ICRAR: SS Cygni measured with public help

[MargaritaMc]

International Centre for Radio Astronomy Research

Astronomers team up with the public to solve decade old puzzle

An extremely precise measurement of the distance to a star system has finally allowed astronomers to solve a decade-old puzzle, confirming understanding of the way exotic objects like black holes interact with nearby stars.

Published today in prestigious journal Science, a team of astronomers headed by Dr James Miller-Jones from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR), have measured the distance to star system SS Cygni to be 372 light years, much closer than a previous measurement made by the Hubble Space Telescope in the 1990s.

The measurement was made possible by amateur astronomers from the American Association of Variable Star Observers (AAVSO) who alerted the team to changes in the compact star system, triggering the team to start observations with two of the world's most accurate radio telescopes.

Dr Miller-Jones' team then measured the annual wobble of the system compared to distant background galaxies, allowing them to measure the distance to SS Cygni with unprecedented precision.

"If you hold your finger out at arm's length and move your head from side to side, you should see your finger appear to wobble against the background. If you move your finger closer to your head, you'll see it starts to wobble more. We did the exact same thing with SS Cygni - we measured how far it moved against some very distant galaxies as the Earth moved around the Sun," Dr James Miller-Jones said.

"The wobble we were detecting is the equivalent of trying to see someone stand up in New York from as far as away as Sydney."

The distance to SS Cygni had previously been measured using the Hubble Space Telescope, producing a puzzling result that was much further than predicted.

"If SS Cygni was actually as far away as Hubble measured then it was far too bright to be what we thought it was, and we would have had to rethink the physics of how systems like this worked," Dr Miller-Jones said.

Dr Miller-Jones said that SS Cygni is a double star system containing a normal low mass star and a white dwarf star.

A white dwarf is the remnant of a star like our Sun that has run out of fuel and collapsed into an object about the size of Earth. Because it's so dense, its strong gravity strips gas off its companion star, which then swirls around the white dwarf.

Occasionally the flow of gas onto the white dwarf will increase dramatically, causing the system to appear up to 40 times brighter in visible light. It's only during these rare periods that the star system emits radio waves, which allow for a much more precise measure of the distance.

"Our key advantage was using radio telescopes to observe the system. In visible light, optical telescopes like Hubble see hundreds of different stars, all of which are moving by different amounts, whereas in radio waves the background we compare against is much further away and therefore doesn't appear to move at all," co-author Assistant Professor Gregory Sivakoff from the University of Alberta, said.

The team used groups of telescopes called the Very Long Baseline Array (VLBA) in the United States and the European Very Long Baseline Interferometry Network (EVN) in Europe and South Africa to pinpoint the exact location of the system relative to the background galaxies.

"The system only emits radio waves for a short period of time. Without the cooperation of our many amateur observers who looked at SS Cygni night after night, we wouldn't have known when to look - their contribution was invaluable," co-author Dr Matthew Templeton from the AAVSO said.

The measured distance of just over 370 light years means the light detected by the team left SS Cygni around the time that famous physicist Sir Isaac Newton was born in the 1600s.

"The pull of gas off a nearby star onto the white dwarf in SS Cygni is the same process that happens when neutron stars and black holes are orbiting with a nearby companion, so a lot of effort has gone in to understanding how this works," Dr Miller-Jones said.

"Our new distance measurement has solved the puzzle of SS Cygni's brightness, it fits our theories after all."

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ICRAR is a joint venture between Curtin University and The University of Western Australia providing research excellence in the field of radio astronomy.

http://news.ualberta.ca/newsarticles/20 ... ce-mystery

[bystander]

Accurate Distance Measurement Resolves Major Astronomical Mystery
National Radio Astronomy Observatory | 2013 May 23

Famed Pair of Stars Closer To Earth Than We Imagined
Universe Today | Elizabeth Howell | 2013 May 25

An Accurate Geometric Distance to the Compact Binary SS Cygni Vindicates Accretion Disc Theory - J.C.A. Miller-Jones et al

• Science 340(6135) 950 (24 May 2013) DOI: 10.1126/science.1237145
  arXiv.org > astro-ph > arXiv:1305.5846 > 24 May 2013

[bystander]

Hobbyist stakeout solves dwarf star enigma
New Scientist | Astrophile | Andrew Fazekas | 2013 May 24

Object type: Stellar pair
Location: 500, no, 370 light years from Earth

[i]A white dwarf strips its companion of material (Credit: Mark Garlick/SPL)[/i]

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Surveillance of the police variety is strictly for professionals. But humble backyard stargazers have carried out a cosmic stakeout on an unpredictable star system, SS Cygni, which periodically explodes. The result is a measurement of its distance from Earth that bests one made by NASA's Hubble telescope – and shores up the leading mechanism for the process that lights up the most common type of black hole.

"Because there are people all over the world we get better coverage in time than if we just focus on observations from one location," says astronomer Gregory Sivakoff of the University of Alberta in Canada, who called on the hobbyists for assistance. "Professionals just can't monitor objects over long periods of time when competition for time on large instruments is fierce, so citizen astronomers are crucial when it comes to monitoring the transient universe."

Tucked away in one corner of the constellation Cygnus, SS Cygni consists of an Earth-sized white dwarf – a remnant of a now-dead sun-like star – plus a companion. The strong gravity from the dwarf strips material from the companion, forming a whirling, flattened accretion disc. As this material accumulates, some regularly ignites, forming outbursts that occur every 49 days or so.

Tied-up telescope

But the distance to SS Cygni as measured by Hubble in 1999 – 520 light years – suggests an inherent disc brightness that, when plugged into the leading model of accretion disc formation, would put it in a permanent state of explosion. "Until this study, the observational distance from Hubble simply did not match up with what the theoretical model was saying it should be," says Sivakoff.

To solve the conundrum, he and his colleagues enlisted 280 stargazers from the American Association of Variable Star Observers (AAVSO), which has been observing the system since it was discovered in the late 19th century. They tipped off Sivakoff's team whenever SS Cygni started to ignite, so that two high-end radio telescopes could be pointed at the baffling stellar pair.

"No one can accurately predict when the next eruption will occur and professional astronomers can't tie up telescopes, satellites or dish arrays waiting for something to happen," says AAVSO's Mike Simonsen, a veteran of more than 80,000 observations of variable stars. "They need someone to monitor these systems and alert them when they begin to behave in the manner they want to observe, in this case, a narrow time window at the beginning of an outburst."

Disc relief

Unlike Hubble, which had to use unpredictable stars in the Milky Way as a stationary reference point, Sivakoff was able to use a more reliable, distant galaxy, leading to a much more precise measurement. This put the distance to SS Cygni at 370 light years – much closer than Hubble's measurement. It also implies a lower inherent brightness for SS Cygni, which, when plugged into the disc model, produces the observed, periodic outbursts.

This suggests the leading model for accretion discs is correct – a relief for astronomers, who use it to explain all kinds of exotic phenomena, including the similar discs that form around the most common type of black hole. Indeed, the only reason we know that so-called stellar black holes exist is the radiation emitted by their accretion discs, which form when they suck material from a companion star.

"By understanding the detailed physics of what's going on around these white dwarf binaries we are getting a better feel of the mechanisms of black holes in galaxies across the universe," says Sivakoff.

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