fermi, also observed this spectacular event. The detectors onboard these satellites have a wide ‘energy range’. Swift BAT can detect photons only if their energy is between 15 and 350 keV, while Konus-Wind instruments detect photons between 20 and 20,000 keV, and Fermi instruments can detect photons of energy between 8 and 300,000,000 keV (300 GeV). Thanks to measurements provided by these missions, we know the electromagnetic emission of GRB130427A in fantastic detail. GRB130427A gamma-ray fluence is 0.0027 erg cm-2 (20-10000 keV; Konus-Wind measurement); since observations of these sources begun in the late 60’s there has only been one GRB, GRB 840304, with confirmed higher fluence. Fermi detected photons coming from GRB130427A for which the energy is in the millions of keV. In particular, it found the highest energy photon ever detected from a GRB, 95 million keV. The energy of such a photon is 40 billion times higher than that of the typical photons of sunlight. Furthermore, these extremely energetic photons were detected for almost 1 day, while they are usually observed only for 1 hour or less in the other few GRBs that present such ultra-energetic emission.
Within 2 minutes of the trigger, XRT and UVOT were already observing GRB130427 in the X-ray and optical bands. At the same time, Swift beamed the position of the source to telescopes and other ground facilities, which could repoint and start observing this GRB.
XRT detected one of the brightest X-ray sources ever found at the position of the GRB, the so called ‘afterglow’. UVOT also detected a bright source, the optical counterpart of the afterglow, shining at 11th magnitude (at this magnitude it could have been seen even by amateur telescopes, if pointed at the right position within the first ~200s). At the same time, several ground radio, infrared and optical observatories detected the source and started to take measurements in matter of minutes. We thus have fantastic measurements of the electromagnetic emission of GRB130427A, ranging from the radio (10-8 keV) to very high energy emission (108 keV): 16 decades of energy. And while the declining optical and X-ray afterglow emissions of typical GRBs are usually visible for a few days, sometimes a couple of weeks, after the trigger before fading below the threshold sensitivity of the instruments, for GRB130427, the very bright X-ray and optical afterglows are still detected 6 weeks after the trigger.
About a day after the trigger, the RAPTOR team reported that the three of their wide-field telescopes, which monitor the full sky, detected an optical source at the position of the GRB roughly at the beginning of the gamma-ray emission. Such source brightened up to magnitude 7.4, enough to be visible with common binoculars. Then it faded to magnitude 10-11, when it was detected by UVOT.
Spectral analysis of the optical emission showed the signature of chemical elements at certain wavelengths (see also the post by Ignacio Ferreras). Such signatures, called “absorption lines” and due to chemical elements absorbing light, enabled us to determine the distance of the event. In fact, the wavelengths of these signatures are not the same as those measured on Earth, but they are larger. Since the red colour is associated with large wavelengths, this phenomenon is called “redshift“. Its origin is the expansion of the Universe: the wavelengths of the electromagnetic radiation is “stretched”. The amount of redshift is linked to the distance of the emitter and the age when the emission was first produced.
The absorption lines in the spectra of GRB130427 were at wavelengths 1.34 times larger than those measured on Earth, which corresponds to a redshift of 0.34. From such measurement, we know that GRB130427 occurred 4 billion years ago. Knowing the distance, we also can compute how many cm2 there on the surface of the sphere centered on this GRB, and we also know that each one of these cm2 was crossed by 0.0027 erg of energy. As a result, we can calculate the total amount of gamma-ray energy produced by the GRB, which sums up to the astonishing value of 1054 erg. GRB130427A dissipated in a few hundreds seconds 1000 times the energy our Sun will produce within its 10-billion years lifespan.
We can also determine what brightness it would have had if placed at the same distance of the brightest star in the sky, Sirius. It would have shone with a magnitude of -36, about 5000 times brighter than the Sun.
The average GRB produces “only” 1052, 1053 erg of energy, and is located at redshift 2-3. Events that unleash 1054 erg are rare, and their occurrence is even rarer at such “low redshift” from us. It has been calculated that it might take several decades before another GRB like 130427A takes place again.
GRB130427A is unprecedented for another reason. So far, GRBs at low redshift were the most common, ‘weak’ events which produce only ~1051 erg or less. When such bursts are observed for a long time in the optical, the signature of a supernova is detected. Such a supernova flags a link between GRBs and the deaths of very massive stars; however, such a link could be safely established only for these weak events. GRB130427 is a 1054 erg behemoth; nobody knew for sure whether a supernova was associated with such events. If no supernova had been found, the current paradigm of the association between GRBs and massive stars would have been deeply shaken. In recent weeks, very big telescopes all over the world took measurements of the optical emission of the afterglow, waiting impatiently to spot the supernova emission. Eventually, the Spanish 10.4-m GTC telescope detected the signature of the supernova in the light of GRB130427A on the night of May 13th.
GRB130427 represents an unparalleled opportunity to deeply study one of Nature’s biggest explosions so close to us. It is a rare combination of very high energy output and closeness. Such an event will be actively studied for years to come and will produce a plethora of papers in the most important scientific journals worldwide.
At MSSL we are greatly contributing to this research and efforts, thanks to the analysis of optical data provided by UVOT as well as to the theoretical interpretation of the observations, and we shall be co-authors of forthcoming scientific works on this outstanding GRB.
Welcome to our first science blog post. I am Dr Samantha Oates, a postdoctoral researcher at MSSL, investigating what we can learn about Gamma-ray Bursts (GRBs) from their X-ray and optical/UV afterglow emission. As our first science post, I shall introduce you to these enigmatic events, why and how we study them, and what we do at MSSL as part of the NASA Swift team.
What are Gamma-ray Bursts?
GRBs are the most extreme explosions in the known Universe. These events release as much as 1055 erg within a few seconds in the form of gamma-rays. This is significantly more energy than the Sun contains and releases over its entire lifetime (~11 billion years). The duration of this prompt gamma-ray emission roughly divides GRBs into long (>2 seconds) and short (<2 seconds) classes. The long GRBs are associated with the deaths of massive stars, while the short GRBs are associated with the merger of two compact objects, such as Neutron Stars or a Neutron star and a Black Hole (see Fig 1). GRBs appear to occur randomly throughout time and space. The furthest detected so far is at redshift 9.4, approximately 13.14 billion years ago (Cucchiara, et al., 2011, ApJ, 736, 7), when the Universe was about 5% of its present age.
Why Study GRBs?
During a GRB, relativistic jets, moving at 99.999% of the speed of light, are produced and a black hole is formed. Both these features are extreme situations that cannot be reproduced on Earth. GRBs thus enable us to examine processes that cannot be studied under laboratory conditions and enable us to test our understanding of physics. GRBs can also help us to understand the formation and structure of matter in the Universe. Because long GRBs signify the deaths of massive short-lived stars, they are thought to trace where stars form in the Universe. And as they can be observed over almost all cosmic time, they can be used to probe star formation, from the present through to the earliest times, even, in principle, before the first galaxies formed. Being able to detect them over cosmic history means we can also use them as standard candles to probe the expansion history of the Universe. In particular, long GRBs can be used to extend the Hubble diagram (distance versus redshift) beyond the limit of Type Ia supernovae (the Noble prize in Physics was awarded in 2011 for research using the Hubble Diagram and Type Ia supernovae). Another exciting prospect is that short GRBs could be the first sources for which gravitational waves are detected and confirmed.
What do we use to observe GRBs?
The unpredictability and the rapid fading nature of GRBs means that specialized instruments are necessary to detect and observe these spectacular events. Here at MSSL, we are involved in NASA’s Swift satellite, a GRB dedicated mission, designed to catch GRBs on the fly. Swift’s wide field detector (the Burst Alert Telescope; BAT) views ~1/6th of the sky for the prompt gamma-ray signal. Upon detection, Swift stops looking at its present target, and within a few tens of seconds has slewed to point its two narrow field instruments at the location of the GRB. These instruments, the X-ray Telescope (XRT) and the Ultra-violet Optical Telescope (UVOT), then observe the accompanying afterglow at X-ray and optical/UV wavelengths. The X-ray and optical/UV emission may be observed typically for a few hours to days after the prompt gamma-rays, and, in a small number of cases, up to several weeks. Swift detects on average 2 bursts per week. The rest of the time Swift performs a set of pre-planned observations, or follows targets of opportunity requested by the astronomical community.
What is MSSL’s involvement in Swift?
The UK has had direct involvement with the design and construction of the both the XRT, at the University of Leicester, and the UVOT here at MSSL. MSSL therefore has a responsibility to help calibrate the UVOT and to provide mission support. Part of that support involves analyzing the UVOT data following a GRB and also becoming “Burst Advocates“. A Burst Advocate is required to report on observations of a GRB made by Swift and other telescopes at the Swift daily planning meeting so that the operations team know whether to continue observing. At MSSL, our responsibility is to analyze UVOT data for GRBs occurring every other week and for one week in ten to be on-call as Burst Advocates.
How do we Chase GRBs?
Since GRBs can occur at anytime day or night and fade very rapidly, there must always be always be someone on-call so that the community can be informed, within minutes, on the location and brightness of the GRB in the gamma-ray, X-ray and optical/UV bands. It is the Burst Advocates responsibility to make sure that all the necessary information is promptly released, enabling the astronomical community to follow the event with other ground-based telescopes. For one week in every ten MSSL are Burst Advocates, this means someone has to be available to respond to Swift alerts, sent by email and text message, at any time, day or night, during these 7 days. While each GRB is very exciting and has the potential to help us understand different aspects of how these events work, I do wish they would wake me up less often at 5 am when I am on duty :-D
Swift has detected over 700 GRBs, since launch in November 2004, and has so far been a very successful and productive mission both for the GRB community and the astronomical community in general. At MSSL, we hope there are many more productive years for Swift to come.