Zooming in on colliding neutron stars with e-MERLIN and EVN
The radio telescopes of the UK’s National Radio Astronomy Facility, e-MERLIN, have been directed low in the southern sky in recent weeks. Their target the aftermath of a collision between two neutron stars 130 million light years away.
The collision between these two super-dense remnants of exploded stars produced ripples in space-time which were detected by the LIGO gravitational wave observatory. Remarkably, other telescopes working across the spectrum from radio to gamma-rays were also able to detect the collision and its aftermath – the first time any cosmic event has been seen in both gravitational waves and by its electromagnetic radiation.
The power of e-MERLIN, working alone and in combination with the wider European VLBI Network of telescopes (EVN), is the ability to zoom in on the fine detail of these gravitational wave events and help pinpoint their precise locations. Observations with e-MERLIN began on August 23, just 6 days after the gravitational wave detection.
As Rob Beswick, Head of e-MERLIN Science Support at the University of Manchester’s Jodrell Bank Centre for Astrophysics, explains, the observations of this particular event are challenging:
“We are reporting today on 22 epochs of observation extending over the period from 6 to 36 days after the collision. This particular source is very low in the southern sky as seen from the UK, which means it only rises above the horizon for a few hours a day, limiting the time we can spend on it and hence making it harder to detect its very faint radio emissions.
“Even so, we are tantalisingly close to obtaining a detection. At the moment, our 1-sigma noise level is about the same as the faint transient signal detected by the Very Large Array 16 days after the outburst. Nevertheless, this places very useful stringent limits on the radio emissions over this period.
“We are continuing to obtain e-MERLIN and EVN observations. In the near future, we will also be able to include the giant Lovell Telescope in our array, greatly increasing its power. As the source evolves and potentially brightens in the coming weeks, this will make a detection far more likely.”
Simon Garrington, Director of e-MERLIN and Associate Director of Jodrell Bank Centre for Astrophysics, added,
“This discovery is hugely exciting and will be the first of many electromagnetic counterparts to gravitational wave sources in the coming years.
“e-MERLIN is a powerful instrument capable of providing rapid and extremely sensitive radio observations of these events with high magnification imaging that rivals that of the Hubble Space Telescope. The detection of radio emissions from these gravitational wave sources is right at the limit of our current capabilities; however planned enhancements to e-MERLIN over the next few years will make future follow-ups routine.
“e-MERLIN and EVN will be crucial tools to zoom in at radio wavelengths, enabling precise (millisecond) localisation of GW events in space. Detailed localisation is likely to prove fundamental to our understanding of the energetic processes at the heart of these titanic events.”
Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovae. As these two neutron stars spiralled together, they emitted gravitational waves that were detectable for about 100 seconds; when they collided, a flash of light in the form of gamma rays was emitted and seen on Earth about two seconds after the gravitational waves. In the days and weeks following the collision, other forms of light, or electromagnetic radiation — including X-ray, ultraviolet, optical, infrared, and radio waves — were detected.
The discovery was made using the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO); the Europe-based Virgo detector; and followed up by some 70 ground- and space-based observatories.
The initial detection of the gravitational signal, named GW170817, was first made on Aug. 17 at 8:41 a.m. Eastern Daylight Time; by the two identical LIGO detectors, located in Hanford, Washington, and Livingston, Louisiana. Virgo, situated near Pisa, Italy, also recovered a signal that allowed scientists to precisely triangulate the position of the source in a relatively small patch in the southern sky.
At nearly the same time that this detection was made the Gamma-ray Burst Monitor on NASA’s Fermi space telescope had detected a burst of gamma rays. LIGO-Virgo analysis software put the two signals together and saw it was highly unlikely to be a chance coincidence, and another automated LIGO analysis indicated that there was a coincident gravitational wave signal in the other LIGO detector.
Rapid gravitational-wave detection by the LIGO-Virgo team, coupled with Fermi’s gamma-ray detection, enabled the launch of follow-up by telescopes around the world. In this follow up an optical transient was identified in the galaxy known as NGC 4993 – which is located over 130 million lights years away from Earth, and in the same region of the southern sky that the gravitational wave detection was made. While at the other end of the electromagnetic spectrum, the first detection of a faint, transient radio emission was reported from the same galaxy by the Jansky Very Large Array (VLA) on Sep. 2, 16 days after the initial detection of the gravitational signal.
The LIGO-Virgo results are published today in the journal Physical Review Letters; additional papers from the LIGO and Virgo collaborations and the astronomical community have been either submitted or accepted for publication in various journals.
e-MERLIN is the UK’s National Radio Astronomy Facility and is operated by the University of Manchester on behalf of the UK’s Science and Technology Facility (STFC). e-MERLIN is an array of seven radio telescopes across the UK, connected to a powerful central correlator at Jodrell Bank Observatory (JBO), The University of Manchester. It is operated as a dedicated radio interferometer producing high-resolution radio images. With a maximum baseline length of 220 km, e-MERLIN provides a unique capability for microJansky sensitivity radio imaging, spectroscopy, polarisation and astrometry. The e-MERLIN/VLBI National Facility provides the UK contribution to the European VLBI Network (EVN), which links telescopes across Europe and China for observations at milliarcsecond resolution.
European VLBI Network telescopes
The telescopes involved in the observation of the neutron stars were part of the EVN and included: Effelsberg Radio Telescope (Max-Planck Institute for Radio Astronomy, Germany), Hartebeesthoek Radio Astronomy Observatory (National Research Foundation, South Africa), Jodrell Bank Observatory (University of Manchester, UK), Medicina Radio Observatory (National Institute for Astrophysics, Italy), Onsala Space Observatory (Chalmers University of Technology, Sweden), Noto Radioastronomical Station (National Institute for Astrophysics, Italy), Toruń Centre for Astronomy (Nicolaus Copernicus University, Poland), Ventspils International Radio Astronomy Centre (Latvian Academy of Sciences, Latvia), Westerbork Synthesis Radio Telescope (ASTRON, the Netherlands), and Yebes Observatory (National Geographic Institute, Spain).
More about e-MERLIN, VLBI, the European VLBI Network and JIVE
e-MERLIN and VLBI are radio interferometers which use multiple large radio telescopes distributed over great distances to observe the same region of sky simultaneously. Data from each telescope is sent to a central "correlator" to produce images with higher resolution than the most powerful optical telescopes.
The e-MERLIN national facility (www.e-merlin.ac.uk) is an interferometric array of 7 large telescopes (including the 76-m Lovell Telescope) spanning the southern half of the UK which are remotely connected to a central facility at Jodrell Bank Observatory via a dedicated high speed fibre network. With a maximum baseline length of 220 km, e-MERLIN provides a unique capability for microJansky sensitivity radio imaging, spectroscopy, polarisation and astrometry at 0.01-0.15-arcsec resolution in broad frequency bands. The e-MERLIN/VLBI National Facility provides the UK contribution to the European VLBI Network. The e-MERLIN National facility is operated by the University of Manchester on behalf of the UK’s Science and Technology Facility Council (STFC; www.stfc.ac.uk)
The European VLBI Network (EVN; www.evlbi.org) is an interferometric array of radio telescopes spread throughout Europe, Asia, South Africa and the Americas that conducts unique, high-resolution, radio astronomical observations of cosmic radio sources. Established in 1980, the EVN has grown into the most sensitive VLBI array in the world, including over 20 individual telescopes, among them some of the world's largest and most sensitive radio telescopes. The EVN is administered by the European Consortium for VLBI, which includes a total of 15 institutes, including the Joint Institute for VLBI ERIC (JIVE).
The Joint Institute for VLBI ERIC (JIVE; www.jive.eu) has as its primary mission to operate and develop the EVN data processor, a powerful supercomputer that combines the signals from radio telescopes located across the planet. Founded in 1993, JIVE is since 2015 a European Research Infrastructure Consortium (ERIC) with six member countries: France, Latvia, the Netherlands, United Kingdom, Spain and Sweden; additional support is received from partner institutes in China, Germany, Italy and South Africa.
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