Motivating Reason to Slow the Factive Turn in Epistemology

In this paper I give a novel argument for the view that epistemic normative reasons (or evidence) need not be facts. I first argue that the nature of normative reasons is uniform, such that our positions about the factivity of reasons should agree across normative realms –– whether epistemic, moral, practical, or otherwise. With that in mind, I proceed in a somewhat indirect way. I argue that if practical motivating reasons are not factive, then practical normative reasons are not factive. If it is possible to act rationally in the light of a falsehood, as I will say, then some good reasons must be falsehoods. The implication of this argument is perhaps surprising: for one to firmly establish the view that epistemic normative reasons are factive, one must discredit the view that practical motivating reasons are not factive

Origin of asymmetries in X-ray emission lines from the blast wave of the 2014 outburst of nova V745 Sco

The symbiotic nova V745 Sco was observed in outburst on 2014 February 6. Its observations by the Chandra X-ray Observatory at days 16 and 17 have revealed a spectrum characterized by asymmetric and blue-shifted emission lines. Here we investigate the origin of these asymmetries through three-dimensional hydrodynamic simulations describing the outburst during the first 20 days of evolution. The model takes into account thermal conduction and radiative cooling and assumes a blast wave propagates through an equatorial density enhancement. From the simulations, we synthesize the X-ray emission and derive the spectra as they would be observed with Chandra. We find that both the blast wave and the ejecta distribution are efficiently collimated in polar directions due to the presence of the equatorial density enhancement. The majority of the X-ray emission originates from the interaction of the blast with the equatorial density enhancement and is concentrated on the equatorial plane as a ring-like structure. Our "best-fit" model requires a mass of ejecta in the outburst $M_{ej} \approx 3\times 10^{-7}\,M_{\odot}$ and an explosion energy $E_b \approx 3 \times 10^{43}$ erg and reproduces the distribution of emission measure vs temperature and the evolution of shock velocity and temperature inferred from the observations. The model predicts asymmetric and blue-shifted line profiles similar to those observed and explains their origin as due to substantial X-ray absorption of red-shifted emission by ejecta material. The comparison of predicted and observed Ne and O spectral line ratios reveals no signs of strong Ne enhancement and suggests the progenitor is a CO white dwarf.Comment: 16 pages, 17 Figures; accepted for publication on MNRA

Parallel electric fields are inefficient drivers of energetic electrons in magnetic reconnection

We present two-dimensional kinetic simulations, with a broad range of initial guide fields, that isolate the role of parallel electric fields ($E_\parallel$) in energetic electron production during collisionless magnetic reconnection. In the strong guide field regime, $E_\parallel$ drives essentially all of the electron energy gain, yet fails to generate an energetic component. We suggest that this is due to the weak energy scaling of particle acceleration from $E_\parallel$ compared to that of a Fermi-type mechanism responsible for energetic electron production in the weak guide-field regime. This result has important implications for energetic electron production in astrophysical systems and reconnection-driven dissipation in turbulence

The Massive Star Forming Region, Cygnus OB2

We present results from a catalogue of 1696 X-ray point sources detected in the massive star forming region (SFR) Cygnus OB2, the majority of which have optical or near-IR associations. We derive ages of 3.5 and 5.25 Myrs for the stellar populations in our two fields, in agreement with recent studies that suggest that the central 1-3 Myr OB association is surrounded and contaminated by an older population with an age of 5-10 Myrs. The fraction of sources with proto-planetary disks, as traced by K-band excesses, are unusually low. Though this has previously been interpreted as due to the influence of the large number of OB stars in Cyg OB2, contamination from an older population of stars in the region could also be responsible. An initial mass function is derived and found to have a slope of Gamma = -1.27, in agreement with the canonical value. Finally we introduce the recently approved Chandra Cygnus OB2 Legacy Survey that will image a 1 square degree area of the Cygnus OB2 association to a depth of 120 ks, likely detecting ~10,000 stellar X-ray sources.Comment: 4 pages, 3 figures. To appear in the proceedings of IAU Symposium 266, Star Clusters: Basic Galactic Building Blocks Throughout Time and Space, eds. R. de Grijs and J. Lepin

The role of three-dimensional transport in driving enhanced electron acceleration during magnetic reconnection

Magnetic reconnection is an important driver of energetic particles in many astrophysical phenomena. Using kinetic particle-in-cell (PIC) simulations, we explore the impact of three-dimensional reconnection dynamics on the efficiency of particle acceleration. In two-dimensional systems, Alfv\'enic outflows expel energetic electrons into flux ropes where they become trapped and disconnected from acceleration regions. However, in three-dimensional systems these flux ropes develop axial structure that enables particles to leak out and return to acceleration regions. This requires a finite guide field so that particles may move quickly along the flux rope axis. We show that greatest energetic electron production occurs when the guide field is of the same order as the reconnecting component: large enough to facilitate strong transport, but not so large as to throttle the dominant Fermi mechanism responsible for efficient electron acceleration. This suggests a natural explanation for the envelope of electron acceleration during the impulsive phase of eruptive flares