2 research outputs found
Late time HST UV and optical observations of AT~2018cow: extracting a cow from its background
The bright, blue, rapidly evolving AT2018cow is a well-studied peculiar
extragalactic transient. Despite an abundance of multi-wavelength data, there
still is no consensus on the nature of the event. We present our analysis of
three epochs of Hubble Space Telescope (HST) observations spanning the period
from 713-1474 days post burst, paying particular attention to uncertainties of
the transient photometry introduced by the complex background in which
AT2018cow resides. Photometric measurements show evident fading in the UV and
more subtle but significant fading in the optical. During the last HST
observation, the transient's optical/UV colours were still bluer than those of
the substantial population of compact, young, star-forming regions in the host
of AT2018cow, suggesting some continued transient contribution to the light.
However, a compact source underlying the transient would substantially modify
the resulting spectral energy distribution, depending on its contribution in
the various bands. In particular, in the optical filters, the complex, diffuse
background poses a problem for precise photometry. An underlying cluster is
expected for a supernova occurring within a young stellar environment or a
tidal-disruption event (TDE) within a dense older one. While many recent works
have focused on the supernova interpretation, we note the substantial
similarity in UV light-curve morphology between AT2018cow and several tidal
disruption events around supermassive black holes. Assuming AT2018cow arises
from a TDE-like event, we fit the late-time emission with a disc model and find
M. Further observations are necessary to
determine the late-time evolution of the transient and its immediate
environment.Comment: 16 pages, 10 figures. Accepted for publication in MNRA
A long-duration gamma-ray burst of dynamical origin from the nucleus of an ancient galaxy
The majority of long-duration (>2 s) gamma-ray bursts (GRBs) arise from the collapse of massive stars, with a small proportion created from the merger of compact objects. Most of these systems form via standard stellar evolution pathways. However, a fraction of GRBs may result from dynamical interactions in dense environments. These channels could also contribute substantially to the samples of compact object mergers detected as gravitational wave sources. Here we report the case of GRB 191019A, a long GRB (a duration of T 90 = 64.4 ± 4.5 s), which we pinpoint close (⪅100 pc projected) to the nucleus of an ancient (>1 Gyr old) host galaxy at z = 0.248. The lack of evidence for star formation and deep limits on any supernova emission disfavour a massive star origin. The most likely route for progenitor formation is via dynamical interactions in the dense nucleus of the host. The progenitor, in this case, could be a compact object merger. These may form in dense nuclear clusters or originate in a gaseous disc around the supermassive black hole. Identifying, to the best of our knowledge, a first example of a dynamically produced GRB demonstrates the role that such bursts may have in probing dense environments and constraining dynamical fractions in gravitational wave populations.</p