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Physical Sciences ︱ A. Miguel Holgado Refining the search for the largest gravitational waves

Ask many astronomers, and they n September 2015, the world of LIMITATIONS IN will tell you that gravitational was changed forever, as LIGO’S DETECTIONS The Fermi Gamma-Ray Space Telescope’s view of the gamma-ray sky. The disk of gamma-ray emission is from our own . A significant fraction waves are the greatest scientific Iripples in , first theorised So far, every LIGO has of the point-like gamma-ray sources above and below the Milky Way are gamma-ray emission from . discovery of the by Einstein over a century ago, were detected has originated from mergers of so far. In his research, Miguel directly observed for the first time. holes and neutron stars with similar been relying on a different technique to Holgado at the University The gravitational wave, discovered to our . However, astronomers detect nanohertz gravitational waves by If gravitational waves pass through a of Illinois studies the clever by a global team of researchers at the predict that far larger waves could exploiting the most reliable timekeepers astronomical techniques which LIGO- collaboration, was created originate from ‘supermassive’ black holes known to astronomy. and the Earth, the arrival times can be used to observe the very as two black holes spiralled into each – objects many millions of times heavier of the pulses are shifted due to the largest of these elusive ripples, other and merged, releasing colossal than the Sun, which are thought to occupy RELIABLE ASTRONOMICAL originating from supermassive amounts of energy. the centres of many . TIMEKEEPERS stretching and squeezing of spacetime. black holes as they orbit around Astronomers were baffled when each other. His results suggest Using kilometre-scale interferometers, As Holgado explains, these waves can they first discovered a number of and the Earth, the observed pulse times- ‘Even though PTAs have yet to detect that these binaries are likely accurate to within the widths of single form when two galaxies merge together, mysterious objects which appeared of-arrival are shifted due to the stretching nanohertz gravitational waves, their upper far rarer than astronomers atoms, physicists picked up the wave causing each of their supermassive black to emit relentlessly regular flashes and squeezing of spacetime,’ he says. limits are of astrophysical relevance and originally thought, but could as it passed through Earth, minutely holes to orbit each other. ‘Some of the of high-energy radiation. The pulses importance,’ Holgado says. ‘A number also help them to refine their stretching and squeezing the fabric of loudest gravitational waves are expected kept time so accurately that some Astronomers are now using these of telescopes have been finding and observation techniques. spacetime as it went. Nearly a dozen from the orbital motion of supermassive researchers were even drawn to extra- theorised shifts to monitor changes in observing active galactic nuclei (AGN), more gravitational wave observations binaries that may form within terrestrial explanations of their origin groups, or arrays, of these distorted pulsar which are the centres of galaxies that are have been made since then, and again the centres of merged galaxies,’ he says. at first. However, as Jocelyn Bell and signals, over periods of several years. ‘In observed to be very bright compared to this year, astronomers are turning their Yet despite their enormous size, these Antony Hewish famously discovered order to detect gravitational waves, we normal galaxies due to a supermassive attention to detecting mergers of black waves are notoriously difficult to detect. in 1967, these signals have a perfectly can time an array of and correlate black hole feeding on hot gas.’ holes, neutron stars, and more. Where the ripple observed by LIGO- natural explanation. the observed shifts of the pulse arrival Virgo in 2015 dramatically completed times for each pulsar,’ Holgado continues. In several of these bright galactic centres, a cycle in fractions of a millisecond, The duo revealed that the pulses are ‘A (PTA) is a particular astronomers have found that the those waves from supermassive black actually beams of radiation, which are type of gravitational wave detector that is they emit dips and rises at regular hole binaries with periods of years continually emitted from the poles of sensitive to nanohertz gravitational waves intervals. As Holgado explains, ‘some to decades can endure for thousands rapidly-rotating neutron stars – small, from supermassive black hole binaries.’ AGN have light curves that show some to millions of years. dense objects which were once the cores periodic behaviour, which is sometimes of massive stars. Once every rotation, A GROWING LIST thought to come from the orbital motion ‘LIGO cannot detect gravitational- one of these beams would point straight OF BINARY CANDIDATES of a supermassive black hole binary.’ waves from these sources because towards the Earth, resulting in a regular This technique is vital for detecting low- the frequencies from in-spiralling sequence of flashes from our perspective. frequency gravitational waves that LIGO If the orbit of the black hole binary is supermassive black hole binaries are is not sensitive to. However, observations inclined relative to our perspective, then expected to be of order nanohertz,’ The pulses are emitted with unwavering of PTA signal shifts have yet to yield each black hole would routinely move Holgado continues. ‘LIGO is only sensitive uniformity like a highly precise concrete evidence of the low-frequency towards and away us as they orbit each to gravitational wave frequencies of order metronome, but as Holgado explains, ripples. All the same, researchers have other. This motion relative to our line of tens to thousands of hertz, which mostly this could appear to change from our now begun the search in earnest; paying sight would cause the bright material An artist’s depiction of a , which is thought correspond to the mergers of stellar- perspective if a pulsar signal with was particular attention to the dense, bright surrounding the black holes to become to be an active supermassive black hole with a relativistic jet pointed along our line of sight. black hole binaries and neutron- affected by a gravitational wave. ‘If clusters of gas which collect at the centres even brighter when the motion is star binaries.’ Astronomers have thus gravitational waves pass through a pulsar of some galaxies. towards us and to become dimmer when

www.researchoutreach.org www.researchoutreach.org the motion is away from us. Another possibility is that if the orbit is completely face-on, i.e., no motion towards nor Behind the Research away from us, then the gaseous material can still fall into the black holes in a manner driven by the orbital motion of A. Miguel Holgado the binary. The rate at which the material falls into the black holes can increase E: [email protected] W: https://amholgado.gitlab.io/ and decrease on orbital timescales, and as this rate changes, so does References the luminosity of the gas that is falling Research Objectives in. Astronomers can, therefore, refine Holgado, A.M., Sesana, A., Sandrinelli, A., Covino, S., Treves, A. Miguel Holgado’s research focuses on the gravitational their search for nanohertz-frequency wave signatures of compact objects such as black holes, A., Liu, X. and Ricker, P. (2018). Pulsar timing constraints on the gravitational waves by pointing their neutron stars, and white dwarfs. Fermi massive black hole binary blazar population. Monthly telescopes at those periodic AGN Notices of the Royal Astronomical Society: Letters, 481(1), positioned behind PTAs. Detail L74-L78.

LACK OF GRAVITATIONAL WAVES Miguel Holgado Sesana, A., Haiman, Z., Kocsis, B., and Kelley, L. (2018). If astronomers were right about 1002 W. Green St., Testing the Binary Hypothesis: Pulsar Timing Constraints supermassive black hole binaries being Urbana IL, 61801 on Supermassive Black Hole Binary Candidates. The An artist’s depiction of a pulsar timing array, where a set of pulsars are observed for evidence of the main cause of periodic AGN, we gravitational waves from the cosmic population of supermassive black hole binaries with year-like Astrophysical Journal, Volune 856, Number 1. might expect to have a rapidly-growing orbital periods. Bio database of nanohertz gravitational wave A. Miguel Holgado completed his undergraduate studies observations by now. Unfortunately, Using PTA upper limits, we have shown at Texas A&M University. He is now a PhD Candidate at however, this is not the case. ‘The growing the University of Illinois at Urbana-Champaign researching Personal Response number of candidates that the telescopes that only a small fraction of blazars may gravitational-wave astrophysics. Miguel will be attending are finding imply a population of binaries the 69th Lindau Nobel Laureate Meeting, which will focus that may be emitting gravitational waves, be binaries. on Physics. There is increased effort towards finding potential which pulsar timing arrays are sensitive supermassive black hole binaries. With pulsar timing arrays improving their upper limits, you must be excited to,’ Holgado continues. ‘However, pulsar towards us, making them particularly far smaller proportion of periodic AGN Funding about the future in this field. timing arrays have not yet detected bright among AGN,’ he explains. ‘NASA’s than astronomers originally thought. NANOGrav Physics Frontier Center, NSF PIRE, gravitational waves. This must mean Fermi Gamma-Ray Space Telescope has Instead, Holgado theorises that a DOE Krell Institute Very much so! Even without a direct detection of nanohertz gravitational waves, we can still do multi- that some fraction of the candidate obtained the most complete census of majority of periodic AGN must consist messenger astrophysics, where electromagnetic supermassive black hole binaries in AGN blazars out to distances farther than what of uneven distributions of light-emitting Collaborators observations from telescopes tell us about the possible may be false detections.’ pulsar timing arrays can observe.’ gas orbiting individual supermassive • Alberto Sesana presence of binaries from periodic AGN and gravitational- black holes. • Angela Sandrinelli wave upper limits tell us what fraction of these binary If astronomers are wrong about Many blazars exhibit the light variations • Stefano Covino candidates may be real. Once pulsar timing arrays finally supermassive black hole binaries which caused astronomers to theorise ‘Using PTA upper limits, we have shown • Aldo Treves detect nanohertz gravitational waves, we hope to gain a being the sole cause of periodic AGN, supermassive black hole binaries in the that only a small fraction of blazars, • Xin Liu clearer picture of how supermassive black hole binaries determining the proportion which are first place, giving Holgado a highly- around 0.01 to 0.1%, may be binaries, • Paul Ricker form and how this is tied to the evolution of galaxies over cosmic time. responsible for them could be vital developed database of observations which means that the quasi-periodicity for gravitational wave observations in to work with. ‘Blazars are notorious for often seen in blazars is likely to be due the future. In his research, Holgado is showing quasi-periodic behaviour in their to periodic motions of the hot gas working to determine just how many light curves at a variety of wavelengths, surrounding the supermassive black hole,’ time-variable AGN can be explained which is also sometimes thought to be he explains. by black hole binaries. due to the presence of a binary,’ he continues. With this information, Holgado Such a low fraction may be somewhat DETECTIONS IN BLAZARS can use PTA upper limits on nanohertz disappointing to some astronomers, When AGN are particularly bright, they gravitational waves to directly quantify the but the result could prove vital in will release vast amounts of energy proportion of periodic blazars which may ensuring potential future observations through colossal jets, which point host binaries. of gravitational waves originating from perpendicular to the galaxy’s disc. supermassive black hole binaries are AGN which display these jets are called LIMITED EVIDENCE FOR genuine. The constraints on the binary ‘’, and in special cases, they point SUPERMASSIVE BINARIES fraction also provide some insights into straight towards Earth, making them ideal So far, Holgado’s results have revealed how efficiently supermassive black hole for our observation. The data gathered that only a small fraction of blazars can binaries from in the centers of galaxies. in observations of these special quasars, host gravitational wave emitting binaries In the future, Holgado’s work could play called ‘blazars’ is of particular interest to that would be consistent with PTA upper an important role in our understanding Holgado. ‘Blazars are a particular type limits. This suggests that supermassive of the dynamic ripples in spacetime which of AGN whose jets are pointed directly black hole binaries are responsible for a we now know to permeate our . Photography/Shutterstock.com DnG

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