3,268 research outputs found
Black Hole Hyperaccretion Inflow-outflow Model. I. Long And Ultra-long Gamma-ray Bursts
Long-duration gamma-ray bursts (LGRBs) and ultra-LGRBs (ULGRBs) originate from collapsars, in the center of which a newborn rotating stellar-mass black hole (BH) surrounded by a massive accretion disk may form. In the scenario of the BH hyperaccretion inflow-outflow model and Blandford-Znajek (BZ) mechanism to trigger gamma-ray bursts (GRBs), the real accretion rate to power a BZ jet is far lower than the mass supply rate from the progenitor star. The characteristics of the progenitor stars can be constrained by GRB luminosity observations, and the results exceed usual expectations. LGRBs lasting from several seconds to tens of seconds in the rest frame may originate from solar-metallicity (Z ∼ 1 Z⊙, where Z and Z⊙ are the metallicities of progenitor stars and the Sun), massive (M ≳ 34 M⊙, where M and M⊙ are the masses of progenitor stars and the Sun) stars or some zerometallicity (Z ∼ 0) stars. A fraction of low-metallicity (Z ≲ 10-2Z⊙) stars, including Population III stars, can produce ULGRBs such as GRB 111209A. The fraction of LGRBs lasting less than tens of seconds in the rest frame is more than 40%, which cannot conform to the fraction of the demanded type of progenitor star. It possibly implies that the activity timescale of the central engine may be much longer than the observed timescale of prompt emission phase, as indicated by X-ray late-time activities. Alternatively, LGRBs and ULGRBs may be powered by a millisecond magnetar central engine. © 2017. The American Astronomical Society. All rights reserved
Bis(1H-benzimidazole-κN 3)bis(4-methylbenzoato-κ 2 O,O′)cobalt(II)
In the title mononuclear complex, [Co(C8H7O2)2(C7H6N2)2], the CoII atom is coordinated by four carboxylate O atoms from two 4-methylbenzoate ligands and two N atoms from two benzimidazole ligands in an octahedral coordination geometry. The molecules are assembled via intermolecular N—H⋯O hydrogen-bonding interactions into a three-dimensional network
Poly[[μ2-aqua-aqua(μ3-3,5-dinitrosalicylato)barium(II)] monohydrate]
In the title coordination polymer, {[Ba(C7H2N2O7)(H2O)2]·H2O}n, the BaII atom is ten-coordinated by seven O atoms from four 3,5-dinitrosalicylatate ligands, two μ2-bridging aqua ligands and one water molecule. The coordination mode is best described as a bicapped square-antiprismatic geometry. The 3,5-dinitrosalicylatate ligands bridge three Ba atoms. Centrosymmetrically related dinuclear barium units, with a Ba⋯Ba separation of 4.767 (5) Å, form infinite chains, which are further self-assembled into a supramolecular network through intermolecular O—H⋯O hydrogen-bonding interactions between O atoms of 3,5-dinitrosalicylatate ligands and water molecules
Searching for Compact Object Candidates from LAMOST Time-Domain Survey of Four K2 Plates
The time-domain (TD) surveys of the Large Sky Area Multi-Object Fiber
Spectroscopic Telescope (LAMOST) yield high-cadence radial velocities, paving a
new avenue to study binary systems including compact objects. In this work, we
explore LAMOST TD spectroscopic data of four K2 plates and present a sample of
six single-lined spectroscopic binaries that may contain compact objects. We
conduct analyses using phase-resolved radial velocity measurements of the
visible star, to characterize each source and to infer the properties of
invisible companion. By fitting the radial velocity curves for the six targets,
we obtain accurate orbital periods, ranging from (0.6-6) days, and
radial velocity semi-amplitudes, ranging from (50-130) km s. We
calculate the mass function of the unseen companions to be between 0.08 and
0.17 . Based on the mass function and the estimated stellar
parameters of the visible star, we determine the minimum mass of the hidden
star. Three targets, J034813, J063350, and J064850, show ellipsoidal
variability in the light curves from K2, ZTF, and TESS surveys. Therefore, we
can put constraints on the mass of the invisible star using the ellipsoidal
variability. We identify no X-ray counterparts for these targets except for
J085120, of which the X-ray emission can be ascribed to stellar activity. We
note that the nature of these six candidates is worth further characterization
utilizing multi-wavelength follow-up observations.Comment: 13 pages, 6 figures, accepted for publication in The Astronomical
Journa
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