Black Hole’s Personality Not as Magnetic as Expected

This 2015 NASA Swift observation of V404 Cygni shows the X-ray echoes bouncing off rings of dust surrounding the binary system after the X-ray nova (Andrew Beardmore/Univ. of Leicester/NASA/Swift)

In 2015, a stellar-mass black hole in a binary star system underwent an accretion event causing it to erupt brightly across the electromagnetic spectrum. Slurping down the plasma from its stellar partner — an unfortunate sun-like star — the eruption became a valuable observation for astronomers and, in a recent study, researchers have used the event to better understand the magnetic environment surrounding the black hole.

The binary system in question is V404 Cygni, located 7,795 light-years from Earth, and that 2015 outburst was an X-ray nova, an eruption that previously occurred in 1989. Detected by NASA’s Swift space observatory and the Japanese Monitor of All-sky X-ray Image (MAXI) on board the International Space Station, the event quickly dimmed, a sign that the black hole had consumed its stellar meal.

Combining these X-ray data with observations by radio, infrared and optical telescopes, an international team of astronomers were able to measure emissions from the plasma close to the black hole’s event horizon as it cooled.

The black hole was formed after a massive star ran out of fuel and exploded as a supernova. Much of the magnetism of the progenitor star would have been retained post-supernova, so by measuring the emissions from the highly charged plasma, astronomers have a tool to probe deep inside the black hole’s “corona.” Like the sun’s corona — which is a magnetically-dominated region where solar plasma interacts with our star’s magnetic field (producing the solar wind and solar flares, for example) — it’s predicted that there should be a powerful interplay between the accreting plasma and the black hole’s coronal magnetism.

As charged particles interact magnetic fields, they experience acceleration radially (i.e. they spin around the magnetic field lines that guide their direction of propagation) and, should the magnetism be extreme (in a solar or, indeed, black hole’s corona), this plasma can be accelerated to relativistic speeds. In this case, synchrotron radiation may be generated. By measuring the radiation across all wavelengths, astronomers can thereby probe the magnetic environment close to a black hole as this radiation is directly related to how powerful a magnetic field is generating it.

A black hole with a magnetic field threading through an accretion disk (ESO)

According to the study, published in the journal Science on Dec. 8, V404 Cygni’s hungry black hole has a much weaker magnetic field than theory would suggest. And that’s a bit of a problem.

The researchers write: “Using simultaneous infrared, optical, x-ray, and radio observations of the Galactic black hole system V404 Cygni, showing a rapid synchrotron cooling event in its 2015 outburst, we present a precise 461 ± 12 gauss magnetic field measurement in the corona. This measurement is substantially lower than previous estimates for such systems, providing constraints on physical models of accretion physics in black hole and neutron star binary systems.”

Black holes are poorly understood, but with the advent of gravitational wave (and “multimessenger”) astronomy and the excitement surrounding the Event Horizon Telescope, in the next few years we’re going to get a lot more intimate with these gravitational enigmas. Why this particular black hole’s magnetic environment is weaker than what would be expected, however, suggests that our theories surrounding black hole evolution are incomplete, so there will likely be some surprises in store.

“We need to understand black holes in general,” said collaborator Chris Packham, associate professor of physics and astronomy at The University of Texas at San Antonio (UTSA), in a statement. “If we go back to the very earliest point in our universe, just after the Big Bang, there seems to have always been a strong correlation between black holes and galaxies. It seems that the birth and evolution of black holes and galaxies, our cosmic island, are intimately linked. Our results are surprising and one that we’re still trying to puzzle out.”

Vast Magnetic Canyon Opens up on the Sun — Choppy Space Weather Incoming?

A “live” view of our sun’s corona (NASA/SDO)

As the sun dips into extremely low levels of activity before the current cycle’s “solar minimum”, a vast coronal hole has opened up in the sun’s lower atmosphere, sending a stream of fast-moving plasma our way.

To the untrained eye, this observation of the lower corona — the sun’s magnetically-dominated multi-million degree atmosphere — may look pretty dramatic. Like a vast rip in the sun’s disk, this particular coronal hole represents a huge region of “open” magnetic field lines reaching out into the solar system. Like a firehose, this open region is blasting the so-called fast solar wind in our direction and it could mean some choppy space weather is on the way.

As imaged by NASA’s Solar Dynamics Observatory today, this particular observation is sensitive to extreme ultraviolet radiation at a wavelength of 193 (19.3 nanometers) — the typical emission from a very ionized form of iron (iron-12, or FeXII) at a temperature of a million degrees Kelvin. In coronal holes, it looks as if there is little to no plasma at that temperature present, but that’s not the case; it’s just very rarefied as it’s traveling at tremendous speed and escaping into space.

The brighter regions represent closed field lines, basically big loops of magnetism that traps plasma at high density. Regions of close fieldlines cover the sun and coronal loops are known to contain hot plasma being energized by coronal heating processes.

When a coronal hole such as this rotates into view, we know that a stream of high-speed plasma is on the way and, in a few days, could have some interesting effects on Earth’s geomagnetic field. This same coronal hole made an appearance when it last rotated around the sun, generating some nice high-latitude auroras. predicts that the next stream will reach our planet on March 28th or 29th, potentially culminating in a “moderately strong” G2-class geomagnetic storm. The onset of geomagnetic storms can generate impressive auroral displays at high latitudes. Although not as dramatic as an Earth-directed coronal mass ejection or solar flare, the radiation environment in Earth orbit will no doubt increase.

The sun as seen right now by the SDO’s HMI instrument (NASA/SDO)

The sun is currently in a downward trend in activity and is expected to reach “solar minimum” by around 2019. As expected, sunspot numbers are decreasing steadily, meaning the internal magnetic dynamo of our nearest star is starting to ebb, reducing the likelihood of explosive events like flares and CMEs. This is all part of the natural 11-year cycle of our sun and, though activity is slowly ratcheting down its levels of activity, there’s still plenty of space weather action going on.

Welcome to my Sinkhole, Premium Martian Real Estate

A sinkhole in Tractus Fossae, created by tectonic activity (HiRISE/NASA)
A sinkhole in Tractus Fossae, created by tectonic activity (HiRISE/NASA)

It might not look like much from space, but this depression in the Martian landscape might be considered to be a priceless feature when viewed by future Mars colonists.

In December 2008, the Mars Reconnaissance Orbiter (MRO) flew silently over the Tharsis bulge, the location of a series of ancient volcanoes. The High Resolution Imaging Science Experiment (HiRISE) captured what appears to be a deep hole. This kind of feature has been seen before, like a Martian pore, deep and foreboding. Usually these sinkholes aren’t as deep as they look, but they are deeper than the surrounding landscape. They are also similar to their terrestrial counterparts in that they have very steep sides (unlike the gentle, eroded slopes of crater rims) and they are caused by a lack of material below. On Earth, sinkholes often form due to water flowing beneath, removing material, causing the overlying rock/soil to slump, forming a sudden hole. In the example above, the sinkhole (or “collapse pit”) was caused by tectonic activity. In this case, it is likely that the material dropped into a void left over by magma-filled dykes (lava tubes from old volcanoes).

A stretched and image processed version of the sinkhole; the bottom of the hole is visible (HiRISE/NASA)
A stretched and image processed version of the sinkhole; the bottom of the hole is visible (HiRISE/NASA)

The result is a hole with very steep sides. It has been suggested that these sink holes may be useful to future Mars colonists, as they can use the natural feature for shelter. On Mars, humans would be subject to an increased dosage of radiation (due to the tenuous Martian atmosphere and lack of a global magnetic field), so it is preferable to find any form of natural shelter to build your habitat. The depth of this kind of sinkhole will afford some protection, and drilling into the cavern side would be even better. Perhaps even put a dome over the top? No need to build walls around your building then. Also, there’s the interesting–if a little frightening–prospect of accessing underground lava tubes. Therefore, colonists won’t need to dig very far to create a subterranean habitat with all the radiation protection they’ll ever need (the insulation would also be impressive).

Although this scenario might be a little far-fetched, and probably only suitable for an established human presence on Mars (after all, the numerous valleys would probably suffice for most permanent habitats drilled into cliff faces), it does go to show that the current missions in orbit around Mars are doing a great job at seeking out some possible housing solutions for our future Mars settlers…

Source: HiRISE, Marspedia