astrobites 2018

Find out the latest thinking about our universe.
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Teaching Machines to find Fast Radio Bursts

Post by bystander » Thu Sep 27, 2018 4:12 pm

Teaching Machines to find Fast Radio Bursts
Astrobites | 2018 Sep 24
Joshua Kerrigan wrote:
Today’s astrobite combines two independently fascinating topics for a very interesting result, machine learning and fast radio bursts (FRBs). The field of Machine Learning is moving at an unprecedented pace with fascinating new results. FRBs have entirely unknown origins and experiments to detect more of them are gearing up. So let’s jump right into it and take a look at how the authors of today’s astrobite got a machine to identify fast radio bursts. ...

Fast Radio Burst 121102 Pulse Detection and Periodicity: A Machine Learning Approach ~ Yunfan Gerry Zhang et al
viewtopic.php?t=38683
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What Should We Assume?

Post by bystander » Thu Sep 27, 2018 4:19 pm

What Should We Assume?
Astrobites | 2018 Sep 25
Emily Sandford wrote:
Everyone warns you: don’t make assumptions, because when you ASSUME, you look as foolish AS a SUM of Elephant seals, or however it goes.

But assumptions are useful, as long as they’re based on facts! On the weekends, for example, I just assume, based on prior experience, that an NYC subway journey will take about twenty minutes longer than it’s supposed to because of crowds and construction. More often than not, I arrive about when I expect based on that assumption.

Of course, if the transit authority magically got its act together, I’d have to update my beliefs—I wouldn’t let an old assumption about slow subways mislead me into showing up awkwardly early for things forever. It would probably only take two or three fast train journeys before I stopped building in that extra 20 minutes. My observations, in other words, would take precedence over my assumptions.

But what if I couldn’t test my assumptions against the real world so effectively? What if I were working from very limited data? In the subway analogy, what if I had moved away from New York years ago, but I were still advising tourists about travel time based on how things used to be? My advice might be better than nothing, but still inadequate or misleading.

Today’s authors investigate: What happens when data are scarce, and you have to let your assumptions guide you? How do you choose your assumptions wisely, so you’re misled as rarely as possible? ...

Improving Orbit Estimates for Incomplete Orbits with a New Approach to Priors –
with Applications from Black Holes to Planets
~ K. Kosmo O'Neil et al
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Going Against the Galactic Flow

Post by bystander » Thu Sep 27, 2018 4:31 pm

Going Against the Galactic Flow
Astrobites | 2018 Sep 26
Tomer Yavetz wrote:
More often than not, when you hear an astrophysicist talking about a galactic rotation curve, you can expect their next sentence to be somehow related to dark matter. Indeed, one of the main reasons to believe that dark matter exists in the first place is the ubiquity of flat rotation curves in observations of galaxies. But today’s article focuses on a couple of galaxies with rotation curves that have an additional quirky feature – a large “counter-rotating” group of stars that seem to be moving in the opposite direction as the rest of the galaxy. ...

The properties of the kinematically distinct components in NGC 448 and NGC 4365 ~ B. Nedelchev et al
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Changing with the Tide

Post by bystander » Thu Sep 27, 2018 4:37 pm

Changing with the Tide
Astrobites | 2018 Sep 27
Mara Zimmerman wrote:
Systems of planets and stars influence each other’s every movement; anyone on Earth needs to only look outside to remind themselves of that. But how do these bodies influence each other over long periods of time? The authors of today’s paper attempt to answer this broad question as it pertains to WASP 12. ...

Understanding WASP-12b ~ Avery Bailey, Jeremy Goodman
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Gravitational Redshift and the Pup

Post by bystander » Wed Oct 03, 2018 4:07 pm

Gravitational Redshift and the Pup:
Measuring the Mass of Sirius B

Astrobites | 2018 Oct 02
Daniel Berke wrote:
White dwarfs are fascinating stellar remnants left over at the end of the lifetimes of many stars. Only about the size of Earth, these tiny objects can potentially be more massive than the Sun. And unlike many of the exotic objects we study in astronomy we have one situated right next door to us! Located a mere 8.6 light-years away, Sirius (the brightest star in the night sky), is actually a binary system. Sirius A, the one we see, is a main sequence A-type star, while its invisible companion Sirius B is a white dwarf. (Sirius B is also affectionately called “the Pup” due to Sirius A being known historically as the “Dog Star.”)

Sirius B was discovered on January 31, 1862, and was recognized as a white dwarf in 1915, only the second one to be discovered (after 40 Eridani B, as related in this astrobite). A year later in 1916 Einstein published his theory of General Relativity, with one of its predictions being that light leaving a star should be affected by gravitational redshift. (This is where light climbing out of a gravitational well loses energy and appears redder.) In 1924 the astronomer Arthur Eddington realized that, since Sirius B was so small and dense, it should show a measurable gravitational redshift. This was measured for the first time in 1925 by Walter Adams at the Mt. Wilson Observatory and considered a big success for General Relativity. (Although we now know that both the predicted and measured shift were about four times too low; it’s speculated that the spectra of Sirius B may have been contaminated by light from Sirius A which is very nearby on the sky.)

By measuring the gravitational redshift of a star with a known radius we can also measure its mass, since for a typical stellar object (i.e., not a neutron star or black hole) the gravitational redshift depends only on those two quantities. We can measure the distance to Sirius B using its parallax very well since it’s so nearby, and by measuring its luminosity (based on its brightness and temperature) we can work out its radius. We can also measure its mass dynamically, by observing how it and Sirius A orbit around their common center of mass and applying Kepler’s laws of orbital motion, but there’s a small problem: estimates of Sirius B’s mass based on measurements of its gravitational redshift have historically differed from its dynamically-measured mass by about 10% (to be clear, this is from new measurements taken after the original ones were realized to be wrong).

To clear up this long-standing confusion, the authors of today’s paper used the Hubble Space Telescope to take spectra of both Sirius A and B in order to perform a differential measurement of the gravitational redshifts of both. Differential measurement is a useful tool as it helps eliminate a lot of systematic errors that might be present in an instrument, since any that exist will affect all observations equally. The gravitational redshift of Sirius A is better known than for Sirius B, so by measuring both, finding the difference between them, and correcting for the known value of Sirius A’s redshift it’s possible to make a more precise and accurate measurement of the gravitational redshift of Sirius B than would be possible by observing it alone. ...

The gravitational redshift of Sirius B ~ Simon R.G. Joyce et al
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Some Like it Hot

Post by bystander » Wed Oct 03, 2018 4:15 pm

Some Like it Hot
Astrobites | 2018 Oct 03
Eckhart Spalding wrote:
Direct imaging has turned up only a handful of planets. However, as observing sensitivities get better in the coming years, the technique will become a powerful probe of planet formation physics. Part of the reason planets are so challenging to image is that planets don’t carry out fusion themselves, so they just slowly cool and become dimmer with time.

How, exactly, do they cool? We need to know this in order to convert the measured luminosity of a planet into meaningful data, like the planet’s mass. For that, we have to mostly rely on evolutionary models to predict the cooling curve. The authors of today’s paper do this by tackling the physics of the accretion process during its most rapid phase, when the growing protoplanet’s gravitational well consumes material as fast as the surrounding disk can supply it.

In the field of planet formation physics, “hot start” and “cold start” and a gradient of “warm” starts in between refer to the starting entropies of planets. These terms do not necessarily indicate the formation mechanism. The authors of today’s paper specifically investigate the core accretion mechanism to see what interior entropies, and by extension luminosities, it can lead to. ...

The Evolution of Gas Giant Entropy During Formation by Runaway Accretion ~ David Berardo et al
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How Many Planets Will We Find Around White Dwarfs?

Post by bystander » Sat Oct 06, 2018 1:56 pm

How Many Planets Will We Find Around White Dwarfs?
Astrobites | 2018 Oct 04
Matthew Green wrote:
Do white dwarfs have planets? The short answer is yes—but they’re difficult to detect by the traditional methods people use to look for exoplanets. In the next decade this might change thanks to a ground-breaking project called the Large Synoptic Survey Telescope, or LSST. Today’s paper uses simulations to estimate what LSST will see. ...

On the detectability of transiting planets orbiting white dwarfs using LSST ~ Jorge Cortes, David M. Kipping
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LISA forewarnings can help LIGO study black holes

Post by bystander » Sat Oct 06, 2018 2:10 pm

LISA forewarnings can help LIGO study black holes
Astrobites | 2018 Oct 05
Lisa Drummond wrote:
We have now detected several gravitational wave events (including one event with an electromagnetic counterpart, GW170817) since the first was detected on September 14th, 2015. So far, all the signals have been produced by compact binary mergers and observed using the ground-based detector LIGO (Laser Interferometer Gravitational Wave Observatory) and, more recently, VIRGO, another ground-based detector in Europe. In the future, space-based detector LISA (Laser Interferometer Space Antenna) will also detect gravitational waves (see Figure 1), and in particular will be sensitive to the lower-frequency band of the gravitational-wave spectrum.

Today’s paper focuses on how we can exploit LIGO and LISA in conjunction to greatly enhance our understanding of the nature of black holes. LISA will be able to detect the early inspiral stages of the compact binary, thereby giving a “forewarning” of when the signal will be detectable in the LIGO band (the waves become higher frequency later on). With weeks to years of advance warning due to LISA, LIGO can be optimised to make the most of the future detection of the predicted signal. ...

Optimizing LIGO with LISA forewarnings to improve black-hole spectroscopy ~ Rhondale Tso, Davide Gerosa, Yanbei Chen
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What is going on in the disk around HD 142527?

Post by bystander » Wed Oct 10, 2018 5:13 pm

A beautiful, yet complicated painting:
What is going on in the disk around HD 142527?

Astrobites | 2018 Oct 09
Arianna Musso Barcucci wrote:
We are currently in a golden era of circumstellar disk imaging: new instruments like ALMA and SPHERE (both located in Chile) are gifting us with stunning images of systems surrounding nearby stars, with their glowing disks, neatly carved gaps, rings, and beautiful spiral arms (check this astrobite for a good review on protoplanetary disks).

While most disks show only one or two of the aforementioned features, HD 142527 decided to make it big: the system around this 5-million-year-old star (Figure 1) is nothing short of spectacular.

But the wide variety of features and phenomena taking place at the same time makes this disk a puzzle to interpret! So far, there is no explanation for the entire artwork at once. ...

Circumbinary, not transitional: On the spiral arms, cavity, shadows,
fast radial flows, streamers and horseshoe in the HD142527 disc
~ Daniel J. Price et al
viewtopic.php?t=35642
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Magnetar Madness in Super Luminous Supernovae

Post by bystander » Wed Oct 10, 2018 5:29 pm

Magnetar Madness in Super Luminous Supernovae
Astrobites | 2018 Oct 10
Lauren Sgro wrote:
There are a lot of scary things out there in space: you know – like giant stars, black holes, aliens. But have you ever heard of a magnetar? On Earth’s surface, we experience a magnetic field less than 1 Gauss. The Sun’s magnetic field is not much more at about 25 Gauss. But a neutron star? Try a trillion Gauss. A magnetar is basically a neutron star on steroids with a magnetic field 1,000 times that of its normal counterpart. (You can read more about magnetars themselves in this astrobite).

Magnetars can be formed in supernovae explosions, just like neutron stars (the details of what makes them so magnetic is up for debate). The authors of today’s paper are studying some strange supernovae, and they want to know if the observations can be explained by modeling those supernovae with a magnetar at the center instead of the ordinary neutron star.

The authors are looking into two types of supernovae events, specifically: a type of hydrogen-rich super luminous supernovae (SLSNe II) and what we call Type II-P supernovae (SNe). Type II-P SNe are also hydrogen-rich, but they have a plateau in their light curve, meaning that as the luminosity declines from its peak, it hits a stretch where it declines at a very slow rate. For the case of SLSNe II, magnetars are typically used to model H-free SLSNe (SLSNe I), while interactions with circumstellar material are used to explain SLSNe II. What happens if a magnetar is used to model SLSNe II instead of space stuff around the progenitor star? ...

Systematic study of magnetar-powered hydrogen-rich supernovae ~ M. Orellana, M.C. Bersten, T.J. Moriya
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59 Binary Neutron Star Merger Simulations

Post by bystander » Sat Oct 13, 2018 3:52 pm

59 (Fifty-nine!) Binary Neutron Star Merger Simulations
Astrobites | 2018 Oct 11
Sanjana Curtis wrote:
Neutron star mergers are absolutely fascinating. These events are not just sources of gravitational waves but of electromagnetic radiation all across the spectrum – and of neutrinos as well. If you missed the amazing multimessenger observations last year that gave us a peek into what binary neutron star (BNS) systems are up to, please check out this bite about GW170817! The observations had major implications for many fundamental questions in astrophysics. The gravitational wave signal from the merger was detected along with the electromagnetic radiation produced. As a result, we were able to confirm that neutron star mergers are a site where heavy elements (those beyond iron) can be made via the r-process.

While all of this has undoubtedly been extremely cool (and we’re holding our collective breath for more data), there’s a lot of work that remains to be done. We need accurate predictions of the quantity and composition of material ejected in mergers in order to fully understand the origin of the heavy elements, and to say whether BNS mergers are the only r-process site. To investigate such questions, we require theoretical models that include all the relevant physics. Today’s paper presents the largest set of NS merger simulations with realistic microphysics to date. By realistic microphysics, we mean that the simulations also take into account what the atoms and subatomic particles are doing. This is done by using nuclear theory based descriptions of the matter in neutron stars, and by including composition and energy changes due to neutrinos (albeit in an approximate way). ...

Binary Neutron Star Mergers: Mass Ejection, Electromagnetic Counterparts and Nucleosynthesis ~ David Radice et al
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