Going Big with LOFAR



Left: A map of Europe, showing the LOFAR stations currently online, including 40 stations in the center of the array in the Netherlands, and 8 international stations. Right: one of the international stations, showing the two types of arrays for detecting radio waves at different frequencies.


It can’t be denied that astronomers like to do things big. From putting the biggest telescopes on the tallest mountains to finding the biggest black holes and the largest structures in the universe, we are always pushing toward the next big thing. So it shouldn’t be a surprise that in our collective passion for all things huge we are now advancing on a new frontier– viewing the universe at the biggest (or rather, longest) wavelengths.


Today, we highlight three recent results from LOFAR (the Low Frequency Array for Radio Astronomy). LOFAR observes light with wavelengths from 1.3 to 30m — that’s light waves as long as a 10-story building! There is a lot to see in this previously unexplored portion of the radio spectrum, from dead stars to active galaxies.

Most radio astronomy happens at wavelengths from a fraction of a centimeter to tens of centimeters (or 1 to 40 gigahertz if you think in frequencies). With great big wavelengths comes the need for great big telescope dishes  (if you want to be able to resolve small structures), since the resolution of a telescope is proportional to the wavelength you are observing at, divided by the telescope’s size. However, there is a limit to how big you can build a telescope dish and still expect to be able to move it around and point it in different directions. As a result, radio astronomy has turned to aperture synthesis (interferometry):  making arrays of smaller antennas which simulate dishes that are kilometers in size. Now, new generations of radio telescopes are working to go even longer– toward wavelengths that are meters long. That means even larger arrays are necessary.

LOFAR (read more about it here!) is made up of 40 stations concentrated in the Netherlands and 8 international stations spread over the continent of Europe (see Figure 1), with more planned. Each station has a two separate arrays of antennas designed to detect both long and even longer-wavelength radio waves. The largest separation between stations (Nancay, France to Onsala, Sweden) is about 1300 km. LOFAR was officially inaugurated in 2010, with regular observing starting last December, so results like those we highlight below are just starting to appear!


The bubbly halo of M87

Title: M87 at meter wavelengths: The LOFAR Picture
Authors: F. de Gasperin, E. Orrú, M. Morgia, et al.
First author’s institution: Max-Planck-Institut für Astrophysik, Garching, Germany


A LOFAR image at 140 MHz (2.1 meters) of the M87 galaxy and its enormous halo, stretching over 80 kiloparsecs. On the right, different halo components are identified, from the inner cocoon (containing the galaxy with its active supermassive black hole and jet) to the Eastern and Western flows of gas from the inner jets as it fills the halo and is confined by the pressure of the surrounding cluster gas (Figures adapted from de Gasperin et al. 2012)


M87 (also discussed in this astrobite) is an extremely active radio galaxy with a huge black hole, lying at the center of the Virgo cluster. By observing this very active galaxy with LOFAR, the authors were looking for very low-energy radio emission in the outskirts of the halo, which would act like a fossil record of past cycles of black hole activity in this galaxy. Instead, they only detected the already-known halo of hot plasma, which is confined by pressure from the surrounding ISM (Figure 2, above). However, the authors did see a clear pattern of steepening spectral index (more flux at longer wavelengths) from the inner cocoon to along the flows of the jets. This is consistent with plasma bubbles forming in the strong jets, expanding adiabatically (causing the spectrum to steepen), and then buoyantly dispersing in the halo. Models of the observed spectral steepening suggest that the overall age of the halo, made up of many generations of these bubbles, is about 40 million years, meaning this galaxy has only been active for a small fraction of it’s billions-of-years-long life.


Rethinking pulsar emission

Title: Synchronous X-ray and Radio Mode Switches: a Rapid Global Transformation of the Pulsar Magnetosphere
Authors: W. Hermsen, J.W.T. Hessels, L. Kuiper et al.
First author’s institution: SRON, Netherlands Institute for Space Research


Left: the brightness of the pulsar’s radio pulse is displayed as a function of time (y-axis), showing the pulsar’s switches between radio Bright (B) and Quiet (Q) modes. Right: when the pulsar is in B mode, there is surprisingly no X-ray emission detected. However, when the pulsar enters its Q mode, there is a strong X-ray pulse (Figures adapted from Hermsen et al. 2013)

Pulsars are the remnants of massive exploding stars, the rapidly spinning and highly magnetic compressed cores left over after a supernova has blasted away  the outer layers of the former star (read more about pulsars in this astrobite). A very strange class of these objects are ‘mode-changing’ pulsars. These pulsars will suddenly change the brightness of their radio emission, while simultaneously decreasing the rate at which their rotation slows as angular momentum is transferred from the pulsar to its surrounding magnetosphere. This odd, linked behavior is not well understood. Using LOFAR and X-ray observations with the XMM-Newton space observatory, the authors observed the transitions between the bright and quiet mode of the pulsar PSR B0943+10. They find that when the pulsar is radio-bright, there is no detectable X-ray emission, however when the pulsar’s radio emission becomes quiet, there is strong, thermal  X-ray emission. The fact that this X-ray emission is pulsed, and that there is no thermal X-ray emission when the radio emission is bright, are both difficult to explain with current models of  X-ray production in pulsars. This result is also challenging to understand because for the pulsar to change both its X-ray and radio-emitting properties simultaneously on such short timescales (a few seconds), the nature of its entire magnetosphere (approximately 10 earth-radii) must also be changing almost instantaneously.


Doing Spectroscopy at the Longest Wavelengths


Left: An image of the Cassiopeia A supernova remnant at 52 MHz (5.8 m), obtained with LOFAR. On the right, a spectrum of the carbon recombination line, tracing neutral gas which is detected in absorption against the radio emission from the supernova. More absorption is detected from the small region of a ‘hot spot’ in the remnant than on average across the entire nebula, indicting that the absorbing gas is not uniform, but has variations in its thickness on small scales (Figures adapted from Askegar et al. 2013).

Title: LOFAR Detections of Low-Frequency Radio Recombination Lines Towards Cassiopeia A
Authors: A. Askegar, J.B.R. Oonk, S. Yatawatta, et al.
First author’s institution: ASTRON, The Netherlands

Cassiopeia A is a supernova remnant (read about them in this astrobite) from the explosion of a star about 300 years ago.
Using LOFAR, the authors observed carbon recombination lines (these are like hydrogen Lyman or Balmer lines, but WAY weaker, from much higher levels of the atom) in Cassiopeia A. Not only do they detect these extremely faint lines, they also measure clear differences in the line properties between the average absorption against the entire remnant, and the absorption against a single bright spot (Figure 4, above). They interpret this to mean that the absorbing gas has a lot of small scale structure, which is perhaps not so surprising for supernova remnants, which are full of knots and filaments. These observations are also an important demonstration that LOFAR can detect such faint lines. At the long wavelengths of LOFAR, it will be possible to observe HUNDREDS of these lines in individual sources, and adding them all together can provide extremely sensitive and high-resolution probes of the kinematics and distribution of the gas in many new sources both near (diffuse galactic gas clouds) and far (external galaxies).

All of these exciting findings come from just a few early observations with LOFAR.  Expect even more and bigger results to appear as LOFAR adds new stations and starts looking deeper into the meter-wavelength sky! LOFAR will be doing everything from searching for the origins of ultra-high energy cosmic rays (LOFAR can also act as a cosmic ray detector!), to looking for transient events (changes in the radio sky, possibly caused by explosions!), to the holy grail: the direct detection of neutral HI from the epoch of Reionization. In the future, we can also look forward to even more observations of the long-wavelength sky with the Square Kilometer Array– that’s right, a collecting area of a cool MILLION square meters! Now, that’s pretty big.

About Betsy Mills

I am a 22nd-grader at UCLA, working with Mark Morris and spending the year at the MPIA in Heidelberg finishing my thesis. I like molecules in space, radio telescopes, the extreme center of our galaxy, getting to look at things no-one else has ever seen before, solving puzzles, and finding creative ways to survive graduate school

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