We put telescopes in space to prevent interference from the atmosphere and peripheral sources of radiation, but sometimes even that is not enough. The NuSTAR X-Ray telescope has had to cope with radiation leaking in from objects near the ones it is looking at, making it harder to detect a signal in the noise. However, astronomers have found a way to use this “nuisance light”, giving NuSTAR a wider effective field of view than expected, and have now published the first paper based on an object studied in this way.
SMC X-1 is a system that contains a neutron star a little more massive than the Sun, which is bright in the X-ray part of the spectrum. However, that brightness varies dramatically, and astronomers are keen to understand how and why. In The Astrophysical Journal, one team provides some answers, using nuisance light that crept into NuSTAR.
SMC X-1's name comes from its location in the Small Magellanic Cloud, one of the closest galaxies outside our own. It's a binary pulsar that shows considerable flaring activity that's been puzzling astronomers for a long time.
SMC X-1's variation is partly explained by its orbit around another, 15 times more massive, star, which sometimes hides it from our perspective, but astronomers suspect something else is going on. The paper isn't able to fully solve this puzzle, but does provide information about the mass SMC X-1 is capturing from its larger and less dense neighbor, causing it to increase its rate of spin.
X-Rays are a particularly hard part of the electromagnetic spectrum to study, because the atmosphere does such a good job of blocking them. X-Ray astronomy only got started when rockets carried the first instruments to great heights, and became a serious field once we could put X-Ray telescopes in orbit. With so few telescopes in space, however, time on them is even shorter than for other important instruments, so we don't get to study SMC X-1 nearly as much as anyone in the field would like.
Things change, however, if you can collect data on multiple objects at once. As Dr Brian Grefenstette of Caltech put it in a statement; "Imagine sitting in a quiet movie theater, watching a drama, and hearing the explosions in the action movie playing next door. In the past, that's what the stray light was like—a distraction from what we were trying to focus on. Now we have the tools to turn that extra noise into useful data, opening an entire new way of using NuSTAR to study the universe."
Grefenstette leads what is called the StrayCats team, extracting information about 80 X-Ray objects that are bright enough, and close enough in the sky to other targets, that NuSTAR has picked up significant amounts of nuisance radiation from them. For some of them, this marks a substantial expansion in the time period over which we can track their brightness, even if we don't have the sort of focused images produced by looking directly at an X-ray source.
SMC X-1 is a particularly good example, because there are so many other X-ray sources in the Magellanic clouds that NuSTAR spends a lot of time looking at objects close to it. At its peaks, it is bright enough that plenty of its light creeps into nearby fields of view. Nevertheless, there are likely to be many other objects that will become the subjects of papers using the techniques first demonstrated here.