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  • richardmitnick 10:05 am on August 23, 2017 Permalink | Reply
    Tags: 2017 HEAD: Day 2, a well-studied tidal disruption event ASASSN-14li, , AGN coronae — the incredibly luminous compact regions that lie directly above the accretion disks of supermassive black holes, AGN's - Active galactic nuclei, , , , , The Very High Energy Universe as Viewed with VERITAS and HAWC   

    From AAS NOVA: ” 2017 HEAD: Day 2″ 

    AASNOVA

    American Astronomical Society

    23 August 2017
    Susanna Kohler

    1
    The gamma-ray excess at the heart of M31. [NASA/DOE/Fermi LAT Collaboration and Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF]

    Session: AGN 1

    Ehud Bahar (Technion) opened the meeting’s first session on active galactic nuclei (AGN) by discussing eclipses of a different kind than the one we observed on Monday. Light from AGN is often obstructed on its path to us by warm, outflowing, intervening material that absorbs some of the AGN’s light. Bahar explained the difference between what he termed “absorbers” and “obscurers”: absorbers are slow and steady outflows from the AGN that change very little over long timescales. These provide us with the opportunity to probe their detailed physics. Obscurers, on the other hand, are fast-moving and transient outflows, briefly causing dramatic drops in the X-ray flux of the AGN.

    2
    Artist’s impression of the tidal disruption event ASASSN-14li, in which a supermassive black hole destroyed a star, launching outflows. [NASA GSFC]

    Two speakers in the session discussed the idea of particularly fast outflows from AGN: Michael Nowak (MIT Kavli Institute) presented data on ultrafast outflows moving at 5–20% of the speed of light from the AGN PG 1211+143 (that’s 15,000–60,000 km/s, as compared to more typical outflow speeds of 100–1,000 km/s), and Erin Kara (University of Maryland) discussed what we can learn from ultrafast outflows from tidal disruption events. Kara’s talk demonstrated how we can use our observations of a well-studied tidal disruption event, ASASSN-14li, to learn about how an accretion disk around a black hole can transition from a super-Eddington (especially high) accretion phase that launches winds to a sub-Eddington (lower) accretion phase in which the wind is shut off.

    Andrew Fabian (University of Cambridge) wrapped up the session by providing an overview of what we know about AGN coronae — the incredibly luminous, compact regions that lie directly above the accretion disks of supermassive black holes. Coronae are the source of the majority of the hard X-ray emission from AGN, and we have used observations of this emission to constrain the size of AGN coronae to a mere 10 gravitational radii. We’ve learned that coronae are extremely hot, at 30–300 keV, and are highly magnetized and dynamic, likely containing outflowing plasma.

    Session: The Very High Energy Universe as Viewed with VERITAS and HAWC

    HAWC High Altitude Cherenkov Experiment, located on the flanks of the Sierra Negra volcano in the Mexican state of Puebla at an altitude of 4100 meters, at WikiMiniAtlas 18°59′41″N 97°18′30.6″W.

    CfA/VERITAS, a major ground-based gamma-ray observatory with an array of four 12m optical reflectors for gamma-ray astronomy in the GeV – TeV energy range. Located at FLWO in AZ, USA

    The session on very high energy observations opened with a talk by Brenda Dingus (LANL). Dingus introduced us to the High Altitude Water Cherenkov (HAWC) gamma-ray observatory, a new observatory located in Mexico that maps the northern sky in high-energy gamma rays. HAWC has a wide field of view, observing roughly 2/3 of the sky each day with long integration times. This means that the observatory is sensitive to the highest energy gamma rays. HAWC has recently released its very first catalog, 2HWC, and this is only the beginning — there is much more science expected from this observatory in the future!

    The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is another high-energy observatory, located in southern Arizona; Philip Kaaret (University of Iowa) provided us with an overview of this set of telescopes. VERITAS has a narrower field of view than HAWC, but its sensitivity and angular resolution are higher, allowing it to probe sources at a deeper level. It’s therefore often used for follow-up observations of known targets.

    So what sources are high-energy observatories like VERITAS and HAWC observing? They hunt for photons from astrophysical sources like supernova remnants, pulsar wind nebulae, active galactic nuclei, gamma-ray bursts, and dark matter annihilation. Oleg Kargaltsev (George Washington University), Sara Buson (NASA GSFC), and Matthew Baring (Rice University) each explained some of the insights we’ve obtained about these objects from observatories like HAWC, VERITAS, Fermi, and MAGIC in conjunction with observatories exploring other wavelengths.

    So what sources are high-energy observatories like VERITAS and HAWC observing? They hunt for photons from astrophysical sources like supernova remnants, pulsar wind nebulae, active galactic nuclei, gamma-ray bursts, and dark matter annihilation. Oleg Kargaltsev (George Washington University), Sara Buson (NASA GSFC), and Matthew Baring (Rice University) each explained some of the insights we’ve obtained about these objects from observatories like HAWC, VERITAS, Fermi, and MAGIC in conjunction with observatories exploring other wavelengths.

    NASA/Fermi Telescope

    MAGIC Cherenkov gamma ray telescope on the Canary island of La Palma, Spain

    Mid-Career Prize Talk: X-ray Winds from Black Holes

    Tuesday afternoon kicked off with the HEAD Mid-Career Prize Talk, given this year by Jon Miller (University of Michigan). Miller spoke in further depth about a topic introduced earlier in the day: winds emitted from black hole disks. He argued that these winds are worth studying because they provide information about how mass is accreted onto black holes, and therefore how the black holes grow and their spins evolve.

    The dense and ionized winds from black-hole disks can potentially carry away more mass than is accreted — and this appears to hold true across the mass scale, from X-ray binaries containing stellar-mass black holes to Seyfert galaxies containing supermassive black holes. Miller discussed the different mechanisms that may launch these winds, and how observations indicate that magnetic driving is important, although other forces may also be at work.

    ESA/Athena spacecraft

    Miller argued that many tests of disk physics are now within reach of data and simulations, such as measurements of disk magnetic fields. He also showed how extreme settings such as tidal disruption events can provide a unique and interesting regime in which to explore disks and winds, as the mass accretion rate in these events changes drastically on observable timescales.

    As a final point, Miller discussed how our understanding of black hole disk winds will change with upcoming observatories. Missions like Xarm, ARCUS, ATHENA, Lynx [no image available], etc. will be transformative; ATHENA, for instance, will be able to produce observations outstripping the sensitivity and resolution of any observations obtained so far with current instrumentation, in “less than the time it took you to have lunch today,” Miller explained.

    2
    Xarm satelite

    3
    NASA/ARCUS

    Session: ISM & Galaxies

    Xian Hou (Yunnan Observatories) opened the session on the interstellar medium (ISM) and galaxies by discussing our view of M31 (the Andromeda galaxy) with the Fermi Large Area Telescope.

    NASA/Fermi LAT

    Andromeda Galaxy Adam Evans

    M31 is the only other large spiral local galaxy — and it’s nearby, providing an excellent opportunity for resolved analysis of high-energy emission from a large, star-forming, spiral galaxy similar to the Milky Way. The >1 GeV emission tracked by Fermi LAT was found to be concentrated only in the inner region of the galaxy; it is not correlated with interstellar gas or star-formation sites. What could be this emission’s source, then? Hou suggests that possibilities include a population of millisecond pulsars in the galactic center, or annihilation/decay of dark matter.

    4
    NuSTAR observations of M31. The bright blue point in the inset is the intermediate-mass pulsar candidate. [NASA/JPL-Caltech]

    NASA NuSTAR X-ray telescope

    Later in the session, Ann Hornschemeier (NASA GSFC) provided a complementary discussion of observations of M31 — this time in the form of NuSTAR’s deep survey of of our nearest galactic neighbor. Hornschemeier reminded us that before NuSTAR, we were unable to spatially resolve hard X-ray sources (energies over 10 keV) in other galaxies. Now, with NuSTAR, we can resolve point sources — and their hard X-ray color can help us to identify whether they are black hole X-ray binaries, neutron-star X-ray binaries, pulsars, etc. A number of neutron stars were identified in globular clusters in M31, as well as a particularly high energy source that is likely an intermediate-mass X-ray pulsar.

    The work done by Francesca Fornasini (Harvard-Smithsonian CfA) and collaborators explores how low-luminosity AGN activity and star formation in its host galaxy are connected. Is there a correlation between these two types of activity? If there’s a positive correlation, we can infer that AGN feedback suppresses star formation; if there is a negative correlation, both types of activity may be fueled by a common mechanism. On the other hand, there may be no correlation at all! Because AGN are variable, and because the relation between AGN activity and star formation rate can vary with other host galaxy properties like stellar mass and redshift, we need a very large sample that covers the whole phase space to test for correlation. Fornasini and collaborators achieve this by building X-ray stacks from data from 123,000 galaxies in the Chandra COSMOS Legacy Survey. Their work is still underway, but thus far it has revealed no correlation between the black-hole accretion rate and the star formation rate of the host galaxies.

    See the full article here .

    Please help promote STEM in your local schools.

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    Stem Education Coalition

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    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 4:11 pm on June 19, 2017 Permalink | Reply
    Tags: , AGN's - Active galactic nuclei, , , , Geometric dependence of AGN types, Hidden Black Holes Revealed?,   

    From AAS NOVA: ” Hidden Black Holes Revealed?” 

    AASNOVA

    American Astronomical Society

    19 June 2017
    Susanna Kohler

    1
    Artist’s illustration of the thick dust torus thought to surround supermassive black holes and their accretion disks. [ESA / V. Beckmann (NASA-GSFC)]

    Supermassive black holes are thought to grow in heavily obscured environments. A new study now suggests that many of the brightest supermassive black holes around us may be escaping our detection as they hide in these environments.

    2
    The geometric dependence of AGN types in the unified AGN model. Type 1 AGN are viewed from an angle where the central engine is visible. In Type 2 AGN, the dusty torus obscures the central engine from view. [Urry & Padovani, 1995]

    A Torus Puzzle

    The centers of galaxies with bright, actively accreting supermassive black holes are known as active galactic nuclei, or AGN. According to a commonly accepted model for AGN, these rapidly growing black holes and their accretion disks are surrounded by a thick torus of dust. From certain angles, the torus can block our direct view of the central engines, changing how the AGN appears to us. AGN for which we can see the central engine are known as Type 1 AGN, whereas those with an obscured central region are classified as Type 2.

    Oddly, the fraction of AGN classified as Type 2 decreases substantially with increasing luminosity; brighter AGN seem to be more likely to be unobscured. Why? One hypothesis is that the torus structure itself changes with changing AGN luminosity. In this model, the torus recedes as AGN become brighter, causing fewer of these AGN to be obscured from our view.

    But a team of scientists led by Silvia Mateos (Institute of Physics of Cantabria, Spain) suggests that we may instead be missing the bigger picture. What if the problem is just that many of the brightest obscured AGN are too well hidden?

    Geometry Matters

    3
    Type 2 AGN fraction vs. torus covering factor for AGN in the authors’ three luminosity bins. The black line shows the 1-to-1 relation describing the expected Type 2 AGN fraction; the black data points show the observed fraction. The red points show the best-fit model including the “missing” AGN, and the inset shows the covering-factor distribution for the missing sources. [Mateos et al. 2017]

    Mateos and collaborators built a sample of nearly 200 X-ray-observed AGN from the Bright Ultra-hard XMM-Newton Survey (BUXS). They then determined the intrinsic fraction of these AGN that were obscured (i.e., classified as Type 2) at a given luminosity, for redshifts between 0.05 ≤ z ≤ 1.

    ESA/XMM Newton

    The team next used clumpy torus models to estimate the distributions of AGN covering factors, the geometric factor that describes the fraction of the sky around the AGN central engine that’s obscured.

    The pointing directions for AGN should be randomly distributed, and geometry then dictates that the covering factor distributions combined over the total AGN population should match the intrinsic fraction of AGN classified as Type 2 AGN. Instead, the sample from BUXS reveals a “missing” population of high-covering-factor tori that we have yet to detect in X-rays.

    Missing Sources

    When they include the missing AGN, Mateos and collaborators find that the total fraction of Type 2 AGN is around 58%. They also show that more of these AGN are missing at higher luminosities. By including the missing ones, the total fraction of obscured AGN therefore has a much weaker dependence on luminosity than we thought — which suggests that the receding torus model isn’t necessary to explain observations.

    Mateos and collaborators’ results support the idea that the majority of very bright, rapidly accreting supermassive black holes at redshifts of z ≤ 1 live in nuclear environments that are extremely obscured. These black holes are so well embedded in their environments that they’ve escaped detection in X-ray surveys thus far.

    Citation

    S. Mateos et al 2017 ApJL 841 L18. doi:10.3847/2041-8213/aa7268

    Related Journal Articles
    See the full article for a list of further references with links.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    1

    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

     
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