space science

New observations of the first black hole ever detected have led astronomers to question what they know about the Universe’s most mysterious objects.

Published today in the journal Science, the research shows the system known as Cygnus X-1 contains the most massive stellar-mass black hole ever detected without the use of gravitational waves.

Cygnus X-1 is one of the closest black holes to Earth. It was discovered in 1964 when a pair of Geiger counters were carried on board a sub-orbital rocket launched from New Mexico.

The object was the focus of a famous scientific wager between physicists Stephen Hawking and Kip Thorne, with Hawking betting in 1974 that it was not a black hole. Hawking conceded the bet in 1990.

In this latest work, an international team of astronomers used the Very Long Baseline Array—a continent-sized radio telescope made up of 10 dishes spread across the United States—together with a clever technique to measure distances in space.

“If we can view the same object from different locations, we can calculate its distance away from us by measuring how far the object appears to move relative to the background,” said lead researcher, Professor James Miller-Jones from Curtin University and the International Centre for Radio Astronomy Research (ICRAR).

“If you hold your finger out in front of your eyes and view it with one eye at a time, you’ll notice your finger appears to jump from one spot to another. It’s exactly the same principle.”

“Over six days we observed a full orbit of the black hole and used observations taken of the same system with the same telescope array in 2011,” Professor Miller-Jones said. “This method and our new measurements show the system is further away than previously thought, with a black hole that’s significantly more massive.”

Co-author Professor Ilya Mandel from Monash University and the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav) said the black hole is so massive it’s actually challenging how astronomers thought they formed.

“Stars lose mass to their surrounding environment through stellar winds that blow away from their surface. But to make a black hole this heavy, we need to dial down the amount of mass that bright stars lose during their lifetimes” he said.

“The black hole in the Cygnus X-1 system began life as a star approximately 60 times the mass of the Sun and collapsed tens of thousands of years ago,” he said. “Incredibly, it’s orbiting its companion star—a supergiant—every five and a half days at just one-fifth of the distance between the Earth and the Sun.

“These new observations tell us the black hole is more than 20 times the mass of our Sun—a 50 per cent increase on previous estimates.”

Xueshan Zhao is a co-author on the paper and a PhD candidate studying at the National Astronomical Observatories—part of the Chinese Academy of Sciences (NAOC) in Beijing.

“Using the updated measurements for the black hole’s mass and its distance away from Earth, I was able to confirm that Cygnus X-1 is spinning incredibly quickly—very close to the speed of light and faster than any other black hole found to date,” she said.

“I’m at the beginning of my research career, so being a part of an international team and helping to refine the properties of the first black hole ever discovered has been a great opportunity.”

Next year, the world’s biggest radio telescope—the Square Kilometre Array (SKA)—will begin construction in Australia and South Africa.

“Studying black holes is like shining a light on the Universe’s best kept secret—it’s a challenging but exciting area of research,” Professor Miller-Jones said.

“As the next generation of telescopes comes online, their improved sensitivity reveals the Universe in increasingly more detail, leveraging decades of effort invested by scientists and research teams around the world to better understand the cosmos and the exotic and extreme objects that exist.

It’s a great time to be an astronomer.”

Accompanying the publication in Science, two further papers focusing on different aspects of this work have also been published today in The Astrophysical Journal.

Original Publication:

‘Cygnus X-1 contains a 21-solar mass black hole – implications for massive star winds’, published in Science on February 18^th, 2021.

Companion Papers:

‘Reestimating the Spin Parameter of the Black Hole in Cygnus X-1’, published in The Astrophysical Journal on February 18th, 2021.

‘Wind mass-loss rates of stripped stars inferred from Cygnus X-1’, published in The Astrophysical Journal on February 18^th, 2021.

Contact Person (ISS): Dr. Valeriu Tudose <tudose[at]spacescience[dot]ro>

Photo Gallery:

An animation showing the Cygnus X-1 system, containing a black hole 21 times the mass of the Sun orbiting a star that’s 41 times the mass of the Sun. Recent observations by radio telescopes have found the system is 20 per cent further away than previously thought, making it the most massive stellar-mass black hole ever detected without the use of gravitational waves. Credit: International Centre for Radio Astronomy Research.

The Pierre Auger Collaboration is releasing 10% of the data recorded using the world’s largest cosmic ray detector. These data are being made available publicly with the expectation that they will be used by a wide and diverse community including professional and citizen-scientists and for educational and outreach initiatives. While the Auger Collaboration has released data in a similar manner for over a decade, the present release is much wider with regard to both the quantity and type of data, making them suitable both for educational purposes and for scientific research. The data can be accessed at www.auger.org/opendata [1]

Operation of the Pierre Auger Observatory, by a Collaboration of about 400 scientists from over 90 institutions in 18 countries across the world, has enabled the properties of the highest-energy cosmic rays to be determined with unprecedented precision. These cosmic rays are predominantly the nuclei of the common elements and reach the Earth from astrophysical sources. The data from the Observatory have been used to show that the highest-energy particles have an extra-galactic origin. The energy spectrum of cosmic rays has been measured beyond 10²⁰ eV, corresponding to a macroscopic value of about 16 joules in a single particle. It has been demonstrated that there is a sharp fall of the flux at high energy, and emerging evidence of emission from particular near-by sources has been uncovered. Analyses of the data have allowed characterisation of the type of particles that carry these remarkable energies, which include elements ranging from hydrogen to silicon. The data can also be used to test particle physics at energies beyond the reach of the LHC.

At the Pierre Auger Observatory [2], located in Argentina, cosmic rays are observed indirectly, through extensive air-showers of secondary particles produced by the interaction of the incoming cosmic ray with the atmosphere. The Surface Detector of the Observatory covers 3000 km² and comprises an array of particle detectors separated by 1500 m. The area is overlooked by a set of telescopes that compose the Fluorescence Detector which is sensitive to the auroral-like light emitted as the air-shower develops, while the Surface Detector is sensitive to muons, electrons and photons that reach the ground. The data from the Observatory comprises the raw ones, obtained directly from these and other instruments, through reconstructed data sets generated by detailed analysis, up to those presented in scientific publications. Some of the data are routinely shared with other observatories to allow analyses with full-sky coverage and to facilitate multi-messenger studies.

As pointed out by the spokesperson, Ralph Engel, “the data from the Pierre Auger Observatory, which was founded more than 20 years ago, are the result of a vast and long-term scientific, human, and financial investment by a large international collaboration. They are of outstanding value to the worldwide scientific community.” By releasing data and analysis programs to the public, the Auger Collaboration upholds the principle that open access to the data will, in the long term, allow the maximum realization of their scientific potential.

The Auger Collaboration has adopted a classification of four levels of complexity of their data, following that used in high-energy physics, and adapted it for its open-access policy:

(Level 1) Open-access publication with additional numerical data provided to facilitate re-use [3];

(Level 2) Regular release of cosmic-ray data in a simplified format, for education and outreach. This began in 2007 when 1% of the data was released and increased to 10% in 2019 [4];

(Level 3) Release of reconstructed cosmic-ray events, selected with the best available knowledge of the detector performance and conditions at the time of data-taking. Example codes derived from those used by the Collaboration for published analyses are also provided [5];

(Level 4) Release of close-to-raw data associated with those events. An event-display, and codes to read these data, are also provided [6].

The last two levels of information are added in the present release [1], which includes data from the two major instruments of the Observatory, the 1500 m array of the Surface Detector and the Fluorescence Detector. The dataset consists of 10% of all the events recorded at the Observatory, subjected to the same selection and reconstruction procedures used by the Collaboration in recent publications. The periods of data recording are the same as used for the physics results presented at the International Cosmic Ray Conference held in 2019. The examples of analyses use updated versions of the Auger data sets, which differ slightly from those used for the publications because of subsequent improvements to the reconstruction and calibration. On the other hand, as the fraction of data which is now available is currently 10% of the actual Auger data sample, the statistical significances of measured quantities are reduced with respect to what can be achieved with the full dataset, but the number of events is comparable to what was used in some of the first scientific publications by the Auger Collaboration.

The Pierre Auger Collaboration is committed to its open data policy, in order to increase the diversity of people accessing scientific data and so the common scientific potential for the future.

Links:

[1] https://www.auger.org/opendata/

[2] https://www.auger.org

[3] https://www.auger.org/index.php/science

[4] https://labdpr.cab.cnea.gov.ar/ED/

[5] https://www.auger.org/opendata/analysis.php

[6] https://www.auger.org/opendata/display.php?evid=81847956000

Photos of the Pierre Auger Observatory (CC BY-SA 2.0):

https://www.flickr.com/photos/134252569@N07/21948576246/in/album-72157656013297308/

This year, the Noble Prize in Physics, announced in October, has been divided between Roger Penrose, from University of Oxford, UK, “for the discovery that black hole formation is a robust prediction of the general theory of relativity” and Reinhard Genzel, from Max-Planck-Institut für Extraterrestrische Physik, Germany, along with Andrea Ghez, from University of California, USA, “for the discovery of a supermassive object, compact in the center of our galaxy”, conform the official press release.

The three laureates that share this years’ Nobel Prize in Physics have contributed to the discovery of the most exotic objects in the Universe, black holes.

In 1965, 10 years after the death of Albert Einstein, Roger Penrose has managed to prove the existence and describe in detail the formation and properties of black holes, starting from the theory of relativity and using revolutionary mathematical methods. Thus, Penrose proved that these super-massive objects that capture everything that falls inside them, and around which the classic laws of physics no longer apply, are a direct consequence to Einstein’s general theory of relativity. The article in which Roger Penrose has published these results it is still considered today as being the second most important contribution to the theory of relativity after the works of Einstein.

Twenty-five years later, in 1990, Reinhard Genzel and Andrea Ghez led two teams of astronomers that have studied, independently from one another, the center of our galaxy, more exactly the region named Sagittarius A*. The two teams observed closely the unusual behavior of nearby stars in that region of the Milky Way and deducted that these were in the vicinity of a supermassive compact object, having a mass of a couple millions times greater than our Sun and occupying a region equivalent to about the size of our Solar System. So far, the only object whose characteristics can explain the topology and dynamics of this region is a supermassive black hole.

The discovery of this object is important not only because it proves Einstein’s theory and Penrose’s calculations, but also because in order to make these observations, the limits of the technology and data processing tools existing at that moment have been pushed further, leading to progress in observational astrophysics.

The Institute of Space Science (ISS) is actively doing research in astrophysics in general, and on massive and supermassive black holes in particular, with contributions such as new concepts and theories about black holes, catalogs containing the mass of black holes and simulations of their formation, growth and evolution. ISS is involved in the space research field also, for instance, through the participation in the LISA space mission, designed and built by the European Space Agency (ESA), mission that aims to study gravitational wave signals coming from the collision of massive objects, including black holes, and to identify the mechanisms of the formation and evolution of black holes, from their birth until now. The Romanian Space Agency (ROSA) continuously supports the Romanian contributions to space research, including the LISA mission, in which our country is being anchored in pioneering research of the study of gravitational waves in space.

Contact person: dr. Laurențiu Caramete <lcaramete[at]spacescience[dot]ro>

The energy spectrum of the highest-energy particles in the Universe, ultra-high energy cosmic rays, has been measured with the Pierre Auger Observatory with an unprecedented precision. In addition to the well-known kink in the energy spectrum, typically referred to as the ankle, a new spectral break is found at somewhat higher energy. This new break in the energy spectrum can be explained by an energy-dependent mass composition of cosmic rays. The results are published in two related papers (Phys. Rev. Lett. 125, 121106, 2020 and Phys. Rev. D 102, 062005, 2020).

This determination of the energy spectrum is unique in having an unprecedented exposure of more than 60,000 km² sr yr, in its method of determining the spectrum free of assumptions about the mass composition of the initial cosmic ray particle, and about details of the hadronic physics of air showers.

Ultra-high energy cosmic rays (UHECRs) are particles that reach energies of up to 10²⁰ eV, the highest energies of individual particles known in the Universe. With our currently available technology, the LHC accelerator would have to be scaled to the size of the orbit of the planet Mercury to reach this energy. The flux of these particles is extremely small. Less than one particle per century arrives on an area of a square-kilometer. There is a long-standing quest to identify the sources of these particles and the processes that give them such exceptional energies.

The Pierre Auger Collaboration, a group of about 400 scientists from 17 countries from all over the world, is operating the world’s largest observatory for cosmic rays: a hybrid detector made of more than 1600 surface water-Cherenkov stations covering a 3,000 km² area, which is overlooked by 27 fluorescence telescopes. Together, the different instruments provide calorimetric measurements of the energies of particle cascades produced by UHECRs in the atmosphere and an indirect evaluation of the mass of the primary particle. Combining the information on the energy spectrum, mass composition and the observed arrival direction distribution, important constraints on the sources of these extraordinary particles can be derived.

Analyzing the data collected by the Pierre Auger Observatory so far, the energy spectrum of UHECRs has been determined with very high statistics. Thanks to the unprecedented precision of the measurement, a new spectral feature, a break in the power law at about 1.3´10¹⁹ eV, has been identified. The results are reported in two recent publications (Phys. Rev. Lett. 125, 121106, 2020 and Phys. Rev. D 102, 062005, 2020) of the Pierre Auger Collaboration and are illustrated in Figure 1, which shows a possible interpretation of the observed flux and composition data of UHECRs in a scenario with sources that inject particles with a mass composition that changes with energy. The shown example represents a particular class of models, in which the acceleration of particles depends only on their rigidity (energy divided by charge). The abundance of nuclear elements appears to be dominated by intermediate-mass nuclei that are released from the sources with a very hard energy spectrum, which is modified by extragalactic propagation effects. In such a model scenario, the new feature in the spectrum would naturally occur due to the change of composition in the energy range of the new spectral break.

The observed energy spectrum also determines the energy density injected as UHECRs by continuously emitting sources into extragalactic space. Interestingly, some classes of Active Galactic Nuclei and Starburst Galaxies, for which indications of anisotropy have been obtained in different analyses of the Pierre Auger Collaboration, are expected to provide this energy production rate: an intriguing step forward in the quest for the UHECR sources.

The Pierre Auger Observatory is currently undergoing a large-scale upgrade by adding scintillation detectors and radio antennas on top of the existing water-Cherenkov detector stations. This will allow the scientists to obtain more information about the UHECR mass composition, extending it to the highest energies where a possible presence of light mass nuclei could open a new window to composition-sensitive searches for sources and studies of cosmic magnetic fields.

Romania has fully joined the Pierre Auger Collaboration in 2014, and its current contribution comes from the following three institutions: Institute of Physics and Nuclear Engineering Horia Hulubei (IFIN-HH), Institute of Space Science (ISS) and University Polytechnic Bucharest (UPB). Since 2019, fluorescence detectors of the Pierre Auger Observatory are fully monitored and operated, upon a common collaborative measurement calendar, also remotely from Romania, which is “from ISS-Măgurele a step forward to the experiment in the Argentinian pampas”. The Auger-group at ISS contributes also to the mass simulation production of Auger measured events using distributed computing in frame of Auger GRID VO (Virtual Organization), as well as to disseminating and awareness of physics studied at Auger, contributing thus to education through science.

Figure 1: All-particle flux of the highest energy cosmic rays as measured with the Pierre Auger Observatory, scaled by E³. The data are compared with a representative model scenario for sources, illustrating the correlation between the energy of the new spectral feature and the energy-dependent mass composition of the particles.

Contact person: Dr. Gina Isar <gina.isar[at]spacescience.ro>, (ISS) Institutional Responsible in Auger