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  • richardmitnick 9:32 am on April 15, 2020 Permalink | Reply
    Tags: , , , , , , FRB 181112,   

    From astrobites: “How It’s Made, Fast Radio Burst Edition” 

    Astrobites bloc

    From astrobites

    1
    Artist’s conception of the localization of a fast radio burst to its host galaxy. [Danielle Futselaar]

    Title: Spectropolarimetric analysis of FRB 181112 at microsecond resolution: Implications for Fast Radio Burst emission mechanism
    Authors: Hyerin Cho et al.
    First Author’s Institution: Gwangju Institute of Science and Technology, Korea
    Status: Published in ApJL

    Fast radio bursts (FRBs) are probably the fastest growing and most interesting field in radio astronomy right now. These extragalactic, incredibly energetic bursts last just a few milliseconds and come in two flavors, singular and repeating. Recently the number of known FRBs has exploded, as the ​Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope has discovered about 20 repeating FRBs (and also redetected the famous FRB 121102) and over 700 single bursts (hinted at here).

    CHIME Canadian Hydrogen Intensity Mapping Experiment -A partnership between the University of British Columbia, the University of Toronto, McGill University, Yale and the National Research Council in British Columbia, at the Dominion Radio Astrophysical Observatory in Penticton, British Columbia, CA Altitude 545 m (1,788 ft)

    However, despite the huge growth in the known FRB population, we still don’t know what the source(s) of these bursts is (are). Today’s paper looks at possible explanations for the properties of one FRB in particular to try to figure out what its source might be.

    Your Friendly Neighborhood FRB

    A number of previous astrobites have discussed the basics of FRBs (here, here, and here for example) but the FRB that the authors of this paper focus on is FRB 181112. FRB 181112 was found with the Australian Square Kilometer Array Pathfinder (ASKAP) and localized to a host galaxy about 2.7 Gpc away from us even though it has not been observed to repeat.

    Australian Square Kilometre Array Pathfinder (ASKAP) is a radio telescope array located at Murchison Radio-astronomy Observatory (MRO) in the Australian Mid West. ASKAP consists of 36 identical parabolic antennas, each 12 metres in diameter, working together as a single instrument with a total collecting area of approximately 4,000 square metres.

    That’s over a hundred times farther away than the closest galaxy cluster, the Virgo Cluster!

    Virgo Supercluster NASA


    Virgo Supercluster, Wikipedia

    One quality of FRB 181112 that makes it particularly interesting to study is that the way ASKAP records data allows the authors to study the polarization of the radio emission. Polarization of light is a measure of how much the electromagnetic wave (here the radio emission) rotates due to any magnetic fields it propagates through. The two types of polarization are linear polarization (Q for vertical/horizontal, or V for ±45°), which occurs if the electromagnetic wave rotates in a plane, and circular (either left- or right-handed depending on the rotation direction) if the light rotates on a circular path. By looking at the polarization of FRB 181112, shown in Figure 1, the authors can determine the strength of the magnetic field it traveled through.

    2
    Figure 1: a) The full polarization profile of FRB 181112 showing four profile components. The black line, I, is the sum of all the polarizations of light, or the total intensity of the burst. The red line, Q, is the profile using only (linearly) horizontally or vertically polarized light; the green line, U, is using only the (linearly) ±45° polarized light; and the blue line, V, is the profile using only circularly polarized light. Negative values describe the direction of the polarization. b) The polarization position angle of the zoomed in profiles from panel (a) seen in panel (c). Variation here suggests the emission is coming from different places in the source. d) A three second time series of the data where the FRB is clearly visible at about 1.8 seconds. [Cho et al. 2020]

    In addition to polarization, the dispersion measure (DM), or difference in time of arrival of the FRB at the telescope between the highest and lowest radio emission frequencies due to its journey through the interstellar medium (ISM), can provide information about the properties of the environment(s) the burst travels through. Each of the four components of FRB 181112 (visible in panel (a) of Figure 1 in three different polarizations, Q, U, and V, as well as total intensity, I) are shown in the bottom row of Figure 2, and each component has a slightly different DM. By looking at how the DM changes, the authors can not only look at different emission processes that could lead these apparent changes, but can also measure how scattered the radio emission of FRB 181112 might be due to the ISM. The intensity of the emission as a function of time and radio frequency for each of the four polarization profiles (I , Q, U, and V) are shown in the top row of Figure 2. The four different components that make up FRB 181112 are shown in total intensity, I, in the bottom row of Figure 2.

    3
    Figure 2: Top row: Intensity of the radio emission of each of the four polarization profiles, I, Q, U, and V (described in Figure 1) as a function of time and radio frequency. Bottom row: Close up of the four different pulse components of the total intensity polarization profile, I, of FRB 181112 as a function of time and radio frequency. All components have been assumed to have a DM of 589.265 pc cm-3 , and a slight slope in the intensity as a function of time and frequency can be seen in pulse 4, indicating it may have a slightly different DM. [Cho et al. 2020]

    Properties of FRB 181112

    4
    Figure 3: Degree of polarization of FRB 181112. The black line (P/I) shows the total polarization, the red line (L/I) shows the linear polarization, and the blue line (V/I) shows the circular polarization. The red and black lines show a large amount of polarization constant in time, while the blue line shows the circular polarization changes over the pulse. [Cho et al. 2020]

    The authors first find that FRB 181112 is highly polarized (see Figures 1 and 3), and while the degree of both the total (P/I) and linear (L/I) polarization is constant across all four components of the pulse, the degree of circular (V/I) polarization varies, as shown in Figure 3. This indicates that the FRB must have either traveled through a relativistic plasma, a cold plasma in the ISM that is moving at relativistic speeds, or that the emission was already highly polarized at the time it was emitted, meaning the source of FRB 181112 would have to be highly magnetized. However if the source of the polarization is due to the plasma in the ISM, the expected polarization would be almost completely linear (Q or U), whereas we observe significant circular polarization (V).

    The authors next analyzed the four different components shown in the bottom row of Figure 2 for variations in DM and find there are some small, but significant differences between each component. These differences could be due to some unmodeled structure in the ISM, again possibly a relativistic plasma, but is unlikely since the burst lasts for only 2 milliseconds. The authors also suggest these differences in DM could be due to gravitational lensing, the radio light being bent around a massive object.

    Gravitational Lensing

    Gravitational Lensing NASA/ESA

    This would mean different components travel through different paths in the ISM, accounting for the different DMs and four different components. However, gravitational lensing cannot explain the high degree of polarization seen in FRB 181112.

    The Million Dollar Question

    So how was FRB 181112 made? What caused the polarization and differences in DM? Well, the authors can’t say anything for certain. They suggest that the most likely model is a relativistic plasma close to the source of the emission, which has polarization properties similar to known magnetars (highly magnetized neutron stars known to emit radio bursts), but none of their models can fully explain all of the different properties of FRB 181112. The source of FRB 181112 remains a mystery for now, but with the huge number of FRBs now being detected, the answer may lie just around the corner.

    See the full article here .


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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 10:48 am on October 2, 2019 Permalink | Reply
    Tags: "Galaxy found to float in a tranquil sea of halo gas", , , , , FRB 181112, ,   

    From UC Santa Cruz: “Galaxy found to float in a tranquil sea of halo gas” 

    UC Santa Cruz

    From UC Santa Cruz

    September 26, 2019
    Tim Stephens
    stephens@ucsc.edu

    Analysis of radio pulses that passed through a galactic halo reveals a surprisingly quiescent halo with very low density and weak magnetic field.

    1
    This illustration shows the radio signal from the fast radio burst FRB 181112 passing through the halo of a foreground galaxy on its way toward the telescopes that detected it on Earth. (Illustration © J. Josephides, Centre for Astrophysics and Supercomputing, Swinburne University of Technology)

    2
    Imaging with the Very Large Telescope (VLT) in Chile shows the host galaxy of the fast radio burst, with the position of the burst depicted by the red ellipses. The brighter galaxy located nearby is in the foreground, and the sight-line to the burst passes through the halo of this foreground galaxy. (Image credit: Prochaska et al., Science 2019)

    3
    The ASKAP radio telescope array in outback Western Australia detected and localized the fast radio burst. (Image credit: CSIRO/Alex Cherney)

    Using one cosmic mystery to probe another, astronomers have analyzed the signal from a fast radio burst, an enigmatic blast of cosmic radio waves lasting less than a millisecond, to characterize the diffuse gas in the halo of a massive galaxy.

    A vast halo of low-density gas extends far beyond the luminous part of a galaxy where the stars are concentrated. Although this hot, diffuse gas makes up more of a galaxy’s mass than stars do, it is nearly impossible to see. In November 2018, astronomers detected a fast radio burst that passed through the halo of a massive galaxy on its way toward Earth, allowing them for the first time to get clues to the nature of the halo gas from an elusive radio signal.

    “The signal from the fast radio burst exposed the nature of the magnetic field around the galaxy and the structure of the halo gas. The study proves a new and transformative technique for exploring the nature of galaxy halos,” said J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz and lead author of a paper on the new findings published online September 26 in Science.

    Astronomers still don’t know what produces fast radio bursts, and only recently have they been able to trace some of these very short, very bright radio signals back to the galaxies in which they originated. The November 2018 burst (named FRB 181112) was detected and localized by the instrument that pioneered this technique, CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope. Follow-up observations with other telescopes identified not only its host galaxy but also a bright galaxy in front of it.

    “When we overlaid the radio and optical images, we could see straight away that the fast radio burst pierced the halo of this coincident foreground galaxy and, for the first time, we had a direct way of investigating this otherwise invisible matter surrounding this galaxy,” said coauthor Cherie Day at Swinburne University of Technology, Australia.

    A galactic halo contains both dark matter and ordinary (“baryonic”) matter, which is expected to be mostly hot ionized gas. While the luminous part of a massive galaxy might be around 30,000 light-years across, its roughly spherical halo is ten times larger. Halo gas fuels star formation as it falls in toward the center of the galaxy, while other processes (such as supernova explosions) can eject material out of the star-forming regions and into the galactic halo. One reason astronomers want to study the halo gas is to better understand these ejection processes, which can shut down star formation.

    “The halo gas is a fossil record of these ejection processes, so our observations can inform theories about how matter is ejected and how magnetic fields are threaded through galaxies,” Prochaska said.

    Contrary to expectations, the results of the new study indicate a very low density and a feeble magnetic field in the halo of this intervening galaxy.

    “This galaxy’s halo is surprisingly tranquil,” Prochaska said. “The radio signal was largely unperturbed by the galaxy, which is in stark contrast to what previous models predict would have happened to the burst.”

    The signal of FRB 181112 consisted of several pulses, each lasting less than 40 microseconds (ten thousand times shorter than the blink of an eye). The short duration of the pulses puts an upper limit on the density of the halo gas, because passage through a denser medium would lengthen the radio signals. The researchers calculated that the density of the halo gas must be less than a tenth of an atom per cubic centimeter (equivalent to several hundred atoms in a volume the size of a child’s balloon).

    “Like the shimmering air on a hot summer’s day, the tenuous atmosphere in this massive galaxy should warp the signal of the fast radio burst. Instead we received a pulse so pristine and sharp that there is no signature of this gas at all,” said coauthor Jean-Pierre Macquart, an astronomer at the International Center for Radio Astronomy Research at Curtin University, Australia.

    The density constraints also limit the possibility of turbulence or clouds of cool gas within the halo (“cool” being a relative term, referring here to temperatures around 10,000 Kelvin, versus the hot halo gas at around 1 million Kelvin). “One favored model is that halos are pervaded by clouds of clumpy gas. We find no evidence for these clouds whatsoever,” Prochaska said.

    The FRB signal also yields information about the magnetic field in the halo, which affects the polarization of the radio waves. Analyzing the polarization as a function of frequency gives a “rotation measure” for the halo, which the researchers found to be very low. “The weak magnetic field in the halo is a billion times weaker than that of a refrigerator magnet,” Prochaska said.

    At this point, with results from only one galactic halo, the researchers cannot say whether the unexpectedly low density and magnetic field strength are unusual or if previous studies of galactic halos have overestimated these properties. ASKAP and other radio telescopes will use fast radio bursts to study many more galactic halos and resolve their properties.

    “This galaxy may be special,” Prochaska said. “We will need to use FRBs to study tens or hundreds of galaxies over a range of masses and ages to assess the full population.”

    In addition to Prochaska, Day, and Macquart, the coauthors of the paper include UCSC graduate student Sunil Simha and researchers at eight other institutions in Australia, the United States, South Korea, and Chile. This work was funded in part by the U.S. National Science Foundation and the Australian Research Council.

    See the full article here .


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  • richardmitnick 2:46 pm on September 26, 2019 Permalink | Reply
    Tags: "Enigmatic radio burst illuminates a galaxy’s tranquil ​halo", , , , , , FRB 181112   

    From European Southern Observatory: Science Release “Enigmatic radio burst illuminates a galaxy’s tranquil ​halo” 

    ESO 50 Large

    From European Southern Observatory

    26 September 2019

    J. Xavier Prochaska
    UCO/Lick Observatory — UC Santa Cruz
    USA
    Tel: +1 (831) 295-0111
    Email: xavier@ucolick.org

    Cherie Day
    Centre for Astrophysics and Supercomputing — Swinburne University of Technology
    Australia
    Tel: +61 4 5946 3110
    Email: cday@swin.edu.au

    Mariya Lyubenova
    ESO Head of Media Relations
    Garching bei München, Germany
    Tel: +49 89 3200 6188
    Email: pio@eso.org

    1
    Astronomers using ESO’s Very Large Telescope [below] have for the first time observed that a fast radio burst passed through a galactic halo. Lasting less than a millisecond, this enigmatic blast of cosmic radio waves came through almost undisturbed, suggesting that the halo has surprisingly low density and weak magnetic field. This new technique could be used to explore the elusive halos of other galaxies.

    Using one cosmic mystery to probe another, astronomers analysed the signal from a fast radio burst to shed light on the diffuse gas in the halo of a massive galaxy [1]. In November 2018 the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope pinpointed a fast radio burst, named FRB 181112.

    Australian Square Kilometre Array Pathfinder (ASKAP) is a radio telescope array located at Murchison Radio-astronomy Observatory (MRO) in the Australian Mid West. ASKAP consists of 36 identical parabolic antennas, each 12 metres in diameter, working together as a single instrument with a total collecting area of approximately 4,000 square metres.

    Follow-up observations with ESO’s Very Large Telescope (VLT) and other telescopes revealed that the radio pulses have passed through the halo of a massive galaxy on their way toward Earth. This finding allowed astronomers to analyse the radio signal for clues about the nature of the halo gas.

    “The signal from the fast radio burst exposed the nature of the magnetic field around the galaxy and the structure of the halo gas. The study proves a new and transformative technique for exploring the nature of galaxy halos,” said J. Xavier Prochaska, professor of astronomy and astrophysics at the University of California Santa Cruz and lead author of a paper presenting the new findings published today​ in the journal ​Science.​

    Astronomers still don’t know what causes fast radio bursts and only recently have been able to trace some of these very short, very bright radio signals back to the galaxies in which they originated. “When we overlaid the radio and optical images, we could see straight away that the fast radio burst pierced the halo of this coincident foreground galaxy and, for the first time, we had a direct way of investigating the otherwise invisible matter surrounding this galaxy,” said coauthor Cherie Day, a PhD student at Swinburne University of Technology, Australia.

    A galactic halo contains both dark and ordinary—or baryonic—matter that is primarily in the form of a hot ionised gas. While the luminous part of a massive galaxy might be around 30 000 light years across, its roughly spherical halo is ten times larger in diameter. Halo gas fuels star formation as it falls towards the centre of the galaxy, while other processes, such as supernova explosions, can eject material out of the star-forming regions and into the galactic halo. One reason astronomers want to study the halo gas is to better understand these ejection processes which can shut down star formation.

    “This galaxy’s halo is surprisingly tranquil,” Prochaska said. “The radio signal was largely unperturbed by the galaxy, which is in stark contrast to what previous models predict would have happened to the burst.”

    The signal of FRB 181112 was comprised of a few pulses, each lasting less than 40 microseconds (10 000 times shorter than the blink of an eye). The short duration of the pulses puts an upper limit on the density of the halo gas because passage through a denser medium would broaden the duration of the radio signal. The researchers calculated that the density of the halo gas must be less than 0.1 atoms per cubic centimeter (equivalent to several hundred atoms in a volume the size of a child’s balloon) [2].

    “Like the shimmering air on a hot summer’s day, the tenuous atmosphere in this massive galaxy should warp the signal of the fast radio burst. Instead we received a pulse so pristine and sharp that there is no signature of this gas at all,” said coauthor Jean-Pierre Macquart, an astronomer at the International Center for Radio Astronomy Research at Curtin University, Australia.

    The study found no evidence of cold turbulent clouds or small dense clumps of cool halo gas. The fast radio burst signal also yielded information about the magnetic field in the halo, which is very weak—a billion times weaker than that of a refrigerator magnet.

    At this point, with results from only one galactic halo, the researchers cannot say whether the low density and low magnetic field strength they measured are unusual or if previous studies of galactic halos have overestimated these properties. Prochaska said he expects that ASKAP and other radio telescopes will use fast radio bursts to study many more galactic halos and resolve their properties.

    “This galaxy may be special,” he said. “We will need to use fast radio bursts to study tens or hundreds of galaxies over a range of masses and ages to assess the full population.” Optical telescopes like ESO’s VLT play an important role by revealing how far away the galaxy that played host to each burst is, as well as whether the burst would have passed through the halo of any galaxy in the foreground.

    Notes

    [1] A vast halo of low-density gas extends far beyond the luminous part of a galaxy where the stars are concentrated. Although this hot, diffuse gas makes up more of a galaxy’s mass than stars do, it is very difficult to study.

    [2] The density constraints also limit the possibility of turbulence or clouds of cool gas within the halo. Cool here is a relative term, referring to temperatures around 10 000°C, versus the hot halo gas at around 1 million degrees.

    More information

    The team is composed of J. Xavier Prochaska (University of California Observatories-Lick Observatory, University of California, USA and Kavli Institute for the Physics and Mathematics of the Universe, Japan), Jean-Pierre Macquart (International Centre for Radio Astronomy Research, Curtin University, Australia), Matthew McQuinn (Astronomy Department, University of Washington, USA), Sunil Simha (University of California Observatories-Lick Observatory, University of California, USA), Ryan M. Shannon (Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Australia), Cherie K. Day (Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Australia and Commonwealth Science and Industrial Research Organisation, Australia Telescope National Facility, Australia), Lachlan Marnoch (Commonwealth Science and Industrial Research Organisation, Australia Telescope National Facility, Australia and Department of Physics and Astronomy, Macquarie University, Australia), Stuart Ryder (Department of Physics and Astronomy, Macquarie University, Australia), Adam Deller (Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Australia), Keith W. Bannister (Commonwealth Science and Industrial Research Organisation, Australia Telescope National Facility, Australia), Shivani Bhandari (Commonwealth Science and Industrial Research Organisation, Australia Telescope National Facility, Australia), Rongmon Bordoloi (North Carolina State University, Department of Physics, USA), John Bunton (Commonwealth Science and Industrial Research Organisation, Australia Telescope National Facility, Australia), Hyerin Cho (School of Physics and Chemistry, Gwangju Institute of Science and Technology, Korea), Chris Flynn (Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Australia), Elizabeth Mahony (Commonwealth Science and Industrial Research Organisation, Australia Telescope National Facility, Australia), Chris Phillips (Commonwealth Science and Industrial Research Organisation, Australia Telescope National Facility, Australia), Hao Qiu (Sydney Institute for Astronomy, School of Physics, University of Sydney, Australia), Nicolas Tejos (Instituto de Fisica, Pontificia Universidad Catolica de Valparaiso, Chile).

    See the full article here .


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