Seeing cosmic X-ray emitters could be a matter of perspective

Known as ultra-bright X-ray sources, the emitters are easy to spot when viewed from the front, but they can be hidden from view if they are even slightly away from Earth.

It’s hard to miss a flashlight beam pointed straight at you. But this beam seen from the side seems much weaker. The same is true for some cosmic objects: like a flashlight, they radiate mainly in one direction, and they look radically different depending on whether the beam is pointing away from Earth (and nearby space telescopes) or directly towards she.

New data from NasaThe NuSTAR space observatory indicates that this phenomenon is true for some of the most important x-ray emitters in the local universe: ultraluminous x-ray sources, or ULX. Most cosmic objects, including stars, emit little x-rays, especially in the high energy range seen by NuSTAR. ULXs, on the other hand, are like X-ray headlights breaking through the dark. To be considered an ULX, a source must have an X-ray brightness about a million times brighter than the total light output of the Sun (at all wavelengths). ULXs are so bright that they can be seen millions of light years away, in other galaxies.

The new study shows that the object known as SS 433, located in the Milky Way galaxy, and only about 20,000 light years from Earth, is a ULX, even though it appears to be about 1,000 times lower than the minimum threshold to be considered such.

This weakness is a perspective tip, according to the study: The high-energy x-rays of SS 433 are initially confined in two gas cones extending outward from opposite sides of the central object. These cones are similar to a mirrored bowl that surrounds a flashlight bulb: they encircle the x-ray light from SS 433 in a narrow beam, until it escapes and is detected by NuSTAR. But because the cones do not point directly at Earth, NuSTAR cannot see the full brightness of the object.

“We have long suspected that some ULXs emit light in narrow columns, rather than in all directions like a bare bulb,” said Matt Middleton, professor of astrophysics at the University of Southampton in the UK and author principal of the study. “In our study, we confirm this hypothesis by showing that the SS 433 would be considered as a ULX by a frontal observer. “

If an ULX relatively close to Earth can hide its true brightness due to its orientation, then there are probably more ULXs – especially in other galaxies – in similar disguise. This means that the total population of ULX is expected to be much larger than what scientists are currently observing.

cone of darkness

About 500 ULX have been found in other galaxies, and their distance from Earth means that it is often almost impossible to tell what type of object generates the x-ray emission. The x-rays are probably coming from a large area. amount of gas heated to extreme temperatures because it is attracted by the gravity of a very dense object. This object can be either a neutron star (the remains of a collapsed star) or a small black hole, which is no more than about 30 times the mass of our Sun. The gas forms a disc around the object, like water circling a drain. Friction in the disc increases the temperature, causing it to radiate, sometimes becoming so hot that the system bursts with x-rays. The faster the material falls on the central object, the brighter the x-rays.

Astronomers suspect that the object at the heart of SS 433 is a black hole about 10 times the mass of our Sun. What is certain is that it cannibalizes a nearby large star, its gravity siphoning off matter at a rapid rate: in a single year, the SS 433 steals the equivalent of about 30 times the mass of Earth. to its neighbor, which makes it the most greedy black hole or neutron star known in our galaxy.

“It’s been known for a long time that this thing eats at a phenomenal rate,” Middleton said. “This is what sets ULX apart from other objects, and is probably the root cause of the abundant amounts of X-rays we see from them.”

The object in SS 433 has eyes bigger than its belly: it steals more material than it can consume. Some of the excess material is blown off the disc and forms two hemispheres on opposite sides of the disc. Inside each is a cone-shaped void that opens into space. These are the cones that collect the light from high-energy x-rays into a beam. Anyone looking straight into one of the cones will see an obvious ULX. Although composed only of gas, the cones are so thick and massive that they act like lead panels in an x-ray screening room and prevent x-rays from passing through them to the side.

Scientists suspected that some ULX might be hidden for this reason. SS 433 provided a unique chance to test this idea because, like a spinning top, it wobbles on its axis – a process astronomers call precession.

Most of the time, the two cones of SS 433 point far from Earth. But because of the way SS 433 precession, a cone periodically tilts slightly toward Earth, so scientists can see some x-ray light coming out of the top of the cone. In the new study, scientists looked at how the x-rays seen by NuSTAR change as the SS 433 moves. They show that if the cone continued to tilt towards Earth so that scientists could look at it directly, they would see enough x-rays to officially call SS 433 an ULX.

Black holes that feed at extreme rates have shaped the history of our universe. Supermassive black holes, which are millions to billions of times the mass of the Sun, can deeply affect their host galaxy when they feed. Early in universe history, some of these massive black holes may have fed as fast as SS 433, releasing enormous amounts of radiation that reshaped local environments. The flows (like the cones in SS 433) redistributed matter that could eventually form stars and other objects.

But because these fast-consuming behemoths reside in incredibly distant galaxies (the one in the heart of the Milky Way doesn’t eat much right now), they remain difficult to study. With SS 433, scientists have found a miniature example of this process, much closer to home and much easier to study, and NuSTAR has provided new information about the activity taking place there.

“When we launched NuSTAR, I didn’t think anyone expected ULX to be such a rich area of ​​research for us,” said Fiona Harrison, Principal Investigator at NuSTAR and Professor of Physics at Caltech in Pasadena, in California. “But NuSTAR is unique in that it can see almost the full range of x-ray wavelengths emitted by these objects, and that gives us insight into the extreme processes that must drive them.”

Reference: “NuSTAR reveals the hidden nature of SS433” by MJ Middleton, DJ Walton, W Alston, T Dauser, S Eikenberry, YF Jiang, AC Fabian, F Fuerst, M Brightman, H Marshall, M Parker, C Pinto, FA Harrison , M Bachetti, D Altamirano, AJ Bird, G Perez, J Miller-Jones, P Charles, S Boggs, F Christensen, W Craig, K Forster, B Grefenstette, C Hailey, K Madsen, D Stern and W Zhang, May 6 2021, Monthly notices from the Royal Astronomical Society.
DOI: 10.1093 / mnras / stab1280

Learn more about the mission

NuSTAR is a Small Explorer mission led by Caltech and managed by NASA’s Jet Propulsion Laboratory, a division of Caltech, for the agency’s Science Missions Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corporation in Dulles, Virginia (now part of Northrop Grumman). The NuSTAR Mission Operations Center is located at University of California, Berkeley, and the official data archive is housed at NASA’s High-Energy Astrophysics Science Archives Research Center. ASI provides the mission ground station and a mirror archive.