Incompressible wind accretion [HEAP]

We present a simple, analytic model for the accretion flow of an incompressible wind onto a gravitating object. This solution corresponds to the Newtonian limit of a previously known relativistic model for a fluid obeying a stiff equation of state for which the sound speed is constant everywhere and equal to the speed of light. The new solution should be useful as a benchmark test for numerical hydrodynamics codes and, moreover, it can be used as an illustrative example in a gas dynamics course.

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E. Tejeda
Mon, 22 May 17

Comments: 7 pages, 3 figures. Submitted. Comments welcome

Investigating the nature of INTEGRAL Gamma-ray Bursts and sub-threshold triggers with Swift follow up [HEAP]

We explore the potential of INTEGRAL to improve our understanding of the low fluence regime for explosive transients, such as GRBs. We probe the nature of the so-called “WEAK” INTEGRAL triggers, when the gamma-ray instruments record intensity spikes that are below the usual STRONG significance thresholds. In a targeted Swift follow-up campaign, we observed 15 WEAK triggers. We find six of these can be classified as GRBs. This includes GRB150305A, a GRB discovered from our campaign alone. We also identified a source coincident with one trigger, IGRW151019, as a candidate AGN. We show that real events such as GRBs exist within the IBAS WEAK trigger population. A comparison of the fluence distributions of the full INTEGRAL IBAS and Swift BAT GRB samples showed that the two are similar. We also find correlations between the prompt gamma-ray and X-ray properties of the two samples, supporting previous investigations. We find that both satellites reach similar, low fluence levels regularly, although Swift is more sensitive to short, low fluence GRBs.

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A. Higgins, R. Starling, D. Gotz, et. al.
Mon, 22 May 17

Comments: 12 pages, 8 figures, accepted by MNRAS

Jet quenching in the neutron star low-mass X-ray binary 1RXS J180408.9$-$342058 [HEAP]

We present quasi-simultaneous radio (VLA) and X-ray ($Swift$) observations of the neutron star low-mass X-ray binary (NS-LMXB) 1RXS J180408.9$-$342058 (J1804) during its 2015 outburst. We found that the radio jet of J1804 was bright ($232 \pm 4 \mu$Jy at $10$ GHz) during the initial hard X-ray state, before being quenched by more than an order of magnitude during the soft X-ray state ($19 \pm 4 \mu$Jy). The source then was undetected in radio (< $13 \mu$Jy) as it faded to quiescence. In NS-LMXBs, possible jet quenching has been observed in only three sources and the J1804 jet quenching we show here is the deepest and clearest example to date. Radio observations when the source was fading towards quiescence ($L_X = 10^{34-35}$ erg s$^{-1}$) show that J1804 must follow a steep track in the radio/X-ray luminosity plane with $\beta > 0.7$ (where $L_R \propto L_X^{\beta}$). Few other sources have been studied in this faint regime, but a steep track is inconsistent with the suggested behaviour for the recently identified class of transitional millisecond pulsars. J1804 also shows fainter radio emission at $L_X < 10^{35}$ erg s$^{-1}$ than what is typically observed for accreting millisecond pulsars. This suggests that J1804 is likely not an accreting X-ray or transitional millisecond pulsar.

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N. Gusinskaia, A. Deller, J. Hessels, et. al.
Mon, 22 May 17

Comments: 11 pages, 4 figures, 1 table, Accepted for publication in MNRAS

Imaging of SNR IC443 and W44 with the Sardinia Radio Telescope at 1.5 GHz and 7 GHz [HEAP]

Observations of supernova remnants (SNRs) are a powerful tool for investigating the later stages of stellar evolution, the properties of the ambient interstellar medium, and the physics of particle acceleration and shocks. For a fraction of SNRs, multi-wavelength coverage from radio to ultra high-energies has been provided, constraining their contributions to the production of Galactic cosmic rays. Although radio emission is the most common identifier of SNRs and a prime probe for refining models, high-resolution images at frequencies above 5 GHz are surprisingly lacking, even for bright and well-known SNRs such as IC443 and W44. In the frameworks of the Astronomical Validation and Early Science Program with the 64-m single-dish Sardinia Radio Telescope, we provided, for the first time, single-dish deep imaging at 7 GHz of the IC443 and W44 complexes coupled with spatially-resolved spectra in the 1.5-7 GHz frequency range. Our images were obtained through on-the-fly mapping techniques, providing antenna beam oversampling and resulting in accurate continuum flux density measurements. The integrated flux densities associated with IC443 are S_1.5GHz = 134 +/- 4 Jy and S_7GHz = 67 +/- 3 Jy. For W44, we measured total flux densities of S_1.5GHz = 214 +/- 6 Jy and S_7GHz = 94 +/- 4 Jy. Spectral index maps provide evidence of a wide physical parameter scatter among different SNR regions: a flat spectrum is observed from the brightest SNR regions at the shock, while steeper spectral indices (up to 0.7) are observed in fainter cooling regions, disentangling in this way different populations and spectra of radio/gamma-ray-emitting electrons in these SNRs.

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E. Egron, A. Pellizzoni, M. Iacolina, et. al.
Mon, 22 May 17

Comments: 13 pages, 9 figures, accepted for publication to MNRAS on 18 May 2017

Impact of ejecta morphology and composition on the electromagnetic signatures of neutron star mergers [HEAP]

The electromagnetic transients accompanying compact binary mergers ($\gamma$-ray bursts, afterglows and ‘macronovae’) are crucial to pinpoint the sky location of gravitational wave sources. Macronovae are caused by the radioactivity from freshly synthesised heavy elements, e.g. from dynamic ejecta and various types of winds. We study macronova signatures by using multi-dimensional radiative transfer calculations. We employ the radiative transfer code SuperNu and state-of-the-art LTE opacities for a few representative elements from the wind and dynamical ejecta (Cr, Pd, Se, Te, Br, Zr, Sm, Ce, Nd, U) to calculate synthetic light curves and spectra for a range of ejecta morphologies. The radioactive power of the resulting macronova is calculated with the detailed input of decay products. We assess the detection prospects for our most complex models, based on the portion of viewing angles that are sufficiently bright, at different cosmological redshifts ($z$). The brighter emission from the wind is unobscured by the lanthanides (or actinides) in some of the models, permitting non-zero detection probabilities for redshifts up to $z=0.07$. We also find the nuclear mass model and the resulting radioactive heating rate are crucial for the detectability. While for the most pessimistic heating rate (from the FRDM model) no reasonable increase in the ejecta mass or velocity, or wind mass or velocity, can possibly make the light curves agree with the observed nIR excess after GRB130603B, a more optimistic heating rate (from the Duflo-Zuker model) leads to good agreement. We conclude that future reliable macronova observations would constrain nuclear heating rates, and consequently help constrain nuclear mass models.

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R. Wollaeger, O. Korobkin, C. Fontes, et. al.
Mon, 22 May 17

Comments: 34 pages, submitted to MNRAS

Where Are the r-modes? Chandra Observations of Millisecond Pulsars [HEAP]

We present the results of {\it Chandra} observations of two non-accreting millisecond pulsars, PSRs J1640$+$2224 (J1640) and J1709$+$2313 (J1709), with low inferred magnetic fields and spin-down rates in order to constrain their surface temperatures, obtain limits on the amplitude of unstable $r$-modes in them, and make comparisons with similar limits obtained for a sample of accreting low-mass X-ray binary (LMXB) neutron stars. We detect both pulsars in the X-ray band for the first time. They are faint, with inferred soft X-ray fluxes ($0.3-3$ keV) of $\approx$ $6\times10^{-15}$ and $3\times 10^{-15}$ erg cm$^{-2}$ s$^{-1}$ for J1640 and J1709, respectively. Spectral analysis assuming hydrogen atmosphere emission gives global effective temperature upper limits ($90\%$ confidence) of $3.3 – 4.3 \times 10^5$ K for J1640 and $3.6 – 4.7 \times 10^5$ K for J1709, where the low end of the range corresponds to canonical neutron stars ($M=1.4 M_{\odot}$), and the upper end corresponds to higher-mass stars ($M=2.21 M_{\odot}$). Under the assumption that $r$-mode heating provides the thermal support, we obtain dimensionless $r$-mode amplitude upper limits of $3.2 – 4.8 \times 10^{-8}$ and $1.8 – 2.8 \times 10^{-7}$ for J1640 and J1709, respectively, where again the low end of the range corresponds to lower-mass, canonical neutron stars ($M=1.4 M_{\odot}$). These limits are about an order of magnitude lower than those we derived previously for a sample of LMXBs, except for the accreting millisecond X-ray pulsar (AMXP) SAX J1808.4$-$3658, which has a comparable amplitude limit to J1640 and J1709.

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S. Mahmoodifar and T. Strohmayer
Mon, 22 May 17

Comments: 9 pages, 4 figures, published in ApJ

Point-source and diffuse high-energy neutrino emission from Type IIn supernovae [HEAP]

Type IIn supernovae (SNe), a rare subclass of core collapse SNe, explode in dense circumstellar media that have been modified by the SNe progenitors at their last evolutionary stages. The interaction of the freely expanding SN ejecta with the circumstellar medium gives rise to a shock wave propagating in the dense SN environment, which may accelerate protons to multi-PeV energies. Inelastic proton-proton collisions between the shock-accelerated protons and those of the circumstellar medium lead to multi-messenger signatures. Here, we evaluate the possible neutrino signal of type IIn SNe and compare with IceCube observations. We employ a Monte Carlo method for the calculation of the diffuse neutrino emission from the SN IIn class to account for the spread in their properties. The cumulative neutrino emission is found to be ~ 10 per cent of the observed IceCube neutrino flux above 60 TeV. Type IIn SNe would be the dominant component of the diffuse astrophysical flux, only if 4 per cent of all core collapse SNe were of this type and 20 to 30 per cent of the shock energy was channeled to accelerated protons. Lower values of the acceleration efficiency are accessible by the observation of a single type IIn SN as a neutrino point source with IceCube using up-going muon neutrinos. Such an identification is possible in the first year following the SN shock breakout for sources within 20 Mpc.

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M. Petropoulou, S. Coenders, G. Vasilopoulos, et. al.
Mon, 22 May 17

Comments: 13 pages, 9 figures, MNRAS accepted