Extreme Pulsar System is 'One in a Billion'
Bill Saxton, NRAO, AUI, NSF
Artist conception of the millisecond pulsar and its companion white dwarf stars.
The Atacama Large Millimeter/Submillimeter Array, or ALMA, is nearing completion after over 30 years of planning and collaborating among astronomers and engineers from several nations. The completed array will consist of 66 antennas and two supercomputers, called correlators, at the backend to collect the signals and make the array function as one large telescope.
On March 12th and 13th, 2013, over a hundred journalists descended upon the array along with politicians, scientists, engineers, and other VIPs to celebrate the official inauguration of the array and tour its facilities. I was there along with 11 other journalists from around North America as guests of the National Radio Astronomy Observatory.
Overview of the telescope's technical capabilities, science goals, and history were given by (from left to right) Michael Thorburn, Head of the ALMA Department of Engineering, Pierre Cox, incoming ALMA director, Thijs de Graauw, current ALMA director, Al Wootten, ALMA Program Scientist for North America, and Ewine van Dishoeck, Professor at Leiden University and former ALMA board member.
The ALMA control room is the hub of activity at the "low site" or Operations Support Facility. From here, telescope operators manage and control all array operations and conduct observing runs. Astronomers who win time on the telescope through a merit-based proposal process do not actually travel to the site or control the telescope in real time. Instead, as with other radio interferometers, they prepare observing scripts based on what science they want to achieve, and their scripts are then scheduled and run by ALMA staff. The data is then made available to the astronomer for a proprietary period of one year, when it is then released to the public.
Everyone gets into the spirit of ALMA, which in Spanish actually means "soul," as an art contest was held for local children. Featured here are some of the submissions and several winners in the six to nine year old group.
54 of the final 66 antennas were at the "high site" or Array Operations Site, at an altitude of 16,500 feet. The 12-meter dishes are contributed by partners in Europe, North America, and East Asia, whereas several smaller, 7-meter dishes from East Asia make up a compact array in the center. The completed array will eventually have the capability of expansion by moving the antennas to different concrete "pads" spread around the Chajnantor Plateau. With a baseline, or distance between antennas, of up to 14 kilometers, ALMA will be able to obtain resolution ten times better than that of the Hubble Space Telescope.
Reporters were allowed a peek at the massive correlator behind glass in the second highest building in the world. The correlator is the computer that brings together signals from all the antennas so that the astronomer can make an image using the full array. We received a thorough explanation of its working from correlator engineer Alejandro Saez who has spent time constructing in in Charlottesville, Virginia, and at the high site in Chile. The correlator has the processing power of 3 million laptops as it has to make calculations for up to 1,125 antenna pairs billions of times per second.
The antennas actually put on a show for us during our three hour tour of the high site, slewing, or moving, back and forth. These antennas are built for perfection, or as close to it as one can come. These sturdy, yet flexible machines must maintain a dish surface accuracy of the width of a human hair and pivot back and forth between a target source and calibrator source on the sky every ten seconds.
Taking a hundred or so journalists to a site at 16,500 feet (5000 meters) altitude is quite risky business. Everyone must pass a basic fitness exam to be cleared for access, though employees must endure a much more rigorous process involving a stress test and heart monitoring to work on site. A team of diligent paramedics were there to hand out oxygen bottles and check for signs of altitude sickness. Here, I got my pulse checked in an ambulance after running around like an excited child a bit too much.
The "front end" of a radio telescope is the place where radio light that is collected by the dish is received, amplified, and transferred to the next stage for digital processing. The large blue drum is a cryostat and holds the most sensitive electronics in the telescope at a cold 4 Kelvin using liquid helium. The silver circles are the actual windows into the feedhorms, the first stage where radio light passes into the front end. Don't think those metal plates are transparent? They are to the radio light that ALMA receives.
Just one of ten sets of electronics that can fit inside those blue drums are on display here. Various stages of the system are cooled to difference temperatures, and engineers needs to wear special gloves and shirts when handling these so that they do not impart a spark of static electricity that could ruin a sensitive (and expensive) piece of equipment. The superconducting receivers used by ALMA are state-of-the-art and were developed specifically for this purpose.
How do you move a giant antenna? You build a giant antenna mover. The on-site guests were treated to an amazing site when one of the two antenna crawlers rumbled down a huge dirt road to pick up one of the dishes that is still under testing. Otto and Lore, as the crawlers are named, have to pick up each 100-ton antenna and place them down on their pads with millimeter accuracy. They move the antennas from the Operations Support Facility up to the high site and back and will be used to change array configurations to change the telescopes resolving power.
Pulsar astronomers scored big with the finding of a triple star system that includes a millisecond pulsar. This unique find has allowed them to precisely measure the physics of this system and has the potential to test Einstein’s general relativity with incredible precision.
The system in question contains a millisecond pulsar, or a neutron star that has been left over from a supernova explosion of a massive star at the end of its lifetime. It is in a 1.6 day binary orbit with a white dwarf, a small hot star left over after a mid-sized star lives out its life. Another white dwarf star completes the triple in a larger orbit with a 327 day orbital period.
Millisecond pulsars spin at incredibly high speed — up to thousands of times per second — and have a “hotspot” that rotates into view, creating a radio pulse. By measuring the exact time at which these pulses arrive at Earth, they can be used as incredibly accurate clocks, allowing for exquisite physical measurements to be made.
Finding a triple system like this is extremely rare, and the stars have to survive several catastrophic events. First, the massive star goes supernova to create the neutron star that becomes the pulsar without too significantly disturbing the nearby mid-sized stellar companion.
The pulsar and the star then continue to interact with each other with the incredibly dense pulsar pulling material off of the star as it expands and cools into a red giant phase. This acquisition of material is what eventually speeds up the rotation of the pulsar. The outer star will also go through a red giant phase, and all three components are still together after this. Scott Ransom, one of the lead investigators of the project, estimates that this alignment is actually “one in a billion.”
The pulsar was discovered in a large survey done with the Green Bank Telescope in West Virginia in the summer of 2007. This survey took advantage of the fact that the entire telescope had ceased normal operations in order for the track supporting the massive telescope to be repaired, as it was beginning to buckle under its weight. But astronomers do not waste time, and the telescope scanned vast swaths of the sky looking for pulsars even while the construction was being done.
This pulsar was discovered by graduate student Jason Boyles in the very last bit of data to be analyzed just two years ago. Follow up observations with the GBT, The Arecibo Radio Telescope in Puerto Rico, and the Westerbork Synthesis Array accurately timed the pulses as they arrived at Earth. The 1.6 day period from the orbit of the closer white dwarf was easily detected, but only after careful analysis and checking was the second companion discovered with a period of 327 days. The companion stars were confirmed to be white dwarfs by observations in the infrared, optical, and ultraviolet.
The timing solution from three radio telescopes for the pulsar, showing the 1.6 day period and part of the 327 day period. These points have error bars far too small to plot thanks to the precise measurements.Ransom et al. 2014.
With a truly unique system, the motions and masses of the stars were measured with exquisite precision, with masses of 1.437 solar masses for the pulsar and 0.1975 and 0.4101 solar masses for the white dwarf stars. Yes, those are FOUR significant figures to those measurements! In fact, the error bars on the plot of the pulsar timing are a million times too small to even see on the chart.
This triple system allows such precise measurements that astronomers are testing a key component on general relativity, the strong equivalence principle. This basically states that mass, and only mass, is the important quantity to determining gravity, even when gravitational fields are incredibly strong, such as around a pulsar. By measuring the precise motions of the two white dwarfs around the pulsar, this principle can be tested. Limits are already being made and general relativity does pass the test so far, but more monitoring will provide an even better answer in the future. general relativity still holds supreme, but the team expects to have more precise measurements worked out over the next year.
Watch a video of the system in action:
This work appeared in the January 5th edition of Nature; and a preprint is available at arXiv.org.