NuStar Doesn’t Waste Anything

When, incidentally, telescopes receive photons from objects not directly observed, it's possible to do "salvage astronomy." This happened with the NuStar satellite telescope. Matteo Bachetti tells Mediainaf how a team of researchers managed to make science out of it.

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For nearly ten years, NASA’s NuStar (Nuclear Spectroscopic Telescope Array) X-ray space observatory has studied high-energy objects and phenomena in the universe – such as star mergers and active black holes at the centers of galaxies. During all the scientific investigations – and partly due to the telescope’s design – scientists have had to deal with straylight entering from the sides of the space telescope, which can interfere with observations by adding photons from other celestial bodies not directly observed.

“Straylight is the term used for light that doesn’t come from the sources being observed, but from other sources not framed by the telescope,” explains Matteo Bachetti, a postdoc researcher at INAF in Cagliari, to Media Inaf. “These other sources are so bright that their light bounces off the telescope’s elements until it ends up in the focal field and is thus recorded in the images.”

Bachetti is the co-author of an article published last week in The Astrophysical Journal, where, however, from an unwanted guest, straylight becomes a real resource: in the study, researchers describe, for the first time, the use of straylight collected by NuStar’s detectors to study a specific object: a neutron star. A true case of salvage astronomy, perfectly in tune with current times and needs.

“Typically, straylight is a problem, a source of noise in the observations of main targets,” the researcher says. “The only case where observing via straylight is truly advantageous is when the source would be too bright for the telescope, to the point of ‘blinding’ it or endangering its safety. It’s quite rare, but it happens: for example, some observations of the Crab pulsar and, if I’m not mistaken, the Sun, very bright sources, have been conducted this way.”

The idea, as we said, comes from Caltech researchers, who decided to take a look at all cases of observations ‘tainted by straylight,’ noticing that there were quite a few high-quality observations that could be used for scientific work. The recently published article, in particular, uses the straylight from a famous pulsar, SMC X-1, to study its rotation and orbital decay. The system – located in one of the two small galaxies orbiting the Milky Way, the Small Magellanic Cloud (from which the acronym SMC) – consists of a neutron star orbiting a ‘living’ star. The brightness of SMC X-1’s X-ray emission seems to vary greatly when seen by telescopes, but decades of direct observations (by NuStar and others) have shown a precise pattern in the fluctuations. There are several reasons why SMC X-1 changes brightness when studied by X-ray telescopes: the brightness at these frequencies decreases, for example, when the neutron star disappears behind the companion star. But what is the advantage of doing it this way, instead of going to observe this pulsar directly?

“Perhaps, in the annual competition to secure observation time, a scientific program focused on SMC X-1 wouldn’t have gotten all the time needed for this work,” Bachetti replies. “With this method, NuStar was able to conduct science with the main targets and with the pulsar in question.”

Of course, straylight data cannot replace direct observations. Besides the fact that straylight is out of focus, many objects that NuStar can observe directly are too faint to appear in the straylight catalog. Observing the frequency and intensity of brightness changes of a neutron star through this method, however, can help scientists decipher what’s happening to these objects. Applications may involve searching for patterns that describe long-term brightness fluctuations of a source, or detecting a catastrophic event or, more simply, strange and unexpected behaviors in these objects.

To conclude, this study first demonstrates the feasibility and reliability of using this new technique to study certain objects like the pulsar SMC X-1, and it opens the door to researchers’ curiosity for identifying new scientific cases in the NuStar database.

And finally, it must be said, it’s no coincidence that this demonstration comes from this observatory. “NuStar has the peculiarity of having the telescope optics separated from the focal plane (where the sensors are) by an open truss (as can be seen in any representation of the telescope),” Bachetti explains. “Light from bright sources can therefore easily enter through the openings in front of the sensors. Other telescopes don’t have this feature, so it’s more difficult. Observations in the galactic plane with NuStar are often affected by straylight because there are many bright sources in this area of the sky. Various projects are underway to use straylight from other sources… stay tuned!”

To learn more:

Read in The Astrophysical Journal the article “Extending the baseline for SMC X-1’s spin and orbital behavior with NuSTAR stray light,” by McKinley C. Brumback (Caltech), B.W. Grefenstette, D.J.K. Buisson, M. Bachetti, R. Connors, J.A. Garcia, A. Jaodand, R. Krivonos, R. Ludlam, K.K. Madsen, G. Mastroserio, J.A. Tomsick, and D. Wik