ABSTRACT
When fast radio burst (FRB) waves propagate through the local (${\lesssim}\rm 1\, pc$) environment of the FRB source, electrons in the plasma undergo large-amplitude oscillations. The ...finite-amplitude effects cause the effective plasma frequency and cyclotron frequency to be dependent on the wave strength. The dispersion measure and rotation measure should therefore vary slightly from burst to burst for a repeating source, depending on the luminosity and frequency of the individual burst. Furthermore, free–free absorption of strong waves is suppressed due to the accelerated electrons’ reduced energy exchange in Coulomb collisions. This allows bright low-frequency bursts to propagate through an environment that would be optically thick to low-amplitude waves. Given a large sample of bursts from a repeating source, it would be possible to use the deficit of low-frequency and low-luminosity bursts to infer the emission measure of the local intervening plasma and its distance from the source. Information about the local environment will shed light on the nature of FRB sources.
AT2018cow: A Luminous Millimeter Transient Ho, Anna Y. Q.; Phinney, E. Sterl; Ravi, Vikram ...
The Astrophysical journal,
01/2019, Volume:
871, Issue:
1
Journal Article
Peer reviewed
Open access
We present detailed submillimeter- through centimeter-wave observations of the extraordinary extragalactic transient AT2018cow. The apparent characteristics-the high radio luminosity, the rise and ...long-lived emission plateau at millimeter bands, and the sub-relativistic velocity-have no precedent. A basic interpretation of the data suggests coupled to a fast but sub-relativistic ( ) shock in a dense ( ) medium. We find that the X-ray emission is not naturally explained by an extension of the radio-submm synchrotron spectrum, nor by inverse Compton scattering of the dominant blackbody UV/optical/IR photons by energetic electrons within the forward shock. By , the X-ray emission shows spectral softening and erratic inter-day variability. Taken together, we are led to invoke an additional source of X-ray emission: the central engine of the event. Regardless of the nature of this central engine, this source heralds a new class of energetic transients shocking a dense medium, which at early times are most readily observed at millimeter wavelengths.
There is a supermassive black hole of mass 4 × 10
solar masses at the centre of the Milky Way
. A large reservoir of hot (10
kelvin) and cooler (10
to 10
kelvin) gas surrounds it within a few parsecs
.... Although constraints on the amount of hot gas in the accretion zone of the black hole-that is, within 10
Schwarzschild radii (0.04 parsecs)-have been provided by X-ray observations
, the mass in cooler gas has been unconstrained. One possible way this cooler gas could be detected is by its emission in hydrogen recombination spectral lines
. Here we report imaging of a 10
-kelvin ionized gas disk within 2 × 10
Schwarzschild radii, using the 1.3-millimetre recombination line H30α. This emission line is double-peaked, with full velocity linewidth of about 2,200 kilometres per second. The emission is centred on the radio source Sagittarius A*, but the redshifted side is displaced 0.11 arcsec (0.004 parsecs at a distance of 8 kiloparsecs) to the northeast and the blueshifted side is displaced a similar distance to the southwest. We interpret these observations in terms of a rotating disk of mass 10
to 10
solar masses and mean hydrogen density of about 10
to 10
per cubic centimetre, with the values being sensitive to the assumed geometry. The emission is stronger than expected, given the upper limit on the strength of the Brγ spectral line of hydrogen. We suggest that the H30α transition is enhanced by maser emission.
ABSTRACT
A star is tidally stretched into an elongated stream after being disrupted by a supermassive black hole (BH). Using an approximate tidal equation, we calculate the stream’s thickness ...evolution along its geodesic, during which we treat the effect of nozzle shocks as a perfect bounce. Self-intersection occurs when the closest approach separation is smaller than the stream thickness. We explore a wide parameter space of orbital angular momenta, inclinations, and BH spins to obtain the properties of stream intersection. Two collision modes are identified: in ∼half of the cases, the collision occurs near the pericentre at an angle close to 0o (‘rear-end’ mode) and the other half have collisions far from the pericentre with collision angles close to 180o (‘head-on’ mode). The intersection typically occurs between consecutive half-orbits with a delay time that spans a wide range (from months up to a decade). The intersection radius generally increases with the orbital angular momentum and depends less strongly on the inclination and BH spin. The thickness ratio of the two colliding ends is of order unity and the transverse separation is a small fraction of the sum of the two thicknesses, so a large fraction of the stream’s mass is shock heated in an offset collision. Many of the numerical results can be analytically understood in a post-Newtonian picture, where we find the reason for stream collision to be a geometric one. Future hydrodynamic simulations including recombination are needed to understand the long-term effects of pressure forces which are neglected here.
Abstract The dynamics of the high-velocity compact molecular cloud CO-0.40-0.22 have been interpreted as evidence for a ∼105 M⊙ black hole within 60 pc of Sgr A*. Recently, Oka et al. have identified ...a compact millimetre-continuum source, CO-0.40-0.22*, with this candidate black hole. Here we present a collation of radio and infrared data at this location. Australia Telescope Compact Array constraints on the radio spectrum, and the detection of a mid-infrared counterpart, are in tension with an Sgr A*-like model for CO-0.40-0.22* despite the comparable bolometric to the Eddington luminosity ratios under the intermediate-mass black hole interpretation. A protostellar-disc scenario is, however, tenable. CO-0.40-0.22(*) is positionally coincident with an arrowhead-shaped infrared-dark cloud (which we call the Freccia Rossa). If the VLSR ≈ 70 km s−1 systemic velocity of CO-0.40-0.22 is common to the entire Freccia Rossa system, we hypothesize that it is the remnant of a high-velocity cloud that has plunged into the Milky Way from the Galactic halo.
We report the discovery of a detached double white dwarf binary with an orbital period of 20.6 minutes, PTF J053332.05+020911.6. The visible object in this binary, PTF J0533+0209B, is a 0.17 M mass ...white dwarf with a helium-dominated atmosphere containing traces of hydrogen. This object exhibits ellipsoidal variations due to tidal deformation, and is the visible component in a single-lined spectroscopic binary with a velocity semi-amplitude of KB = 618.7 6.9 km s−1. We have detected significant orbital decay due to the emission of gravitational radiation, and we expect that the Laser Interferometer Space Antenna (LISA) will detect this system with a signal to noise of after four years of operation. Because this system already has a well-determined orbital period, radial velocity semi-amplitude, temperature, atmospheric composition, surface gravity, and orbital decay rate, a LISA signal will help fully constrain the properties of this system by providing a direct measurement of its inclination. Thus, this binary demonstrates the synergy between electromagnetic and gravitational radiation for constraining the physical properties of an astrophysical object.
ABSTRACT PSR J2129−0429 is a "redback" eclipsing millisecond pulsar binary with an unusually long 15.2 hr orbit. It was discovered by the Green Bank Telescope in a targeted search of unidentified ...Fermi gamma-ray sources. The pulsar companion is optically bright (mean mR = 16.6 mag), allowing us to construct the longest baseline photometric data set available for such a system. We present 10 years of archival and new photometry of the companion from the Lincoln Near-Earth Asteroid Research Survey, the Catalina Real-time Transient Survey, the Palomar Transient Factory, the Palomar 60 inch, and the Las Cumbres Observatory Global Telescope. Radial velocity spectroscopy using the Double-Beam Spectrograph on the Palomar 200 inch indicates that the pulsar is massive: 1.74 0.18 . The G-type pulsar companion has mass 0.44 0.04 , one of the heaviest known redback companions. It is currently 95 1% Roche-lobe filling and only mildly irradiated by the pulsar. We identify a clear 13.1 mmag yr−1 secular decline in the mean magnitude of the companion as well as smaller-scale variations in the optical light curve shape. This behavior may indicate that the companion is cooling. Binary evolution calculations indicate that PSR J2129−0429 has an orbital period almost exactly at the bifurcation period between systems that converge into tighter orbits as black widows and redbacks and those that diverge into wider pulsar-white dwarf binaries. Its eventual fate may depend on whether it undergoes future episodes of mass transfer and increased irradiation.
ABSTRACT
The Zwicky Transient Facility has begun to discover binary systems with orbital periods that are less than 1 h. Combined with dedicated follow-up systems, which allow for high-cadence ...photometry of these sources, systematic confirmation and characterization of these sources are now possible. Here, we report the discovery of ZTF J190125.42+530929.5, a 40.6-min orbital period, eclipsing double white dwarf binary. Both photometric modelling and spectroscopic modelling confirm its nature, yielding an estimated inclination of $i = 86.2^{+0.6}_{-0.2}\, \rm deg$ and primary and secondary effective temperatures of $\textrm{{T}}_\textrm{eff} = 28\,000^{+500}_{-500}$ and $17\,600^{+400}_{-400}\, \mathrm{ K}$, respectively. This system adds to a growing list of sources for future gravitational-wave detectors and contributes to the demographic analysis of double degenerates.
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