Gamma-ray burst buried in dust
Gamma-ray burst buried in dust (artist’s impression)
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ALMA probes environment around dark gamma-ray bursts
Observations from the Atacama Large
Millimeter/submillimeter Array (ALMA) have for the first time directly
mapped out the molecular gas and dust in the host galaxies of gamma-ray
bursts (GRBs) — the biggest explosions in the Universe. In a complete
surprise, less gas was observed than expected, and correspondingly much
more dust, making some GRBs appear as “dark GRBs”. This work will appear
in the journal Nature on 12 June 2014 and is the first ALMA science
result on GRBs to appear. It shows ALMA’s potential to help us to better
understand these objects.
Gamma-ray bursts
(GRBs) are intense bursts of extremely high energy observed in distant
galaxies — the brightest explosive phenomenon in the Universe. Bursts
that last more than a couple of seconds are known as long-duration
gamma-ray bursts (LGRBs) [1] and are associated with supernova explosions — powerful detonations at the ends of the lives of massive stars.
In just a matter of seconds, a typical burst releases as much energy
as the Sun will in its entire ten-billion-year lifetime. The explosion
itself is often followed by a slowly fading emission, known as an
afterglow, which is thought to be created by collisions between the
ejected material and the surrounding gas.
However, some gamma-ray bursts mysteriously seem to have no afterglow
— they are referred to as dark bursts. One possible explanation is that
clouds of dust absorb the afterglow radiation.
In recent years, scientists have been working to better understand
how GRBs form by probing their host galaxies. Astronomers expected that
the massive stars that were GRB progenitors would be found in active
star-forming regions in these galaxies, which would be surrounded by a
large amount of molecular gas — the fuel for star formation. However,
there had been no observational result to back up this theory, leaving a
long-standing mystery.
For the first time, a Japanese team of astronomers led by Bunyo
Hatsukade from the National Astronomical Observatory of Japan, have used
ALMA to detect the radio emission from molecular gas in two dark LGRB
hosts — GRB 020819B and GRB 051022
— at about 4.3 billion and 6.9 billion light-years, respectively.
Although such radio emission had never been detected in the GRB host
galaxies, ALMA made it possible with its unprecedentedly high
sensitivity [2].
Kotaro Kohno, a professor at the University of Tokyo and a member of the research team, said, “We
have been searching for molecular gas in GRB host galaxies for over ten
years using various telescopes around the world. As a result of our
hard work, we finally achieved a remarkable breakthrough using the power
of ALMA. We are very excited with what we have achieved.”
Another remarkable achievement made possible by the high resolution
of ALMA was uncovering the distribution of molecular gas and dust in GRB
host galaxies. Observations of the GRB 020819B revealed a remarkably
dust-rich environment in the outskirts of the host galaxy, whereas
molecular gas was found only around its centre. This is the first time
that such a distribution among GRB host galaxies has been revealed [3].
“We didn’t expect that GRBs would occur in such a dusty
environment with a low ratio of molecular gas to dust. This indicates
that the GRB occurred in an environment quite different from a typical
star-forming region,” says Hatsukade. This suggests that massive
stars that die as GRBs change the environment in their star-forming
region before they explode.
The research team believes that a possible explanation for the high
proportion of dust compared to molecular gas at the GRB site is the
difference in their reactions to ultraviolet radiation. Since the bonds
between atoms which make up molecules are easily broken by ultraviolet
radiation, molecular gas cannot survive in an environment exposed to the
strong ultraviolet radiation produced by the hot, massive stars in its
star-forming region, including the one that would eventually explode as
the observed GRB. Although a similar distribution is also seen in the
case of GRB 051022, this has yet to be confirmed due to the lack of
resolution (as the GRB 051022 host is located further away than the GRB
020819B host). In any case, these ALMA observations support the
hypothesis that it is dust that absorbs the afterglow radiation, causing
the dark gamma-ray bursts.
“The results obtained this time were beyond our expectations. We
need to carry out further observations with other GRB hosts to see if
this could be general environmental conditions of a GRB site. We are
looking forward to future research with the improved capability of ALMA,” says Hatsukade.
Notes
[1] Long-duration gamma-ray bursts
(LGRBs), bursts lasting for over two seconds, account for about 70% of
observed GRBs. Developments over the past decade have recognised a new
class of GRBs with bursts of less than two seconds, the short-duration
GRBs, likely due to merging neutron stars and not associated with
supernovae or hypernovae.
[2] The sensitivity of ALMA in this observation was
about five times better than other similar telescopes. Early scientific
observations with ALMA began with a partial array in 2011 (eso1137).
These observations were done with an array only consisting of 24–27
antennas with separations of up to only 125 metres. The completion of
the last of the 66 antennas (eso1342)
offers great promise of what ALMA may be capable of revealing in the
near future, as the antennas can be arranged in different
configurations, with maximum distances between antennas varying from 150
metres to 16 kilometres.
[3] The proportion of dust mass to molecular gas mass
is about 1% in the interstellar medium in the Milky Way and nearby
star-forming galaxies, but it is ten or more times higher in the region
surrounding GRB 020819B.
More information
The Atacama Large Millimeter/submillimeter
Array (ALMA), an international astronomy facility, is a partnership of
Europe, North America and East Asia in cooperation with the Republic of
Chile. ALMA is funded in Europe by the European Southern Observatory
(ESO), in North America by the U.S. National Science Foundation (NSF) in
cooperation with the National Research Council of Canada (NRC) and the
National Science Council of Taiwan (NSC) and in East Asia by the
National Institutes of Natural Sciences (NINS) of Japan in cooperation
with the Academia Sinica (AS) in Taiwan. ALMA construction and
operations are led on behalf of Europe by ESO, on behalf of North
America by the National Radio Astronomy Observatory (NRAO), which is
managed by Associated Universities, Inc. (AUI) and on behalf of East
Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint
ALMA Observatory (JAO) provides the unified leadership and management
of the construction, commissioning and operation of ALMA.
This research was presented in a paper in Nature (12 June 2014) as an
article titled “Two gamma-ray bursts from dusty regions with little
molecular gas”, by B. Hatsukade et al.
The team is composed of B. Hatsukade (NAOJ, Tokyo, Japan), K. Ohta
(Department of Astronomy, Kyoto University, Kyoto, Japan), A. Endo
(Kavli Institute of NanoScience, TU Delft, The Netherlands), K.
Nakanishi (NAOJ; JAO, Santiago, Chile; The Graduate University for
Advanced Studies (Sokendai), Tokyo, Japan), Y. Tamura (Institute of
Astronomy [IoA], University of Tokyo, Japan ), T. Hashimoto (NAOJ) and
K. Kohno (IoA; Research Centre for the Early Universe, University of
Tokyo, Japan).
ESO is the foremost intergovernmental astronomy organisation in
Europe and the world’s most productive ground-based astronomical
observatory by far. It is supported by 15 countries: Austria, Belgium,
Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy,
the Netherlands, Portugal, Spain, Sweden, Switzerland and the United
Kingdom. ESO carries out an ambitious programme focused on the design,
construction and operation of powerful ground-based observing facilities
enabling astronomers to make important scientific discoveries. ESO also
plays a leading role in promoting and organising cooperation in
astronomical research. ESO operates three unique world-class observing
sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO
operates the Very Large Telescope, the world’s most advanced
visible-light astronomical observatory and two survey telescopes. VISTA
works in the infrared and is the world’s largest survey telescope and
the VLT Survey Telescope is the largest telescope designed to
exclusively survey the skies in visible light. ESO is the European
partner of a revolutionary astronomical telescope ALMA, the largest
astronomical project in existence. ESO is currently planning the
39-metre European Extremely Large optical/near-infrared Telescope, the
E-ELT, which will become “the world’s biggest eye on the sky”.
Links
- Research paper
- More about ALMA
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- ALMA brochure
- The movie ALMA — In Search of our Cosmic Origins
- The ALMA Photo Book In Search of our Cosmic Origins – The Construction of the Atacama Large Millimeter/submillimeter Array
- More press releases with ALMA
Contacts
Bunyo HatsukadeNational Astronomical Observatory of Japan
Japan
Tel: +81-422-34-3900 (ext. 3173)
Email: bunyo.hatsukade@nao.ac.jp
Masaaki Hiramatsu
National Astronomical Observatory of Japan
Japan
Tel: +81-422-34-3630
Email: hiramatsu.masaaki@nao.ac.jp
Lars Lindberg Christensen
ESO education and Public Outreach Department
Garching bei München, Germany
Tel: +49 89 3200 6761
Cell: +49 173 3872 621
Email: lars@eso.org
Source: ESO
