arXiv subjects

Nicolas Scepi

Publications and source records attributed to Nicolas Scepi.

22 records · Page 2Linked to original sources

arXiv2020

Magnetic field transport in compact binaries

Dwarf novae (DNe) and low mass X-ray binaries (LMXBs) show eruptions that are thought to be due to a thermal-viscous instability in their accretion disk. These eruptions provide constraints on angular momentum transport mechanisms. We explore the idea that angular momentum transport could be controlled by the dynamical evolution of the large scale magnetic field. We study the impact of different prescriptions for the magnetic field evolution on the dynamics of the disk. This is a first step in confronting the theory of magnetic field transport with observations. We develop a version of the disk instability model that evolves the density, the temperature and the large scale vertical magnetic flux together. We take into account the accretion driven by turbulence or by a magnetized outflow. To evolve the magnetic flux, we use a toy model with physically motivated prescriptions depending mainly on the local magnetization. We find that allowing magnetic flux to be advected inwards provides the best agreement with DNe lightcurves. This leads to a hybrid configuration with an inner magnetized disk, driven by angular momentum losses to an MHD outflow, sharply transiting to an outer weakly-magnetized turbulent disk, where the eruptions are triggered. The dynamical impact is equivalent to truncating a viscous disk so that it does not extend down to the compact object, with the truncation radius dependent on the magnetic flux and evolving as $\dot{M}^{-2/3}$. Models of DNe and LMXBs lightcurves typically require the outer, viscous disk to be truncated in order to match observations. There is no generic explanation for this truncation. We propose that it is a natural outcome of the presence of large-scale magnetic fields in both DNe and LMXBs, the magnetic flux accumulating towards the center to produce a magnetized disk with a fast accretion timescale.

astro-ph.HE↗

arXiv2019

Magnetic wind-driven accretion in dwarf novae

Dwarf novae (DNe) and X-ray binaries exhibit outbursts thought to be due to a thermal-viscous instability in the accretion disk. The disk instability model (DIM) assumes that accretion is driven by turbulent transport, customarily attributed to the magneto-rotational instability (MRI). Recent results point out that MRI turbulence alone fails to reproduce the light curves of DNe. We aim to study the impact of wind-driven accretion on the light curves of DNe. Local and global simulations show that magneto-hydrodynamic winds are present when a magnetic field threads the disk, even for relatively high ratios of thermal pressure to magnetic pressure ($β\approx 10^{5}$). These winds are very efficient in removing angular momentum but do not heat the disk; they do not behave as MRI-driven turbulence. We add wind-driven transport in the angular momentum equation of the DIM, assuming a fixed magnetic configuration: dipolar or constant with radius. We use prescriptions for the wind torque and the turbulent torque derived from shearing box simulations. The wind torque enhances the accretion of matter, resulting in light curves that look like DNe outbursts when assuming a dipolar field with a moment $μ\approx10^{30}\,\mathrm{G\,cm^{3}}$. In the region where the wind dominates, the disk is cold, optically thin and the accretion speed is sonic. This acts as if the inner disk was truncated, leading to higher quiescent X-ray luminosities from the white dwarf boundary layer than expected with the standard DIM. The disk is stabilized if the wind-dominated region is large enough, potentially leading to `dark' disks emitting little radiation. Wind-driven accretion can play a key role in shaping the light curves of DNe and X-ray binaries. Future studies will need to include the time evolution of the magnetic field threading the disk to fully assess its impact on the dynamics of the accretion flow.

astro-ph.HE↗

arXiv2018

Turbulent and wind-driven accretion in dwarf novae threaded by a large scale magnetic field

Dwarf novae (DNe) are accreting white dwarfs that show eruptions due to a thermal-viscous instability in the accretion disk. The outburst timescales constrain $α$, the ratio of the viscous stress to the thermal pressure, and so the mechanism of angular momentum transport. The eruptive state has $α\approx0.1$ while the quiescent state has $α\approx0.03$. Turbulent transport due to the magneto-rotational instability (MRI) is generally considered to be the source of angular momentum transport in DNe. Here, we perform 3D local magnetohydrodynamic (MHD) shearing box simulations including vertical stratification, radiative transfer and a net constant vertical magnetic flux to investigate how transport changes between the outburst and quiescent states of DNe. We find that a constant $B_z$ provides a higher $α$ in quiescence than in outburst, in opposition to what is expected. Including resistivity quenches MRI turbulence in quiescence, suppressing transport, unless the magnetic field is high enough, which again leads to $α\approx0.1$. A major difference between simulations with a net poloidal flux and simulations without is that angular momentum transport in the former is shared between turbulent and wind-driven transport. We find that wind-driven transport dominates in quiescence even for low magnetic fields $\sim 1$ G. This can have a major impact on observational signatures since wind-driven transport does not heat the disk. Furthermore, wind transport cannot be reduced to an $α$ prescription. We provide fits for $α$ and the wind torque with $β$, ratio of thermal to magnetic pressure. We conclude that the evolution of the thermal-viscous instability, and its consequences on the outburst cycles of CVs, needs to be seriously revised to take into account that most of the accretion energy may be carried away by a wind instead of being locally dissipated.

astro-ph.HE↗

arXiv2015

Galaxy clusters in the SDSS Stripe 82 based on galaxy photometric redshifts

Based on a recent photometric redshift galaxy catalogue, we have searched for galaxy clusters in the Stripe~82 region of the Sloan Digital Sky Survey by applying the Adami & MAzure Cluster FInder (AMACFI). Extensive tests were made to fine-tune the AMACFI parameters and make the cluster detection as reliable as possible. The same method was applied to the Millennium simulation to estimate our detection efficiency and the approximate masses of the detected clusters. Considering all the cluster galaxies (i.e. within a 1 Mpc radius of the cluster to which they belong and with a photoz differing by less than 0.05 from that of the cluster), we stacked clusters in various redshift bins to derive colour-magnitude diagrams and galaxy luminosity functions (GLFs). For each galaxy with absolute magnitude brighter than -19.0 in the r band, we computed the disk and spheroid components by applying SExtractor, and by stacking clusters we determined how the disk-to-spheroid flux ratio varies with cluster redshift and mass. We detected 3663 clusters in the redshift range 0.15<z<0.70, with estimated mean masses between 10^13 and a few 10^{14 solar masses. By stacking the cluster galaxies in various redshift bins, we find a clear red sequence in the (g'-r') versus r' colour-magnitude diagrams, and the GLFs are typical of clusters, though with a possible contamination from field galaxies. The morphological analysis of the cluster galaxies shows that the fraction of late-type to early-type galaxies shows an increase with redshift (particularly in high mass clusters) and a decrease with detection level, i.e. cluster mass. From the properties of the cluster galaxies, the majority of the candidate clusters detected here seem to be real clusters with typical cluster properties.

astro-ph.CO↗