Something’s cooking

As it sometimes happen in astronomy, very peculiar signals that could potentially represent a groundbreaking discovery turns out to come not from the far reaches of space, but from a much more local source, like the case of the potassium flaring stars. These incidents illustrate the rigorous testing and investigations that occurs when unexpected signals are present in the data, before a solid discovery can be claimed.

Another example of such a story involves the Parkes Observatory Radio Telescope and the search for fast radio bursts. Fast radio bursts are high-energy astrophysical signals lasting only a few milliseconds, and they are believed to originate from outside of the Milky Way. They were first observed in 2001 and to date only around 20 of these bursts have been detected. Their origin is still a mystery, but astronomers have proposed merging black holes, merging neuron stars, flaring magnetars or collapsing pulsars as the source of these bursts, but these explanations are all somewhat speculative. It is naturally of great interest to discover more of these bursts, to hopefully learn more about their origin.

When observing at radio frequencies, especially when looking for something as rare as fast radio bursts, it is important to rule out any kind of radio interference originating from Earth that could potentially mimic the astrophysical signal. A certain type of interference observed at Parkes has been labelled “perytons” after the mythological Peryton creature. They are short-lived burst of radiation at around 1.4GHz, with a shape that is somewhat similar to a bona fide fast radio burst signal.

While it was pretty clear to the astronomers that the perytons arose from some source on Earth (they were observed over a large field of view), it was not evident what was causing them. The radio astronomers at Parkes Observatory began a meticulous investigation to pin this down. One hint to the source was that they only appeared when the telescope was pointed in certain directions (towards the Visitors Center and the staff building). Furthermore, they mostly appeared between 9am and 6pm, clustering just around lunchtime.

The hunt then began for equipment localized in those two buildings that might emit signals in the right frequency range. Immediately, microwave ovens were suspected as the source, as the magnetron inside these operates at 2.4GHz. This is not too far from the 1.4GHz perytons detected with the radio dish. Several tests were performed, to see if there were malfunctioning microwave ovens that would emit at the right frequency. This initial effort did not produce any perytons, but by further experimentation it was found that if the door of the microwave was opened prematurely, a short 1.4GHz radio burst would escape from the oven. So something had definitely been cooking in the data. Take-away lesson: Save those microwave popcorn for when you’re not observing.

While this may seem like a silly investigation, it nevertheless underlines an important scientific principle, namely that when seeing a strange new signal, it is important to rule out sources of terrestrial origin, before a discovery can be claimed. This particular case also has the advantage that perytons have now been properly characterized and this can be used when trying to identify proper astrophysical signals. While peryton signals looks similar to fast radio bursts, they are not identical. Thus investigations like these helps astronomers in their search for more of these mysterious signals.

For the interested readers, a full paper describing the investigation of the perytons can be found here.

The potassium flare stars

In astronomy, we sometimes make discoveries that really make us scratch our heads. Observations that appear to contradict what we believe we know about the Universe. These kind of discoveries are always exciting, as it gives us an opportunity to learn new things about the Universe, and gain a deeper understanding of the world around us. It is certainly something that most astronomers hope for, at some point in their career. That whatever you are researching into, does something unexpected. I mean, who wouldn’t want to have your name associated with a new astrophysical phenomenon?

Thus, one can imagine the excitement, when, in 1962, the French astronomers Daniel Barbier and Nina Morguleff looked at a spectrum they had taken of the G5 dwarf star HD117034 (yes, we’re good with names), and observed two bright emission lines of neutral potassium. A spectra taken the following night, showed no sign of these emission lines. They quickly wrote a summary of their observations and announced the discovery of this new class of “potassium flare stars” in the Astrophysical Journal.

It is not an unknown phenomenon that low-mass stars exhibit flares. These intense, short, outbursts of energy we know even from our own Sun, the solar flares, which are typically followed by coronal mass ejections. However, flaring stars often show emission lines from multiple elements, like silicon, iron and oxygen. Thus, it was a surprise to discover this new class of stars, that seemed to show emission only in the same two potassium lines. Over the next couple of years, another two potassium flaring stars were discovered, both observed from Observatorie de Haute-Provence, as was the case for the initial discovery. Again, the potassium emission lines were only visible in a single spectrum. Adding to the mystery was the fact that the three flaring stars were of wildly different spectral types, namely a G5 dwarf, a K7 dwarf and a B9 dwarf, where particularly the latter stands out, as B-type stars pumps out a lot of energy, meaning that potassium should be fully ionized and no emission lines from neutral potassium would be expected.

Nevertheless, since this had now been observed in different stars at different times, it would appear that a real, transient, astrophysical phenomenon had been discovered, although no one was able to explain it. This spawned a lot of interest, and an extensive search was carried out by Robert F. Wing, Manuel Peimbert and Hyron Spinrad at Lick Observatory, to discover more of these mysterious objects. Although they surveyed 162 stars, not a single potassium flare was observed. Thus, the head-scratching began. Had the astronomers who discovered the three flare stars just been incredibly lucky and made a serendipitous discovery? Or might the explanation be something a bit more down to Earth?

It had previously been suggested that the emission lines could be originating in the Earth’s atmosphere, but in this case the emission lines should have been very narrow, which was not what was observed. The astronomers at Lick Observatory then began looking for other sources of potassium emission. After some experimentation, it was found that the observed emission spectra matched quite closely the spectra of ordinary matches, if the light from the matches were able to reach the spectrograph. Several different types of matches were used, and all produced similar results. The same thing was also tested at the Observatorie de Haute-Provence, and it was found that, in certain positions, the light from the matches did indeed reach the spectrograph. The full paper can be read here.

Thus it would appear that the potassium flaring stars was in reality a potassium flaring member of the observatory staff, who had lit up briefly in the science literature. Following this, I bet this observatory may have been the first to see a smoking ban on the premises. Or at least anywhere near the spectrograph.

This is also a perfect example of how astronomy, and science in general, works when something weird and unexpected it found (remember the super luminal neutrinos?). Such findings need to be independently confirmed and checked rigorously, to rule out other potential explanations. In many cases these things can be explained by already known phenomenon, or an overlooked mistake somewhere, but sometimes genuinely new things are discovered, which is amazing!

As a side-note, the extensive testing of the spectra of matches also showed that French and American matches were very similar products, in terms of composition, so there’s no blaming foreign matches when you can’t light your fire.