News Article | April 17, 2017
The international Event Horizon Telescope (EHT) Collaboration, which is imaging for the first time the black-hole candidate at the center of our Milky Way, has a major research focus in Germany. A significant contribution to this experiment is part of "BlackHoleCam", a German-Dutch experiment founded in 2014. The research group of Prof. Rezzolla at the Institute for Theoretical Physics at the Goethe University Frankfurt is part of the collaboration. BlackHoleCam is supported by the European Research Council via an ERC Synergy Grant of 14 Million Euros. Due to the strong pull of gravity, not even light can escape from black holes, whose surface, i.e., the event horizon, cannot be observed directly. However, the boundary which separates photons that are trapped from those that can escape from the incredible gravitational pull is called the black-hole "shadow", because it would appear as a shadow against a bright lit background. It is such a shadow that is the target of series of observations presently ongoing of Sgr A*, the name of the black-hole candidate in our Milky Way. During the observations, the researchers will analyze the radio emission emitted by Sgr A*, whose mass is 4.5 million times that of our Sun and whose shadow is about half of the size of the distance between the Sun and the Earth. Despite being so massive, Sgr A* is also very far from us, at a 26,000 light years, making the angular size of the shadow extremely small. Measuring the emission from this surface is therefore equivalent to imaging an apple on the surface of the Moon. To accomplish this ambitious project several radio telescopes across the globe are connected and thus form a virtual telescope with a diameter comparable to the Earth. This technique is called Long Baseline Interferometry (VLBI). The work of BlackHoleCam is lead by Prof. Luciano Rezzolla (ITP, Frankfurt), Prof. Michael Kramer (Max Planck Institute for Radio Astronomy, Bonn), and by Prof. Heino Falcke (Radboud-University Nijmegen, Netherlands); all of them are important contributors of the EHT collaboration. In the current observations of Sgr A*, network of radiotelescopes from Europe, the United States of America, Middle- and South America, and the South Pole telescope are participating at the same time. During the observations, each telescope records the data on hard disks which are shipped after the end of the campaign to one of the high-performance computer centers in the US or to Bonn. In these centers the individual data of the telescopes are combined by supercomputers and an image can be reconstructed. This shadow image can be regarded as the starting point for the theoretical research of Prof. Rezzolla's group. Besides predicting theoretically what type of image scientists is expected to observe, the group in Frankfurt is also working on determining whether it will be possible to establish if Einstein's theory of general relativity is the correct theory of gravity. There are several other theories of gravity besides the well-known one by Einstein and the observations of the black-hole shadow may help to identify the true one. Because of this, scientists in Frankfurt analyze the size and the geometry of the shadow and compare them to synthetic images generated on supercomputers which model accretion flows onto black holes.. These images are computed by solving the equations of relativistic magneto-hydrodynamics and tracing the orbit of photons around black holes in different theories of gravity using state-of-the art numerical tools developed in the group of Prof. Rezzolla. Comparing the synthetic shadow to the observed one may shed light on the existence of one of the most extreme predictions of Einstein's theory of gravity: the existence of black holes. However, as Prof. Rezzolla remarks, "These observations represent a major step forward in the international attempt of understanding the nature of the dark and compact object at the centre of our Galaxy. However, they are just the first step and it is likely that many more observations of increasing precision will be necessary for finally settling this fundamental issue".
News Article | April 15, 2017
Ten nights of staunch observation may have led astronomers to successfully peer inside a black hole and take an image of its event horizon, or its point of no return. The mass of data collected is now on its way to two supercomputers in the United States and Germany to confirm in early 2018 if it is indeed the very first capture of the renowned gravitational sinkhole. The ultimate goal for researchers was getting a picture of a region surrounding the black hole. This is the event horizon, or the boundary beyond which not even light can escape the object’s massive grasp. Albert Einstein’s general theory of relativity, born in 1915 and which details how gravity affects the cosmos, has the existence of extremely massive black holes as one of its first predictions. “They are the ultimate endpoint of space and time, and may represent the ultimate limit of our knowledge,” said radio astronomer Heino Falcke of Radboud University in the Netherlands, adding that the first images will turn black holes from mythical things to concrete evidence that scientists can actually study. Einstein’s theory notes that all the information crossing a black hole’s event horizon gets lost forever. Yet according to quantum mechanics, information can never be lost. Back in the 1970s, astrophysicist Stephen Hawking found that black holes can disappear, and so information can be lost forever. The theoretical structure that quantum mechanics puts forward is therefore compromised if the particles’ information could indeed be lost in the black hole. All the scientific inquiry and ambition has led to the widely ambitious Event Horizon Telescope, an international collaboration linking eight observatories to create a virtual telescope dish as wide as Earth. While the method is nothing new, it is the first time that a project is done on a large scale. The radio-dish network went to work on a 10-day window starting April 4, peering at two supermassive black holes: Sagittarius A*, lying at the core of the Milky Way and 4 million times as huge as our sun; and the Messier 87, a black hole in a neighboring galaxy some 53 million light-years away. The telescope has investigated the vicinity of each of the monster black holes before, but this marks the first time the network comprised the South Pole telescope as well as the Atacama Large Millimeter/submillimeter Array (ALMA) telescope located in Chile. ALMA, for one, increases the Event Horizon Telescope’s acuity 10 times, allowing it to find something as tiny as a golf-sized object on the moon — and potentially the small event horizons of the black holes. The image returned is hoped to demonstrate the flow of material moving in and out of the black hole. “What we expect to see is an asymmetric image where you have a circular dark region. That’s the black hole shadow,” MIT research scientist Vincent Fish told Newsweek, adding the presence of the photon ring or a spherical area of space where gravity is so potent that photons are forced to travel in orbits. The weather proved to be a crucial factor in the mission as astronomers observe black holes in millimeter radio waves, which water absorbs as well as emits. This means precipitation could cloud the observations. Mitigating this issue involves placing radio telescopes at high altitudes, although rain, clouds, or snow could still take the observatory offline. Even high-altitude winds could shut down a given telescope. Thus Fish and his fellow scientists met every day to decide on when to activate the large network and assess weather conditions at every site. Constant weather monitoring and communication among astronomers were done. The researchers have collected about one petabyte of data, which equates to MP3 songs playing continuously for more than 2,000 years without any repeats. Two research institutes, the MIT Haystack and the Max Planck Institute for Radio Astronomy in Germany, are receiving the said data. The telescopes’ recorded information is stored on 1,024 drives, which will be mailed to the research institutes’ processing centers. Hard drives coming from the South Pole telescope, too, cannot be sent out until the end of winter in the region, or by the end of October. Despite the long wait and other external factors, the team remains optimistic. Falcke said that even if the images emerge as “crappy and washed out,” they can help test basic predictions of Einstein’s theory in the extreme-physics environment of a black hole. Knowing the black hole’s mass and distance, explained Fish, means one should see the shadow and ring, and that the latter will have a specific diameter and will be quite circular. “If the shape isn’t circular or the wrong size, then relativity has made a prediction that has failed,” he said. © 2017 Tech Times, All rights reserved. Do not reproduce without permission.
News Article | April 17, 2017
Last Tuesday, the weather at the Event Horizon Telescope (EHT) command center in Cambridge, Massachusetts couldn’t have been much worse: it was rainy and barely 40 degrees F. But that wasn’t important. What mattered were the conditions in Hawaii, Arizona, Spain, central Mexico, northern Chile, and at the South Pole. Was the weather good in all those places simultaneously? If so, they had a shot at imaging a black hole. For this experiment, some 50 astronomers from around the world had traveled to high-frequency radio telescopes on four continents: ALMA and the Atacama Pathfinder Experiment in Chile; the Large Millimeter Telescope in Mexico; the Submillimeter Telescope in Arizona; the Submillimeter Array and the James Clerk Maxwell Telescope in Hawaii; the IRAM 30-meter telescope on Pico Veleta in Spain. A crew wintering over in Antarctica, whom EHT astronomers trained months earlier, would operate the South Pole Telescope. Using a technique called Very Long Baseline Interferometry, the astronomers would unite these geographically distant stations into a virtual Earth-size telescope capable of imaging the “shadow” that certain supermassive black holes cast against the glow of surrounding, superheated matter. A prime target is Sagittarius A*, the 4 million-solar-mass giant at the center of the Milky Way. Earlier in the day, Shep Doeleman, the director of the EHT, gathered with a handful of colleagues to oversee the global effort here at the project’s command center, a modified faculty office at Harvard University’s Black Hole Initiative. Astronomers in the field checked in via phone, Slack, and Webex. They had until 4 pm local time to make the go/no-go decision. That night—the first of five opportunities in a ten-night window—the decision was straightforward. The skies were clear in all the right places. At some sites, the weather could hardly have been better. It was almost as if the universe had decided the time was right to reveal this particular secret. “The tau is 0.07 at Mauna Kea, and it’s the middle of the afternoon,” said Shep Doeleman. “Tau” measures the optical depth of a medium—in this case, the Earth’s atmosphere. The higher a medium’s tau, the more opaque it is to starlight. “It just goes down from there,” as night approaches. “That doesn’t happen very often.” “Twenty seconds until we hit,” said Jason SooHoo, an MIT Haystack Observatory technician who was remotely monitoring the data recorders at some of the sites. Doeleman turned to SooHoo. “Count down from five.” “Absolutely,” Doeleman said. “I want to hear it.” “Okay,” SooHoo said. “Five. Four. Three. Two. One. Alright, things should be recording.” And so the Event Horizon Telescope’s long-planned bid to directly image a black hole began. The collaboration has been observing annually with smaller assemblages of telescopes since 2006, but this is the first year they have enough participating stations—and, thus, a powerful enough telescope array—to see actual images. I’ve been following the EHT for five years, and it has been rare to see an observing campaign go smoothly. Things were different this week. Sure, there were technical glitches, but, as far we know, nothing catastrophic—no mechanical problems knocking a telescope out of commission for an entire night, no faulty cables or disk drives failing to capture data. And with the weather, they were just lucky. To increase the chances that the astronomers get good weather at all sites, the participating telescopes allotted the EHT five nights to use over the course of 10 days. The weather was good enough at every site that the EHT observed the first three nights of the window consecutively. Such fortune is rare in observational astronomy. The team observed for so many hours during those three days, in fact, that by the end crew fatigue became a key limiting consideration. After taking two nights off to rest, troubleshoot some technical issues, and wait out some less-than-ideal weather, the team fired off another two consecutive nights, concluding this year’s campaign this morning, Tuesday, April 11—several days ahead of schedule. The cruel irony of Very Long Baseline Interferometry is that the astronomers won’t know for months what their telescopes have seen. First, they must ship hard drives from all of the telescopes back to Haystack Observatory in Massachusetts and the Max Planck Institute for Radio Astronomy in Bonn, Germany. That will take some time; the South Pole Telescope’s hard drives are stuck in Antarctica until October, when the prohibitively harsh austral winter has ended and routine flights resume to and from the bottom of the world. Next, the astronomers will feed data from those hard drives into supercomputers, pooling data gathered at all eight telescopes into a single, correlated set. If the data is good, those common detections will accumulate until images begin to emerge. Next, they’ll have to interpret those images. Have they seen what they expected to see, or something they don’t understand? After checking and rechecking their findings and interpretations, they’ll start the final stage: writing papers and submitting them for peer review. That process could easily take a year. The results, however, could reverberate for much longer. As cool as it will be to have a pretty picture of a black hole, the details are important. The shape of the shadow, its size, its relation to the surrounding accretion disk, and other parameters might expose limitations of Einstein’s theory of general relativity, reveal deep secrets about the nature of spacetime, and help point the way toward a long-sought quantum theory of gravity. And no matter how smoothly the Event Horizon Telescope’s inaugural full-array observation might have gone, these are not secrets we should expect the universe to give up easily.
Shao L.,Max Planck Institute for Radio Astronomy |
Shao L.,Peking University
Physical Review Letters | Year: 2014
The standard model extension is an effective field theory introducing all possible Lorentz-violating (LV) operators to the standard model and general relativity (GR). In the pure-gravity sector of minimal standard model extension, nine coefficients describe dominant observable deviations from GR. We systematically implemented 27 tests from 13 pulsar systems to tightly constrain eight linear combinations of these coefficients with extensive Monte Carlo simulations. It constitutes the first detailed and systematic test of the pure-gravity sector of minimal standard model extension with the state-of-the-art pulsar observations. No deviation from GR was detected. The limits of LV coefficients are expressed in the canonical Sun-centered celestial-equatorial frame for the convenience of further studies. They are all improved by significant factors of tens to hundreds with existing ones. As a consequence, Einstein's equivalence principle is verified substantially further by pulsar experiments in terms of local Lorentz invariance in gravity. © 2014 American Physical Society.
Lacki C.,Institute for Advanced Study |
Beck R.,Max Planck Institute for Radio Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2013
Equipartition arguments provide an easy way to find a characteristic scale for the magnetic field from radio emission by assuming that the energy densities in cosmic rays and magnetic fields are the same. Yet most of the cosmic ray content in star-forming galaxies is in protons, which are invisible in radio emission. Therefore, the argument needs assumptions about the proton spectrum, typically that of a constant proton/electron ratio. In some environments, particularly starburst galaxies, the reasoning behind these assumptions does not necessarily hold: secondary pionic positrons and electrons may be responsible for most of the radio emission, and strong energy losses can alter the proton/electron ratio. We derive an equipartition expression that should work in a hadronic loss-dominated environment like starburst galaxies. Surprisingly, despite the radically different assumptions from the classical equipartition formula, numerically the results for starburst magnetic fields are similar. We explain this fortuitous coincidence using the energetics of secondary production and energy loss times. We show that these processes cause the proton/electron ratio to be ~100 for GHz-emitting electrons in starbursts. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
Lee K.J.,Max Planck Institute for Radio Astronomy
Classical and Quantum Gravity | Year: 2013
In this paper, we focus on testing gravity theories in the radiative regime using pulsar timing array observations. After reviewing current techniques to measure the dispersion and alternative polarization of gravitational waves, we extend the framework to the most general situations, where the combinations of a massive graviton and alternative polarization modes are considered. The atlas of the Hellings-Downs functions is completed by the new calculations for these dispersive alternative polarization modes. We find that each mode and corresponding graviton mass introduce characteristic features in the Hellings-Downs function. Thus, in principal, we can not only detect each polarization mode, measure the corresponding graviton mass, but also discriminate the different scenarios. In this way, we can test gravity theories in the radiative regime in a generalized fashion, and such method is a direct experiment, where one can address the gauge symmetry of the gravity theories in their linearized limits. Although current pulsar timing still lacks enough stable pulsars and sensitivity for such practices, we expect that future telescopes with larger collecting areas could make such experiments feasible. © 2013 IOP Publishing Ltd.
Schnitzeler D.H.F.M.,CSIRO |
Schnitzeler D.H.F.M.,Max Planck Institute for Radio Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2012
An accurate picture of how free electrons are distributed throughout the Milky Way leads to more reliable distances for pulsars and more accurate maps of the magnetic field distribution in the Milky Way. In this paper we test eight models of the free electron distribution in the Milky Way that have been published previously, and we introduce four additional models that explore the parameter space of possible models further. These new models consist of a simple exponential thick-disc model, and updated versions of the models by Taylor & Cordes and Cordes & Lazio with more extended thick discs. The final model we introduce uses the observed Hα intensity as a proxy for the total electron column density, also known as the dispersion measure (DM). Since accurate maps of Hα intensity are now available, this final model can in theory outperform the other models. We use the latest available data sets of pulsars with accurate distances (through parallax measurements or association with globular clusters) to optimize the parameters in these models. In the process of fitting a new scale height for the thick disc in the model by Cordes & Lazio, we discuss why this thick disc cannot be replaced by the thick disc that Gaensler et al. advocated in a recent paper. In the second part of our paper we test how well the different models can predict the DMs of these pulsars at known distances. We base our test on the ratios between the modelled and observed DMs, rather than on absolute deviations, and we identify systematic deviations between the modelled and observed DMs for the different models. For almost all models the ratio between the predicted and the observed DM cannot be described very well by a Gaussian distribution. We therefore calculate the deviations N between the modelled and observed DMs instead, and compare the cumulative distributions of N for the different models. Almost all models perform well, in that they predict DMs within a factor of 1.5-2 of the observed DMs for about 75 per cent of the lines of sight. This is somewhat surprising since the models we tested range from very simple models that only contain a single exponential thick disc to very complex models like the model by Cordes & Lazio. We show that the model by Taylor & Cordes that we updated with a more extended thick disc consistently performs better than the other models we tested. Finally, we analyse which sightlines have DMs that prove difficult to predict by most models, which indicates the presence of local features in the interstellar medium between us and the pulsar. © 2012 CSIRO. Monthly Notices of the Royal Astronomical Society © 2012 RAS.
Noutsos A.,Max Planck Institute for Radio Astronomy
Space Science Reviews | Year: 2012
Faraday rotation towards polarised pulsars and extragalactic sources is the best observable for determining the configuration of the magnetic field of the Galaxy in its plane and also at high latitudes. The Galactic magnetic field plays an important role in numerous astrophysical processes, including star formation and propagation of ultrahigh-energy cosmic rays; it is also an important component in measurements of the cosmological microwave background. This review article provides a brief overview of the latest advancements in the field, from an observer's point of view. The most recent results based on pulsar rotation measures are discussed, which show that we have begun to confidently resolve the main features of the Galactic magnetic field on kiloparsec scales, both in the Solar neighbourhood and at larger distances. As we are currently in great anticipation of polarisation observations with new, state-of-the-art telescopes and hardware, a brief overview of how much this field of research will benefit from the upcoming pulsar surveys is also given. © 2011 Springer Science+Business Media B.V.
Beck R.,Max Planck Institute for Radio Astronomy
Space Science Reviews | Year: 2012
Radio synchrotron emission, its polarization and its Faraday rotation are powerful tools to study the strength and structure of magnetic fields in galaxies. Unpolarized emission traces turbulent fields which are strongest in spiral arms and bars (20-30 μG) and in central starburst regions (50-100 μG). Such fields are dynamically important, e.g. they can drive gas inflows in central regions. Polarized emission traces ordered fields which can be regular or anisotropic random, generated from isotropic random fields by compression or shear. The strongest ordered fields of 10-15 μG strength are generally found in interarm regions and follow the orientation of adjacent gas spiral arms. Ordered fields with spiral patterns exist in grand-design, barred and flocculent galaxies, and in central regions of starburst galaxies. Faraday rotation measures (RM) of the diffuse polarized radio emission from the disks of several spiral galaxies reveal large-scale patterns, which are signatures of regular fields generated by a mean-field dynamo. However, in most spiral galaxies observed so far the field structure is more complicated. Ordered fields in interacting galaxies have asymmetric distributions and are an excellent tracer of past interactions between galaxies or with the intergalactic medium. Ordered magnetic fields are also observed in radio halos around edge-on galaxies, out to large distances from the plane, with X-shaped patterns. Future observations of polarized emission at high frequencies, with the EVLA, the SKA and its precursors, will trace galactic magnetic fields in unprecedented detail. Low-frequency telescopes (e.g. LOFAR and MWA) are ideal to search for diffuse emission and small RMs from weak interstellar and intergalactic fields. © 2011 Springer Science+Business Media B.V.
Ohnaka K.,Max Planck Institute for Radio Astronomy
Astronomy and Astrophysics | Year: 2013
Aims. We present a high-spatial and high-spectral resolution observation of the well-studied K giant Aldebaran with AMBER at the Very Large Telescope Interferometer (VLTI). Our aim is to spatially resolve the outer atmosphere (so-called MOLsphere) in individual CO first overtone lines and derive its physical properties, which are important for understanding the mass-loss mechanism in normal (i.e., non-Mira) K-M giants. Methods. Aldebaran was observed between 2.28 and 2.31 μm with a projected baseline length of 10.4 m and a spectral resolution of 12 000. Results. The uniform-disk diameter observed in the CO first overtone lines is 20-35% larger than is measured in the continuum. We have also detected a signature of inhomogeneities in the CO-line-forming region on a spatial scale of ∼45 mas, which is more than twice as large as the angular diameter of the star itself. While the MARCS photospheric model reproduces the observed spectrum well, the angular size in the CO lines predicted by the MARCS model is significantly smaller than observed. This is because the MARCS model with the parameters of Aldebaran has a geometrical extension of only ∼2% (with respect to the stellar radius). The observed spectrum and interferometric data in the CO lines can be simultaneously reproduced by placing an additional CO layer above the MARCS photosphere. This CO layer is extended to 2.5 ± 0.3 R· with CO column densities of 5 × 1019-2 × 1020 cm-2 and a temperature of 1500 ± 200 K. Conclusions. The high spectral resolution of AMBER has enabled us to spatially resolve the inhomogeneous, extended outer atmosphere (MOLsphere) in the individual CO lines for the first time in a K giant. Our modeling of the MOLsphere of Aldebaran suggests a rather small gradient in the temperature distribution above the photosphere up to 2-3 R*. © 2013 ESO.