 Subjects -> ASTRONOMY (Total: 94 journals)
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- LIGO tells us LINERs are not optically thick RIAFs
- Authors: Ford K; McKernan B.
Abstract: ABSTRACTLow ionization nuclear emission-line regions (LINERs) are a heterogeneous collection of up to one-third of galactic nuclei in the local Universe. It is unclear whether LINERs are simply the result of low accretion rates onto supermassive black holes (BHs) or whether they include a large number of optically thick radiatively inefficient but super-Eddington accretion flows (RIAFs). Optically thick RIAFs are typically discs of large-scale height or quasi-spherical gas flows. These should be dense enough to trap and merge a large number of the stellar mass BHs, which we expect to exist in galactic nuclei. Electromagnetic observations of photospheres of accretion flows do not allow us to break model degeneracies. However, gravitational wave observations probe the interior of accretion flows where the merger of stellar mass BHs can be greatly accelerated over the field rate. Here, we show that the upper limits on the rate of BH mergers observed with LIGO demonstrate that most LINERs cannot be optically thick RIAFs.
PubDate: Wed, 09 Oct 2019 00:00:00 GMT
DOI: 10.1093/mnrasl/slz116
Issue No: Vol. 490, No. 1 (2019)
- Super-giant pulses from the Crab pulsar: energy distribution and
occurrence rate- Authors: Bera A; Chengalur J.
Abstract: ABSTRACTWe present statistical analysis of a fluence-limited sample of over 1100 giant pulses from the Crab pulsar, with fluence > 130 Jy ms at ∼1330 MHz. These were detected in ∼260 h of observation with the National Centre for Radio Astrophysics (NCRA) 15 m radio telescope. We find that the pulse-energy distribution follows a power law with index $\rm \alpha \approx -3$ at least up to a fluence of ∼5 Jy s. The power-law index agrees well with that found for lower-energy pulses in the range 3–30 Jy ms. The fluence distribution of the Crab pulsar hence appears to follow a single power law over ∼3 orders of magnitude in fluence. We do not see any evidence for the flattening at high fluences reported by earlier studies. We also find that, at these fluence levels, the rate of giant-pulse emission varies by as much as a factor of ∼5 on time-scales of a few days, although the power-law index of the pulse-energy distribution remains unchanged. The slope of the fluence distribution for Crab giant pulses is similar to that recently determined for the repeating FRB 121102. We also find an anti-correlation between the pulse fluence and the pulse width, so that more energetic pulses are preferentially shorter.
PubDate: Tue, 24 Sep 2019 00:00:00 GMT
DOI: 10.1093/mnrasl/slz140
Issue No: Vol. 490, No. 1 (2019)
- G4.8+6.2, a possible kilonova remnant'
- Authors: Liu Y; Zou Y, Jiang B, et al.
Abstract: ABSTRACTKilonova explosions typically release ∼1050–51 erg in kinetic energy, which is sufficient to constitute a kilonova remnant (KNR); however, it has not yet been confirmed. In this work, we investigate the probable association between G4.8+6.2 and the guest star of AD 1163, which is recorded by the Korean ancient astronomers. Although the evidence available is insufficient to draw a definite conclusion, it is at least theoretically self-consistent that the guest star of AD 1163 was a historical kilonova associated with G4.8+6.2, considering the possible short visible timescale of AD 1163, the relatively high Galactic latitude of G4.8+6.2, and that G4.8+6.2 is spatially coincident with the guest star of AD 1163. Further observation of G4.8+6.2 is needed to test our hypothesis. If our interpretation is correct, our results indicate that young KNRs should have a large diameter and low surface brightness, unlike other young supernova remnants.
PubDate: Fri, 20 Sep 2019 00:00:00 GMT
DOI: 10.1093/mnrasl/slz141
Issue No: Vol. 490, No. 1 (2019)
- Cosmic-ray-driven outflow from the Galactic Centre and the origin of
magnetized radio filaments- Authors: Yusef-Zadeh F; Wardle M.
Abstract: AbstractRadio, X-ray, and infrared observations of the inner few hundred parsecs of the Galactic Centre have highlighted two characteristics of the interstellar medium. The cosmic-ray ionization rate derived from molecular ions such as H$^+_3$ is at least two to three orders of magnitude higher than in the Galactic disc. The other is bipolar X-ray and radio emission away from the Galactic plane. These features are consistent with a scenario in which high cosmic-ray pressure drives large-scale winds away from the Galactic plane. The interaction of such a wind with stellar wind bubbles may explain the energetic non-thermal radio filaments found throughout the Galactic Centre. Some of the implications of this scenario is the removal of gas driven by outflowing winds, acting as a feedback to reduce the star formation rate in the central molecular zone (CMZ), and the distortion of azimuthal magnetic field lines in the CMZ to vertical direction away from the plane. The combined effects of the wind and the vertical magnetic field can explain why most magnetized filaments run perpendicular to the galactic plane. This proposed picture suggests our Milky Way nucleus has recently experienced starburst or black hole activity, as recent radio and X-ray observations indicate.
PubDate: Fri, 13 Sep 2019 00:00:00 GMT
DOI: 10.1093/mnrasl/slz134
Issue No: Vol. 490, No. 1 (2019)
- Group pre-processing versus cluster ram-pressure stripping: the case of
ESO156−G029- Authors: Džudžar R; Kilborn V, Murugeshan C, et al.
Abstract: ABSTRACTWe report on observations of ESO156−G029, member of a galaxy group which is positioned at the virial radius of cluster Abell 3193. ESO156−G029 is located ∼1.4 Mpc in projected distance from the brightest cluster galaxy NGC1500. We show that ESO156−G029 has disturbed gas kinematics and a highly asymmetric neutral hydrogen (H i) distribution, which are consequences of group pre-processing, and possibly of ram pressure. Based on the current data we propose a scenario in which ESO156−G029 had a minor gas-rich merger in the past and now starts to experience ram pressure. We infer that the galaxy will undergo rapid evolution once it gets closer to the cluster centre (less than 0.5 Mpc) where ram pressure is strong enough to begin stripping the H i from the galaxy.
PubDate: Sat, 07 Sep 2019 00:00:00 GMT
DOI: 10.1093/mnrasl/slz139
Issue No: Vol. 490, No. 1 (2019)
- The discovery of secondary lobes in the Seyfert galaxy NGC 2639
- Authors: Sebastian B; Kharb P, O’Dea C, et al.
Abstract: ABSTRACTWe report the discovery of a secondary pair of radio lobes in the Seyfert galaxy NGC 2639 with polarization-sensitive observations with the Karl G. Jansky Very Large Array (VLA). The presence of these lobes, which are aligned nearly perpendicular to the known set of radio lobes observed in the east–west direction, has not been reported previously in the literature. The in-band rotation measure image shows gradients in both the lobes indicative of organized magnetic field structures on kpc-scales. The magnetic field structure is aligned with the jet/lobe direction in both the lobes. Based on the settled optical morphology of the host galaxy, it is likely that a minor merger that did not disrupt the host galaxy structure is responsible for the observed features in NGC 2639. This also explains the near 90° change in the jet direction; the current jet direction being the result of a new accretion disc formed by the minor merger, whose direction was a result of the angular momentum of the inflowing merger gas.
PubDate: Thu, 05 Sep 2019 00:00:00 GMT
DOI: 10.1093/mnrasl/slz136
Issue No: Vol. 490, No. 1 (2019)
- Graphene oxide nanoparticles in the interstellar medium
- Authors: Sarre P.
Abstract: ABSTRACTDust particles play a major role in the formation, evolution and chemistry of interstellar clouds, stars, and planetary systems. Commonly identified forms include amorphous and crystalline carbon-rich particles and silicates. Also present in many astrophysical environments are polycyclic aromatic hydrocarbons (PAHs), detected through their infrared emission, and which are essentially small flakes of graphene. Astronomical observations over the past four decades have revealed a widespread unassigned ‘extended red emission’ (ERE) feature which is attributed to luminescence of dust grains. Numerous potential carriers for ERE have been proposed but none has gained general acceptance. In this Letter it is shown that there is a strong similarity between laboratory optical emission spectra of graphene oxide (GO) and ERE, leading to this proposal that emission from GO nanoparticles is the origin of ERE and that these are a significant component of interstellar dust. The proposal is supported by infrared emission features detected by the Infrared Space Observatory (ISO) and the Spitzer Space Telescope.
PubDate: Thu, 29 Aug 2019 00:00:00 GMT
DOI: 10.1093/mnrasl/slz131
Issue No: Vol. 490, No. 1 (2019)
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