arXiv subjects

J. H. Gillanders

Publications and source records attributed to J. H. Gillanders.

23 records · Page 2Linked to original sources

arXiv2022

Modelling the spectra of the kilonova AT2017gfo -- I: The photospheric epochs

The kilonova (KN) associated with the binary neutron star (BNS) merger GW170817 is the only known electromagnetic counterpart to a gravitational wave source. Here we produce a sequence of radiative transfer models (using $\textsc{tardis}$) with updated atomic data, and compare them to accurately calibrated spectra. We use element compositions from nuclear network calculations based on a realistic hydrodynamical simulation of a BNS merger. We show that the blue spectrum at +1.4 days after merger requires a nucleosynthetic trajectory with a high electron fraction. Our best-fitting model is composed entirely of first $r$-process peak elements (Sr & Zr) and the strong absorption feature is reproduced well by Sr$\,\textsc{ii}$ absorption. At this epoch, we set an upper limit on the lanthanide mass fraction of $X_{\textsc{ln}} \lesssim 5 \times 10^{-3}$. In contrast, all subsequent spectra from $+2.4 - 6.4$ days require the presence of a modest amount of lanthanide material ($X_{\textsc{ln}} \simeq 0.05^{+0.05}_{-0.02}$), produced by a trajectory with $Y_{\rm e} = 0.29$. This produces lanthanide-induced line blanketing below 6000$\,$Å, and sufficient light $r$-process elements to explain the persistent strong feature at $\sim 0.7 - 1.0 \, \micron$ (Sr$\,\textsc{ii}$). The composition gives good matches to the observed data, indicating that the strong blue flux deficit results in the near-infrared (NIR) excess. The disjoint in composition between the first epoch and all others indicates either ejecta stratification, or the presence of two distinct components of material. This further supports the `two-component' kilonova model, and constrains the element composition from nucleosynthetic trajectories. The major uncertainties lie in availability of atomic data and the ionisation state of the expanding material.

astro-ph.HE↗

arXiv2022

SN 2020kyg and the rates of faint Iax Supernovae from ATLAS

We present multi-wavelength follow-up observations of the ATLAS discovered faint Iax supernova SN 2020kyg that peaked at an absolute magnitude of $M_g \approx -14.9 \pm 0.2$, making it another member of the faint Iax supernova population. The bolometric light curve requires only $\approx 7 \times 10^{-3}$ M$_{\odot}$ of radioactive $^{56}$Ni, with an ejected mass of $M_{\rm ej} \sim 0.4$ M$_{\odot}$ and a low kinetic energy of $E \approx 0.05 \pm 0.02 \times 10^{51}$ erg. We construct a homogeneous volume-limited sample of 902 transients observed by ATLAS within 100 Mpc during a 3.5 year span. Using this sample, we constrain the rates of faint Iax ($M_r \gtrsim -16$) events within 60 Mpc at $12^{+14}_{-8}\%$ of the SN Ia rate. The overall Iax rate, at $15^{+17}_{-9}\%$ of the Ia rate, is dominated by the low-luminosity events, with luminous SNe Iax ($M_r \lesssim -17.5$) like 2002cx and 2005hk accounting for only $0.9^{+1.1}_{-0.5}\%$ of the Ia rate (a 2$σ$ upper limit of approximately 3\%). We favour the hybrid CONe WD + He star progenitor channel involving a failed deflagration of a near Chandrasekhar mass white dwarf, expected to leave a bound remnant and a surviving secondary companion, as a candidate explanation for faint Iax explosions. This scenario requires short delay times, consistent with the observed environments of SNe Iax. Furthermore, binary population synthesis calculations have suggested rates of $1-18\%$ of the SN Ia rate for this channel, consistent with our rate estimates.

astro-ph.HE↗

arXiv2021

An analysis of the spectroscopic signatures of layering in the ejecta of type Iax supernovae

Investigations of some type Iax supernovae have led to the suggestion that their ejecta must be layered to some degree. Such an ejecta structure has been argued as inconsistent with the well-mixed composition predicted by pure deflagrations. Based on explosion models, we create toy models in which the ejecta are artificially stratified and progressively mixed until a uniform composition is obtained. We find that models that are heavily mixed, containing burned and unburned material at all velocities, produce reasonably good agreement with SN 2012Z, for which a layered structure has been suggested. We also discuss how existing ejecta compositions determined for type Iax supernovae do not necessarily contradict pure deflagration models and may be consistent with a steeper density profile. We investigate previous claims that differences in line profile shapes may be due to strong blending, by presenting a series of models with different plasma states. These models indicate that blending could indeed explain differences in the observed profiles. Alternatively, stratification could also explain such differences, however all of our models indicate that this does not necessarily require stratification in abundance. Sufficient stratification in ionisation state can be achieved even for a well-mixed model. Based on our analysis, we demonstrate that there is insufficient evidence to suggest the ejecta of type Iax supernovae must be layered and therefore argue the pure deflagration scenario is not ruled out, even for the brightest type Iax supernovae. Our analysis does not indicate the ejecta cannot be layered to some degree, but observations within days of explosion are necessary to determine the extent to which the outer ejecta could be layered.

astro-ph.HE↗

arXiv2020

Observational constraints on the optical and near-infrared emission from the neutron star-black hole binary merger S190814bv

On 2019 August 14, the LIGO and Virgo interferometers detected a high-significance event labelled S190814bv. Preliminary analysis of the GW data suggests that the event was likely due to the merger of a compact binary system formed by a BH and a NS. ElectromagNetic counterparts of GRAvitational wave sources at the VEry Large Telescope (ENGRAVE) collaboration members carried out an intensive multi-epoch, multi-instrument observational campaign to identify the possible optical/near infrared counterpart of the event. In addition, the ATLAS, GOTO, GRAWITA-VST, Pan-STARRS and VINROUGE projects also carried out a search on this event. Our observations allow us to place limits on the presence of any counterpart and discuss the implications for the kilonova (KN) possibly generated by this NS-BH merger, and for the strategy of future searches. Altogether, our observations allow us to exclude a KN with large ejecta mass $M\gtrsim 0.1\,\mathrm{M_\odot}$ to a high ($>90\%$) confidence, and we can exclude much smaller masses in a subsample of our observations. This disfavours the tidal disruption of the neutron star during the merger. Despite the sensitive instruments involved in the campaign, given the distance of S190814bv we could not reach sufficiently deep limits to constrain a KN comparable in luminosity to AT 2017gfo on a large fraction of the localisation probability. This suggests that future (likely common) events at a few hundreds Mpc will be detected only by large facilities with both high sensitivity and large field of view. Galaxy-targeted observations can reach the needed depth over a relevant portion of the localisation probability with a smaller investment of resources, but the number of galaxies to be targeted in order to get a fairly complete coverage is large, even in the case of a localisation as good as that of this event.

astro-ph.SR↗

arXiv2019

Determining the $^{56}$Ni distribution of type Ia supernovae from observations within days of explosion

Recent studies have shown how the distribution of $^{56}$Ni within the ejecta of type Ia supernovae can have profound consequences on the observed light curves. Observations at early times can therefore provide important details on the explosion physics in thermonuclear supernovae. We present a series of radiative transfer calculations that explore variations in the $^{56}$Ni distribution. Our models also show the importance of the density profile in shaping the light curve, which is often neglected in the literature. Using our model set, we investigate the observations that are necessary to determine the $^{56}$Ni distribution as robustly as possible within the current model set. We find that this includes observations beginning at least $\sim$14 days before $B$-band maximum, extending to approximately maximum light with a high ($\lesssim$3 day) cadence, and in at least one blue and one red band are required (such as $B$ and $R$, or $g$ and $r$). We compare a number of well-observed type Ia supernovae that meet these criteria to our models and find that the light curves of $\sim$70-80\% of objects in our sample are consistent with being produced solely by variations in the $^{56}$Ni distributions. The remaining supernovae show an excess of flux at early times, indicating missing physics that is not accounted for within our model set, such as an interaction or the presence of short-lived radioactive isotopes. Comparing our model light curves and spectra to observations and delayed detonation models demonstrates that while a somewhat extended $^{56}$Ni distribution is necessary to reproduce the observed light curve shape, this does not negatively affect the spectra at maximum light. Investigating current explosion models shows that observations typically require a shallower decrease in the $^{56}$Ni mass towards the outer ejecta than is produced for models of a given $^{56}$Ni mass.

astro-ph.HE↗