Microquasars: The “Elusive” Gamma-Ray Emitters

Artistic view of a microquasar
Credit: NASA/ CXC/M.Weiss

Microquasars are Galactic binary systems composed of a star and a compact object (a black hole or a neutron star) that eats up matter from its companion, usually via an accretion disk, giving rise to relativistic jets, i.e. beams of particles moving almost at the speed of light. These jets, which can be either intermittent or persistent structures depending on the specific state of the system, emanate from the vicinity of the compact object and can expand light years away from the binary system. 

The word “microquasar” was used for the first time in 1992 to describe the Galactic binary system 1E1740.7–2942, characterized by radio-emitting double-sided jets [1]. The jets resembled the relativistic collimated outflows launched by quasars (active galaxies with supermassive black holes at the centre that devour its surrounding material), although, in the latter case, the powerful jets reach distances of up to millions of light years. Thus, we can say that microquasars, as their name suggests, are the little siblings of the quasars, sharing multiple similarities. One of the advantages of studying microquasars is that, given their smaller size, processes inside the system and jets happen on a shorter timescale, allowing scientists to analyze rapid variabilities in their emission.

Microquasars’ outflows are efficient sites of extreme particle acceleration and are responsible for transient and persistent non-thermal radiation, spanning from radio to gamma-ray energies. Nevertheless, the emission at GeV and TeV energies from microquasars has only been sporadically observed up to this point, making these systems a class of non-thermal emitters that is actually “elusive” in the gamma-ray energy range. With its improved sensitivity compared to the current gamma-ray instruments, CTA will be fundamental to the study of these systems and the physical processes inside the jets. In particular, two microquasars, SS 433 and Cygnus X-1, have been drawing attention over the past few years.

 Prolonged observations of SS 433 with the High Altitude Water Cherenkov (HAWC) observatory were able to resolve two lobes at energies of ~20 TeV related the terminal parts of its jets, where the relativistic outflows interact with the surrounding environment [2]. According to the authors, to produce such a TeV signal, the system needs to accelerate particles up to PeV energies along the jets and, therefore, SS 433 might be a so-called Galactic PeVatron. Furthermore, a recent study with the Fermi-LAT has reported sub-TeV persistent emission from a site lying in the proximity of the eastern lobe [3]. Still some mysteries remain: What is the maximum energy to which the particles are accelerated in the jets? Does gamma-ray emission occur near or inside the binary system? What are the exact acceleration sites and mechanisms? CTA’s excellent angular resolution will play a key role in answering these questions.

 In the Cygnus region, three microquasars have been observed above 50 MeV: Cygnus X-1, Cygnus X-3 and V404 Cygni (see e.g. [4,5]). The case of Cygnus X-1 is intriguing. At GeV energies, short-time transient emission [6] and persistent emission coming from the jets [7] have been detected, while at TeV energies, only a hint during a short hard X-ray flare has been reported by MAGIC [8]. Therefore, even though theoretically predicted, a clear TeV component has not yet been detected. According to recent simulations, the CTA-North array, located in La Palma (Spain), would detect a short transient event, similar to the hint reported by MAGIC, in just a few minutes, and would be able to characterize the TeV persistent emission from the jet with a set of prolonged observations (see Figure 1).

Figure 1: CTA-North (100 GeV – 1 TeV) simulations for Cygnus X-1. Panel a.: After 30 minutes observation of a transient event, similar to the hint reported by MAGIC, CTA would clearly detect a TeV signal. Panel b.: 5h (grey triangles) and 50h observation (black points), assuming that the spectrum follows the Fermi-LAT 4FGL power-law; CTA would detect persistent emission after a few hours. Panel c.: 50h observation, assuming that the spectrum is consistent with the theoretical jet leptonic model of [9]; CTA would need more than 50 hours to detect a persistent TeV signal. The MAGIC upper limits (violet squares) in panels b and c are referred to ~83 hours of observation [10].

With CTA, we expect to unveil the timing of a possible TeV flare in a multi-wavelength context, the maximum limit of acceleration along the jets, the nature of the emission mechanisms (leptonic/hadronic) responsible for the very high-energy gamma-ray radiation and more.  Particularly, CTA’s unprecedented sensitivity between 20 GeV to 300 TeV will allow us to delve into these sources like never before: at the lowest energies, we will be able to comprehend the physics mechanisms between the GeV and TeV gamma-ray component (e.g., in Cygnus X-1) and, at the highest energies, we will be able to open a new window at the high end of the electromagnetic spectrum to study the jet-medium interaction (e.g., in SS 433). Thanks to CTA’s improved angular resolution, lower energy threshold and fast telescope repositioning to respond to external triggers for transient events, a better understanding of the physics of extreme particle acceleration in microquasars will finally be well within our grasp.

Written by: Giovanni Piano

References:

[1] Mirabel, I. F. et al., Nature 358, 215 (1992)
[2] Abeysekara, A. U. et al. (HAWC Collaboration), Nature 562, 82 (2018)
[3] Li, Jian et al., https://doi.org/10.1038/s41550-020-1164-6, Nat Astron (2020)
[4] Tavani, M. et al., Nature 462, 620 (2009)
[5] Piano, G. et al., ApJ 839, id. 84 (2017)
[6] Sabatini, S. et al., ApJL 712, L10 (2010)
[7] Zanin, R. et al., A&A 596, id. A55 (2016)
[8] Albert, J. et al., ApJ 665, L51 (2007)
[9] Zdziarski, A. A. et al., MNRAS 471, 3657 (2017)
[10] Ahnen, M. L. et al., MNRAS 472, 3474 (2017)

Source: Cherenkov Telescope Array/News

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Cyg X-3's Little Friend: A Stellar Circle of Life

Cygnus X-3

Credit: X-ray: NASA/CXC/SAO/M.McCollough et al, Radio: ASIAA/SAO/SMA  

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A snapshot of the life cycle of stars has been captured where a stellar nursery is reflecting X-rays from a source powered by an object at the endpoint of its evolution. This discovery, described in our latest press release, provides a new way to study how stars form.

This composite image shows X-rays from NASA's Chandra X-ray Observatory (white) and radio data from the Smithsonian's Submillimeter Array (red and blue). The X-ray data reveal a bright X-ray source to the right known as Cygnus X-3, a system containing either a black hole or neutron star (a.k.a. a compact source) left behind after the death of a massive star. Within that bright source, the compact object is pulling material away from a massive companion star. Astronomers call such systems "X-ray binaries."

In 2003, astronomers presented results using Chandra's high-resolution vision in X-rays to identify a mysterious source of X-ray emission located very close to Cygnus X-3 on the sky (smaller white object to the upper left). The separation of these two sources is equivalent to the width of a penny about 800 feet away. A decade later, astronomers reported the new source is a cloud of gas and dust. 

In astronomical terms, this cloud is rather small - about 0.7 light years in diameter or under the distance between the Sun and Pluto's orbit.

Astronomers realized that this nearby cloud was acting as a mirror, reflecting some of the X-rays generated by Cygnus X-3 towards Earth. They nicknamed this object the "Little Friend" due to its close proximity to Cygnus X-3 on the sky and because it also demonstrated the same 4.8-hour variability in X-rays seen in the X-ray binary.

To determine the nature of the Little Friend, more information was needed. The researchers used the Submillimeter Array (SMA), a series of eight radio dishes atop Mauna Kea in Hawaii, to discover the presence of molecules of carbon monoxide. This is an important clue that helped confirm previous suggestions that the Little Friend is a Bok globule, small, dense, very cold clouds where stars can form. The SMA data also reveal the presence of a jet or outflow within the Little Friend, an indication that a star has started to form inside. The blue portion shows a jet moving towards us and the red portion shows a jet moving away from us.

These results were published in The Astrophysical Journal Letters, and the paper is also available online. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

Fast Facts for Cyg X-3's Little Friend:

Scale: Image is 1.4 arcmin across (about 8.15 light years)
Category: Normal Stars & Star Clusters, Neutron Stars/X-ray Binaries
Coordinates (J2000): RA 20h 32m 25.50s | Dec +40° 57' 27.70"
Constellation: Cygnus
Observation Date: 26 Jan 2006
Observation Time: 13 hours 46 min
Obs. ID: 6601
Instrument: ACIS
References: McCollough, M. et al, 2016, ApJL, 830, L36; arXiv:1610.01923
Color Code: X-ray (Purple); Radio (Blue, Red)
Distance Estimate: About 20,000 light years

Source: NASA’s Chandra X-ray Observatory

Integral Spots Matter a Millisecond from Doom

An artist's impression of the Cygnus X-1 black hole system. Gas from a nearby supergiant star spirals down into the black hole but a small fraction is diverted by magnetic fields into jets that shoot back into space. Credits: ESA

ESA’s Integral gamma-ray observatory has spotted extremely hot matter just a millisecond before it plunges into the oblivion of a black hole. But is it really doomed? These unique observations suggest that some of the matter may be making a great escape.

No one would want to be so close to a black hole. Just a few hundred kilometres away from its deadly surface, space is a maelstrom of particles and radiation. Vast storms of particles are falling to their doom at close to the speed of light, raising the temperature to millions of degrees.

Ordinarily, it takes just a millisecond for the particles to cross this final distance but hope may be at hand for a small fraction of them.

Thanks to the new Integral observations, astronomers now know that this chaotic region is threaded by magnetic fields.

This is the first time that magnetic fields have been identified so close to a black hole. Most importantly, Integral shows they are highly structured magnetic fields that are forming an escape tunnel for some of the doomed particles.

The Imager on Board the Integral Satellite (IBIS) was first activated and put through its paces in November 2002. It captured this image during that test phase and shows not only Cygnus X-1 (centre) but also Cygnus X-3 (upper left). High-energy sources are shown with an 'X' followed by a number according to their strength. Cygnus X-3 is the third brightest high-energy emitter in the constellation of Cygnus, the Swan. Instead of a black hole, Cygnus X-3 is thought to be a neutron star (a tiny dead stellar core) pulling its companion star to pieces. Taken on 16 November 2002, the new IBIS observations support this theory. Cygnus X-1 is about 10 000 light years from Earth and one of the brightest high-energy emitters in the sky. It was discovered in 1966 and is thought to be a black hole, ripping its companion star to pieces. The companion star, HDE 226868, is a blue supergiant with a surface temperature of around 31 000 K. It orbits the black hole once every 5.6 days. Credits: ESA. Original image by the Integral IBIS team. Image processing by ESA/ECF.

Philippe Laurent, CEA Saclay, France, and colleagues made the discovery by studying the nearby black hole, Cygnus X-1, which is ripping a companion star to pieces and feeding on its gas.

Their evidence points to the magnetic field being strong enough to tear away particles from the black hole’s gravitational clutches and funnel them outwards, creating jets of matter that shoot into space. The particles in these jets are being drawn into spiral trajectories as they climb the magnetic field to freedom and this is affecting a property of their gamma-ray light known as polarisation.

A gamma ray, like ordinary light, is a kind of wave and the orientation of the wave is known as its polarisation. When a fast particle spirals in a magnetic field it produces a kind of light, known as synchrotron emission, which displays a characteristic pattern of polarisation. It is this polarisation that the team have found in the gamma rays. It was a difficult observation to make.

“We had to use almost every observation Integral has ever made of Cygnus X-1 to make this detection,” says Laurent.

This is an artist’s impression of ESA’s orbiting gamma-ray observatory, Integral.

Credits: ESA

Amassed over seven years, these repeated observations of the black hole now total over five million seconds of observing time, the equivalent of taking a single image with an exposure time of more than two months. Laurent’s team added them all together to create just such an exposure.

“We still do not know exactly how the infalling matter is turned into the jets. There is a big debate among theoreticians; these observations will help them decide,” says Laurent.

Jets around black holes have been seen before by radio telescopes but such observations cannot see the black hole in sufficient detail to know exactly how close to the black hole the jets originate. That makes these new observations invaluable.

"This discovery of polarized emission from a black hole jet is a unique result demonstrating that Integral, which is covering the high-energy band in ESA's wide spectrum of scientific missions, continues to produce key results more than eight years after its launch," says Christoph Winkler, ESA Integral Project Scientist.

Contact for further information

Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Email: markus.bauer@esa.int
Tel: +31 71 565 6799
Mob: +31 61 594 3 954

Philippe Laurent
Integral/IBIS Instrument Scientist
IRFU / Service d'Astrophysique, CEA Saclay
Laboratoire APC
Email: philippe.laurent@cea.fr
Tel: +33 1 69 08 80 66 / +33 1 57 27 60 72

Christoph Winkler
ESA Integral Project Scientist
Email: cwinkler@rssd.esa.int
Tel: +31 71 565 3591

Notes for editors

Polarized Gamma-ray Emission from the Galactic Black Hole Cygnus X-1 by P. Laurent et al. is published online by Science today and will appear in a future issue of the printed journal.

Fermi Telescope Peers Deep into Microquasar

In Cygnus X-3, an accretion disk surrounding a black hole or neutron star orbits close to a hot, massive star. Gamma rays (purple, in this illustration) likely arise when fast-moving electrons above and below the disk collide with the star's ultraviolet light. Fermi sees more of this emission when the disk is on the far side of its orbit. Credit: NASA's Goddard Space Flight Center

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Brighter colors indicate greater numbers of gamma rays detected in this Fermi LAT view of a region centered on the position of Cygnus X-3 (circled). The brightest sources are pulsars. Credit: NASA/DOE/Fermi LAT Collaboration

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This image locates the view around Cygnus X-3 within Fermi's all-sky map.

Credit: NASA/DOE/Fermi LAT Collaboration

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NASA's Fermi Gamma-ray Space Telescope has made the first unambiguous detection of high-energy gamma-rays from an enigmatic binary system known as Cygnus X-3. The system pairs a hot, massive star with a compact object -- either a neutron star or a black hole -- that blasts twin radio-emitting jets of matter into space at more than half the speed of light.

Astronomers call these systems microquasars. Their properties -- strong emission across a broad range of wavelengths, rapid brightness changes, and radio jets -- resemble miniature versions of distant galaxies (called quasars and blazars) whose emissions are thought to be powered by enormous black holes.

"Cygnus X-3 is a genuine microquasar and it's the first for which we can prove high-energy gamma-ray emission," said Stéphane Corbel at Paris Diderot University in France.

The system, first detected in 1966 as among the sky's strongest X-ray sources, was also one of the earliest claimed gamma-ray sources. Efforts to confirm those observations helped spur the development of improved gamma-ray detectors, a legacy culminating in the Large Area Telescope (LAT) aboard Fermi.

At the center of Cygnus X-3 lies a massive Wolf-Rayet star. With a surface temperature of 180,000 degrees F, or about 17 times hotter than the sun, the star is so hot that its mass bleeds into space in the form of a powerful outflow called a stellar wind. "In just 100,000 years, this fast, dense wind removes as much mass from the Wolf-Rayet star as our sun contains," said Robin Corbet at the University of Maryland, Baltimore County.

Every 4.8 hours, a compact companion embedded in a disk of hot gas wheels around the star. "This object is most likely a black hole, but we can't yet rule out a neutron star," Corbet noted.

Fermi's LAT detects changes in Cygnus X-3's gamma-ray output related to the companion's 4.8-hour orbital motion. The brightest gamma-ray emission occurs when the disk is on the far side of its orbit. "This suggests that the gamma rays arise from interactions between rapidly moving electrons above and below the disk and the star's ultraviolet light," Corbel explained.

When ultraviolet photons strike particles moving at an appreciable fraction of the speed of light, the photons gain energy and become gamma rays. "The process works best when an energetic electron already heading toward Earth suffers a head-on collision with an ultraviolet photon," added Guillaume Dubus at the Laboratory for Astrophysics in Grenoble, France. "And this occurs most often when the disk is on the far side of its orbit."

Through processes not fully understood, some of the gas falling toward Cygnus X-3's compact object instead rushes outward in a pair of narrow, oppositely directed jets. Radio observations clock gas motion within these jets at more than half the speed of light.

Between Oct. 11 and Dec. 20, 2008, and again between June 8 and Aug. 2, 2009, Cygnus X-3 was unusually active. The team found that outbursts in the system's gamma-ray emission preceded flaring in the radio jet by roughly five days, strongly suggesting a relationship between the two.

The findings, published today in the electronic edition of Science, will provide new insight into how high-energy particles become accelerated and how they move through the jets.

Related Links:

Fermi Telescope Caps First Year With Glimpse of Space-Time
Gamma-Rays from High-Mass X-Ray Binaries

Francis Reddy
NASA's Goddard Space Flight Center