| 1. |
- Si, Ran, et al.
(författare)
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A First Spectroscopic Measurement of the Magnetic-field Strength for an Active Region of the Solar Corona
- 2020
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Ingår i: Astrophysical Journal Letters. - : Institute of Physics (IOP). - 2041-8205 .- 2041-8213. ; 898:2
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Tidskriftsartikel (refereegranskat)abstract
- For all involved in astronomy, the importance of monitoring and determining astrophysical magnetic-field strengths is clear. It is also a well-known fact that the corona magnetic fields play an important part in the origin of solar flares and the variations of space weather. However, after many years of solar corona studies, there is still no direct and continuous way to measure and monitor the solar magnetic-field strength. We present here a scheme that allows such a measurement, based on a careful study of an exotic class of atomic transitions, known as magnetic induced transitions, in Fe9+. In this contribution we present a first application of this methodology and determine a value of the coronal field strength using the spectroscopic data from Hinode.
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| 2. |
- Hayakawa, Hisashi, et al.
(författare)
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The extreme space weather event in 1903 october/november : An outburst from the quiet sun
- 2020
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 897:1
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Tidskriftsartikel (refereegranskat)abstract
- While the Sun is generally more eruptive during its maximum and declining phases, observational evidence shows certain cases of powerful solar eruptions during the quiet phase of solar activity. Occurring in the weak Solar Cycle 14 just after its minimum, the extreme space weather event in 1903 October-November is one of these cases. Here, we reconstruct the time series of geomagnetic activity based on contemporary observational records. With the mid-latitude magnetograms, the 1903 magnetic storm is thought to be caused by a fast coronal mass ejection (≈1500 km s-1) and is regarded as a superstorm with an estimated minimum of the equivalent disturbance storm time index (Dst') of ≈-531 nT. The reconstructed time series has been compared with the equatorward extension of auroral oval (≈44. 1 in invariant latitude) and the time series of telegraphic disturbances. This case study shows that potential threats posed by extreme space weather events exist even during weak solar cycles or near their minima.
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| 3. |
- Cliver, E. W., et al.
(författare)
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Solar Longitude Distribution of High-energy Proton Flares : Fluences and Spectra
- 2020
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 900:1
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Tidskriftsartikel (refereegranskat)abstract
- The distribution of the longitudes of solar flares associated with the high-energy proton events called ground level events (GLEs) can be approximated by a Gaussian with a peak at ∼W60, with a full range from ∼E90 to ∼W150. The longitudes of flares associated with the top third (24 of 72) of GLEs in terms of their >430 MeV fluences (F 430) are primarily distributed over E20-W100 with a skew toward disk center. This 120 span in longitude is comparable to the latitudinal spans of powerful coronal mass ejections (CMEs) from limb flares. Only 5 of 24 strong GLEs are located within the W40-80 zone of good magnetic connection to Earth. GLEs with hard spectra, i.e., a spectral index SI30/200(= log(F 30/F 200)) < 1.5, also tend to avoid W40-80 source regions. Three-fourths of such events (16 of 21) arise in flares outside this range. The above tendencies favor a CME-driven shock source over a flare-resident acceleration process for high-energy solar protons. GLE spectra show a trend, with broad scatter, from hard spectra for events originating in eruptive flares beyond the west limb to soft spectra for GLEs with sources near central meridian. This behavior can be explained in terms of: (1) dominant near-Sun quasi-perpendicular shock acceleration of protons for far western (>W100) GLEs; (2) quasi-parallel shock acceleration for well-connected (W40-80) GLEs, and (3) proton acceleration/trapping at CME-driven bow shocks from central meridian (E20-W20) that strike the Earth.
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| 4. |
- Harper, Graham M., et al.
(författare)
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Sofia-exes observations of betelgeuse during the great dimming of 2019/2020
- 2020
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 893:1
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Tidskriftsartikel (refereegranskat)abstract
- In 2019 October Betelgeuse began a decline in V-band brightness that went beyond the minimum expected from its quasi-periodic ∼420 day cycle, becoming the faintest in recorded photometric history. Observations obtained in 2019 December with Very Large Telescope/SPHERE have shown that the southern half of the star has become markedly fainter than in 2019 January, indicating that a major change has occurred in, or near, the photosphere. We present Stratospheric Observatory for Infrared Astronomy (SOFIA) Echelon Cross Echelle Spectrograph (EXES) high spectral-resolution observations of [Fe II] 25.99 μm and [S I] 25.25 μm emission lines from Betelgeuse obtained during the unprecedented 2020 February V-band brightness minimum to investigate potential changes in the circumstellar flow. These spectra are compared to observations obtained in 2015 and 2017 when the V magnitude was typical of brighter phases. We find only very small changes in the gas velocities reflected by either of the line profiles, no significant changes in the flux to continuum ratios, and hence no significant changes in the [Fe ii]/[S i] flux ratios. There is evidence that absorption features have appeared in the 2020 continuum. The Alfvén wave-crossing time from the upper photosphere is sufficiently long that one would not expect a change in the large-scale magnetic field to reach the circumstellar [Fe ii] and [S i] line-forming regions, 3 < R (R ∗) < 20. However, the light-crossing time is of order a few hours and a reduction in luminosity may reduce the dust-gas heating rate and [O i] 63.19 μm emission, which has contributions from R > 20R ∗, where significant circumstellar oxygen-rich dust is observed.
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| 5. |
- Jacobson-Galán, Wynn V., et al.
(författare)
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Late-time Observations of Calcium-rich Transient SN 2019ehk Reveal a Pure Radioactive Decay Power Source
- 2021
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 908:2
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Tidskriftsartikel (refereegranskat)abstract
- We present multiband Hubble Space Telescope imaging of the calcium-rich supernova (SN) SN 2019ehk at 276-389 days after explosion. These observations represent the latest B-band to near-IR photometric measurements of a calcium-rich transient to date and allow for the first opportunity to analyze the late-time bolometric evolution of an object in this observational SN class. We find that the late-time bolometric light curve of SN 2019ehk can be described predominantly through the radioactive decay of 56Co for which we derive a mass of M(56Co) = (2.8 ± 0.1) × 10-2 M o. Furthermore, the rate of decline in bolometric luminosity requires the leakage of γ-rays on timescale t γ = 53.9 ± 1.30 days, but we find no statistical evidence for incomplete positron trapping in the SN ejecta. While our observations cannot constrain the exact masses of other radioactive isotopes synthesized in SN 2019ehk, we estimate a mass ratio limit of M(57Co)/M(56Co) ≤ 0.030. This limit is consistent with the explosive nucleosynthesis produced in the merger of low-mass white dwarfs, which is one of the favored progenitor scenarios in early-time studies of SN 2019ehk.
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| 6. |
- Abdalla, H., et al.
(författare)
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Probing the Magnetic Field in the GW170817 Outflow Using HESS Observations
- 2020
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Ingår i: Astrophysical Journal Letters. - : Institute of Physics Publishing (IOPP). - 2041-8205 .- 2041-8213. ; 894:2, s. 1-5
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Tidskriftsartikel (refereegranskat)abstract
- The detection of the first electromagnetic counterpart to the binary neutron star (BNS) merger remnant GW170817 established the connection between short gamma-ray bursts and BNS mergers. It also confirmed the forging of heavy elements in the ejecta (a so-called kilonova) via the r-process nucleosynthesis. The appearance of nonthermal radio and X-ray emission, as well as the brightening, which lasted more than 100 days, were somewhat unexpected. Current theoretical models attempt to explain this temporal behavior as either originating from a relativistic off-axis jet or a kilonova-like outflow. In either scenario, there is some ambiguity regarding how much energy is transported in the nonthermal electrons versus the magnetic field of the emission region. Combining the Very Large Array (radio) and Chandra (X-ray) measurements with observations in the GeV-TeV domain can help break this ambiguity, almost independently of the assumed origin of the emission. Here we report for the first time on deep H.E.S.S. observations of GW170817/GRB 170817A between 124 and 272 days after the BNS merger with the full H.E.S.S. array of telescopes, as well as on an updated analysis of the prompt (<5 days) observations with the upgraded H.E.S.S. phase-I telescopes. We discuss implications of the H.E.S.S. measurement for the magnetic field in the context of different source scenarios.
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| 7. |
- Brandenburg, Axel, et al.
(författare)
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Hemispheric Handedness in the Galactic Synchrotron Polarization Foreground
- 2020
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 896:1
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Tidskriftsartikel (refereegranskat)abstract
- The large-scale magnetic field of the Milky Way is thought to be created by an alpha omega dynamo, which implies that it should have opposite handedness north and south of the Galactic midplane. Here we attempt to detect a variation in handedness using polarization data from the Wilkinson Microwave Anisotropy Probe. Previous analyzes of the parity-even and parity-odd parts of linear polarization of the global dust and synchrotron emission have focused on quadratic correlations in spectral space of, and between, these two components. Here, by contrast, we analyze the parity-odd polarization itself and show that it has, on average, opposite signs in northern and southern Galactic hemispheres. Comparison with a Galactic mean-field dynamo model shows broad qualitative agreement and reveals that the sign of the observed hemispheric dependence of the azimuthally averaged parity-odd polarization is not determined by the sign of alpha, but by the sense of differential rotation.
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| 8. |
- Coppejans, D. L., et al.
(författare)
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A Mildly Relativistic Outflow from the Energetic, Fast-rising Blue Optical Transient CSS161010 in a Dwarf Galaxy
- 2020
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 895:1
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Tidskriftsartikel (refereegranskat)abstract
- We present X-ray and radio observations of the Fast Blue Optical Transient CRTS-CSS161010 J045834-081803 (CSS161010 hereafter) at t = 69-531 days. CSS161010 shows luminous X-ray (L-x similar to 5 x 10(39) erg s(-1)) and radio (L-nu similar to 10(29) erg s(-1) Hz(-1)) emission. The radio emission peaked at similar to 100 days post-transient explosion and rapidly decayed. We interpret these observations in the context of synchrotron emission from an expanding blast wave. CSS161010 launched a mildly relativistic outflow with velocity Gamma beta c >= 0.55c at similar to 100 days. This is faster than the non-relativistic AT 2018cow (Gamma beta c similar to 0.1c) and closer to ZTF18abvkwla (Gamma beta c >= 0.3c at 63 days). The inferred initial kinetic energy of CSS161010 (E-k greater than or similar to 10(51) erg) is comparable to that of long gamma-ray bursts, but the ejecta mass that is coupled to the mildly relativistic outflow is significantly larger (similar to 0.01-.1 M-circle dot). This is consistent with the lack of observed gamma-rays. The luminous X-rays were produced by a different emission component to the synchrotron radio emission. CSS161010 is located at similar to 150 Mpc in a dwarf galaxy with stellar mass M-* similar to 10(7) M-circle dot and specific star formation rate sSFR similar to 0.3 Gyr(-1). This mass is among the lowest inferred for host galaxies of explosive transients from massive stars. Our observations of CSS161010 are consistent with an engine-driven aspherical explosion from a rare evolutionary path of a H-rich stellar progenitor, but we cannot rule out a stellar tidal disruption event on a centrally located intermediate-mass black hole. Regardless of the physical mechanism, CSS161010 establishes the existence of a new class of rare (rate < 0.4% of the local core-collapse supernova rate) H-rich transients that can launch mildly relativistic outflows.
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| 9. |
- Currie, Thayne, et al.
(författare)
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SCExAO/CHARIS Direct Imaging Discovery of a 20 au Separation, Low-mass Ratio Brown Dwarf Companion to an Accelerating Sun-like Star
- 2020
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 904:2
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Tidskriftsartikel (refereegranskat)abstract
- We present the direct imaging discovery of a substellar companion to the nearby Sun-like star, HD 33632 Aa, at a projected separation of similar to 20 au, obtained with SCExAO/CHARIS integral field spectroscopy complemented by Keck/NIRC2 thermal infrared imaging. The companion, HD 33632 Ab, induces a 10.5 sigma astrometric acceleration on the star as detected with the Gaia and Hipparcos satellites. SCExAO/CHARIS JHK (1.1-2.4 mu m) spectra and Keck/NIRC2 L-p (3.78 mu m) photometry are best matched by a field L/T transition object: an older, higher-gravity, and less dusty counterpart to HR 8799 cde. Combining our astrometry with Gaia/Hipparcos data and archival Lick Observatory radial velocities, we measure a dynamical mass of 46.4 8 M-J and an eccentricity of e < 0.46 at 95% confidence. HD 33632 Ab's mass and mass ratio (4.0% 0.7%) are comparable to the low-mass brown dwarf GJ 758 B and intermediate between the more massive brown dwarf HD 19467 B and the (near-)planet-mass companions to HR 2562 and GJ 504. Using Gaia to select for direct imaging observations with the newest extreme adaptive optics systems can reveal substellar or even planet-mass companions on solar system-like scales at an increased frequency compared to blind surveys.
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| 10. |
- Fremling, Christoffer, et al.
(författare)
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SNIascore : Deep-learning Classification of Low-resolution Supernova Spectra
- 2021
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 917:1
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Tidskriftsartikel (refereegranskat)abstract
- We present SNIascore, a deep-learning-based method for spectroscopic classification of thermonuclear supernovae (SNe Ia) based on very low-resolution (R similar to 100) data. The goal of SNIascore is the fully automated classification of SNe Ia with a very low false-positive rate (FPR) so that human intervention can be greatly reduced in large-scale SN classification efforts, such as that undertaken by the public Zwicky Transient Facility (ZTF) Bright Transient Survey (BTS). We utilize a recurrent neural network architecture with a combination of bidirectional long short-term memory and gated recurrent unit layers. SNIascore achieves a SNIascore simultaneously performs binary classification and predicts the redshifts of secure SNe Ia via regression (with a typical uncertainty of z = 0.01 to z = 0.12). For the magnitude-limited ZTF BTS survey (approximate to 70% SNe Ia), deploying SNIascore reduces the amount of spectra in need of human classification or confirmation by approximate to 60%. Furthermore, SNIascore allows SN Ia classifications to be automatically announced in real time to the public immediately following a finished observation during the night.
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