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  • richardmitnick 7:11 pm on March 6, 2019 Permalink | Reply
    Tags: , , , , , , TDEs-Tidal disruption events, The transient AT2018zr triggered a ZTF alert on 6 March 2018, With many more events like AT2018zr we can hope to build a large sample of flares that will finally shed light on TDE processes, ZTF began its first major public observing survey in mid-March 2018,   

    From AAS NOVA: “First Disrupted Star for a New Survey” 


    From AAS NOVA

    6 March 2019
    Susanna Kohler

    Artist’s impression of a glowing stream of material produced when a star is shredded by a supermassive black hole. [NASA/JPL-Caltech]

    What happens when a black hole makes a meal out of a passing star? So far, we’ve only detected a few dozen candidate tidal disruption events to help us answer this question — but now a new player is in the observing game.

    Snacks for Black Holes

    When a star passes within the tidal radius of a supermassive black hole, things don’t end well for the star. After the unfortunate object is torn apart by gravitational forces, some of the resulting debris accretes onto the black hole, causing a multi-wavelength flare.

    To date, we’ve observed this flare emission from several dozen candidate tidal disruption events (TDEs), but many of them were only noticed significantly after the moment of disruption, when the flare emission is already ramping back down again. We also have only a handful of detections of TDEs across multiple wavelengths.

    Zwicky Transient Facility (ZTF) instrument installed on the 1.2m diameter Samuel Oschin Telescope at Palomar Observatory in California. Courtesy Caltech Optical Observatories

    In short, TDE observations thus far — though tantalizing — aren’t yet enough to help us complete the picture of what happens when a star is torn apart by a supermassive black hole. Clearly, the next step is to gather many more such observations! Luckily, a new tool has recently come online that will help us do exactly that: the Zwicky Transient Facility (ZTF)

    A New Player

    ZTF is a wide-field optical survey that hunts for transient objects in our night sky. ZTF images image the entire northern sky once every three nights, and the plane of the Milky Way twice a night. By scanning the same regions frequently, the survey can detect and monitor rapidly changing objects — like a suddenly rising tidal disruption flare.

    ZTF began its first major public observing survey in mid-March 2018. In the weeks before that, ZTF was still in its commissioning phase, testing the camera and the alert pipeline. It was in this time that the survey detected its first tidal disruption event candidate, AT2018zr.

    ZTF optical and Swift ultraviolet and optical light curves for AT2018zr. The data capture both the sudden rise and gradual decay of the flare. [van Velzen et al. 2019]

    NASA Neil Gehrels Swift Observatory

    Early View of Destruction

    The transient AT2018zr triggered a ZTF alert on 6 March 2018. In the weeks that followed, it was observed by additional telescopes across a number of wavelength bands. In a new publication led by Sjoert van Velzen (University of Maryland and New York University), team members detailed the ZTF and multi-wavelength follow-up observations of AT2018zr.

    By reprocessing earlier ZTF image frames, van Veltzen and collaborators found that ZTF had actually captured this tidal disruption event starting in early February, 50 days before the peak of the flare light curve. These detailed optical observations, combined with the broadband follow-up, provide an unusually complete view of this flare.

    The host of AT2018zr, as observed by the Sloan Digital Sky Survey before the TDE occurred. [SDSS]

    SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude2,788 meters (9,147 ft)

    Harbingers of Data to Come

    With many more events like AT2018zr, we can hope to build a large sample of flares that will finally shed light on TDE processes. ZTF is conveniently poised to start producing those observations; estimates suggest that, now that ZTF is fully operational, it will produce ~30 TDE detections per year.

    What’s more, ZTF is providing researchers with a chance to test clever analysis techniques in advance of an even larger flood of data: the upcoming Large Synoptic Survey Telescope (LSST) is expected to detect ~1,000 TDEs per year!


    LSST Camera, built at SLAC

    LSST telescope, currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

    While only one event, AT2018zr is likely something more — the beginning of a new era for TDE observations.

    “The First Tidal Disruption Flare in ZTF: From Photometric Selection to Multi-wavelength Characterization,” Sjoert van Velzen et al 2019 ApJ 872 198.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition


    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

  • richardmitnick 1:33 pm on August 11, 2018 Permalink | Reply
    Tags: , , , , , Quasars are now known to be supermassive black holes feeding on surrounding gas not stars., TDEs-Tidal disruption events, , Zwicky Transit Facility at California’s Palomar Observatory   

    From Wired: “Star-Swallowing Black Holes Reveal Secrets in Exotic Light Shows” 

    Wired logo

    From Wired

    Joshua Sokol

    Black holes, befitting their name and general vibe, are hard to find and harder to study. You can eavesdrop on small ones from the gravitational waves that echo through space when they collide—but that technique is new, and still rare. You can produce laborious maps of stars flitting around the black hole at the center of the Milky Way or nearby galaxies. Or you can watch them gulp down gas clouds, which emit radiation as they fall.

    Now researchers have a new option. They’ve begun corralling ultrabright flashes called tidal disruption events (TDEs), which occur when a large black hole seizes a passing star, shreds it in two and devours much of it with the appetite of a bear snagging a salmon. “To me, it’s sort of like science fiction,” said Enrico Ramirez-Ruiz, an astrophysicist at University of California, Santa Cruz, and the Niels Bohr Institute.

    During the past few years, though, the study of TDEs has transformed from science fiction to a sleepy cottage industry, and now into something more like a bustling tech startup.

    Automated wide-field telescopes that can pan across thousands of galaxies each night have uncovered about two dozen TDEs. Included in these discoveries are some bizarre and long-sought members of the TDE zoo. In June, a study in the journal Nature described an outburst of X-ray light in a cluster of faraway stars that astronomers interpreted as a midsized black hole swallowing a star. That same month, another group announced in Science that they had discovered what may be brightest ever TDE, one that illuminated faint gas at the heart of a pair of merging galaxies.

    These discoveries have taken place as our understanding of what’s really happening during a TDE comes into sharper focus. At the end of May, a group of astrophysicists proposed [The Astrophysical Journal Letters] a new theoretical model for how TDEs work. The model can explain why different TDEs can appear to behave differently, even though the underlying physics is presumably the same.

    Astronomers hope that decoding these exotic light shows will let them conduct a black hole census. Tidal disruptions expose the masses, spins and sheer numbers of black holes in the universe, the vast majority of which would be otherwise invisible. Theorists are hungry, for example, to see if TDEs might unveil any intermediate-mass black holes with weights between the two known black hole classes: star-size black holes that weigh a few times more than the sun, and the million- and billion-solar-mass behemoths that haunt the cores of galaxies. The Nature paper claims they may already have.

    A numerical simulation of the core of a star as it’s being consumed by a black hole. Video by Guillochon and Ramirez-Ruiz

    Researchers have also started to use TDEs to probe the fundamental physics of black holes. They can be used to test whether black holes always have event horizons—curtains beyond which nothing can return—as Einstein’s theory of general relativity predicts.

    Meanwhile, many more observations are on the way. The rate of new TDEs, now about one or two per year, could jump up by an order of magnitude [Stellar Tidal Disruption Events in General Relativity]even by the end of this year because of the Zwicky Transient Facility, which started scanning the sky over California’s Palomar Observatory in March.

    Zwicky Transit Facility at California’s Palomar Observatory schematic

    Zwicky Transit Facility at California’s Palomar Observatory

    And with the addition of planned observatories, it may increase perhaps another order of magnitude in the years to come.Researchers have also started to use TDEs to probe the fundamental physics of black holes. They can be used to test whether black holes always have event horizons—curtains beyond which nothing can return—as Einstein’s theory of general relativity predicts.

    “The field has really blossomed,” said Suvi Gezari at the University of Maryland, one of the few stubborn pioneers who staked their careers on TDEs during leaner years. She now leads the Zwicky Transient Facility’s TDE-hunting team, which has already snagged unpublished candidates in its opening months, she said. “Now people are really digging in.”

    Searching for Star-Taffy

    In 1975, the British physicist Jack Hills first dreamed up a black-hole-eats-star scenario as a way to explain what powers quasars—superbright points of light from the distant universe. (Quasars are now known to be supermassive black holes feeding on surrounding gas, not stars.) But in 1988, the British cosmologist Martin Rees realized [Nature]that black holes snacking on a star would exhibit a sharp flare, not a steady glow. Looking for such flares could let astronomers find and study the black holes themselves, he argued.

    Nothing that fit the bill turned up until the late 1990s. That’s when Stefanie Komossa, at the time a graduate student at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, found massive X-ray flares [Discovery of a giant and luminous X-ray outburst from the optically inactive galaxy pair RXJ1242.6-1119] from the centers of distant galaxies that brightened and dimmed according to the Rees predictions.

    The astronomical community responded to these discoveries—based on just a few data points—with caution. Then in the mid-2000s, Gezari, then beginning a postdoc at the California Institute of Technology, searched for and discovered her own handful of TDE candidates. She looked for flashes of ultraviolet light, not X-rays as Komossa had. “In the old days,” Gezari said, “I was just trying to convince people that any of our discoveries were actually due to a tidal disruption.”

    Soon, though, she had something to sway even the doubters. In 2010, Gezari discovered an especially clear flare, rising and falling as modelers predicted. She published it in Nature in 2012, catching other astronomers’ attention. In the years since, large surveys in optical light, sifting through the sky for changes in brightness, have taken over the hunt. And like Komossa’s and Gezari’s TDEs, which had both been fished out of missions designed to look for other things, the newest batch showed up as bycatch. “It was, oh, why didn’t we think about looking for these?” said Christopher Kochanek, an astrophysicist at Ohio State University who works on a project designed to search for supernovas [ASAS-SN OSU All-Sky Automated Survey for Supernovae].

    Now, with a growing number of TDEs in hand, astrophysicists are within arm’s reach of Rees’s original goal: pinpointing and studying gargantuan black holes. But they still need to learn to interpret these events, divining their basic physics. Unexpectedly, the known TDEs fall into separate classes [A unified model for tidal disruption events]. Some seem to emit mostly ultraviolet and optical light, as if from gas heated to tens of thousands of degrees. Others glow fiercely with X-rays, suggesting temperatures an order of magnitude higher. Yet presumably they all have the same basic physical root.

    To be disrupted, an unlucky star must venture close enough to a black hole that gravitational tides exceed the internal gravity that binds the star together. In other words, the difference in the black hole’s gravitational pull on the near and far sides of the star, along with the inertial pull as the star swings around the black hole, stretches the star out into a stream. “Basically it spaghettifies,” said James Guillochon, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics.

    The outer half of the star escapes away into space. But the inner half—that dense stream of star-taffy—swirls into the black hole, heating up and releasing huge sums of energy that radiate across the universe.

    With this general mechanism understood, researchers had trouble understanding why individual TDEs can look so distinct. One longstanding idea appeals to different phases of the star-eating process. As the star flesh gets initially torn away and stretched into a stream, it might ricochet around the black hole and slam into its own tail. This process might heat the tail up to ultraviolet-producing temperatures—but not hotter. Then later—after a few months or a year—the star would settle into an accretion disk, a fat bagel of spinning gas that theories predict should be hot enough to emit X-rays.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

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