Godzilla, a monster lurks in the Sunburst galaxy

Abstract

We model the strong lensing effect in the galaxy cluster PSZ1 G311. 65-18.48 (z=0.443) with an improved version of the hybrid method WSLAP+. We extend the number of constraints by including the position of critical points, which are combined with the classic positional constraints of the lensed galaxies. We pay special attention to a transient candidate source (Tr) previously discovered in the giant Sunburst arc (z=2.37). Our lens model predicts Tr to be within a fraction of an arcsecond from the critical curve, having a larger magnification factor than previously found, but still not large enough to explain the observed flux and lack of counterimages. Possible candidate counterimages are discussed that would lower the magnification required to explain Tr, but extreme magnification factors (µ > 1000) are still required, even in that case. The presence of a small mass perturber with a mass comparable to a dwarf galaxy (M ∼ 10 8 M ) near the position of Tr is needed in order to explain the required magnification and morphology of the lensed galaxy. We discuss how the existence of this perturber could potentially be used to constrain models of dark matter. The large apparent brightness and unresolved nature of the magnified object implies a combination of extreme magnification and a very luminous and compact source (r < 0.3 pc). Possible candidates are discussed, including an hyperluminous star or an accretion disc around an intermediate-mass black hole (IMBH). Based on spectral information, we argue that a luminous blue variable (LBV) star caught during an outburst is the most likely candidate. Owing to the extreme magnification and luminosity of this source we dub it Godzilla.

Unscrambling the lensed galaxies in JWST images behind SMACS0723

“…These substructures that can now be resolved by JWST facilitate the identification of families of strongly lensed images (a family consists of all the counter-images from the same background galaxy). The greater sensitivity of JWST to small flux fluctuations and improved spatial resolution will also allow us to search for fainter caustic or micro-caustic crossing events of stars at z < 2 such as Icarus (Kelly et al 2018), or Warhol (Chen et al 2019;Kaurov et al 2019), stars between 2 < z < 5 such as Godzilla (Diego et al 2022), stars at z ≈ 6 such as Earendel (Welch et al 2022) or even further up to the first stars as proposed in Windhorst et al (2018).…”

A fast algorithm for simulating light curves of stars at extreme magnification affected by microlensing

“…Due to its brightness (m AB ≈ 22), Godzilla represents the first star at z > 1 for which a spectrum was obtained (Vanzella et al 2020). The MUSE spectrum shows peculiar features also present in massive luminous blue variable stars like a P-cygni profile (Diego et al 2022). An interesting fact regarding Godzilla is that no microlensing flux fluctuations have been observed so far.…”

“…Although a previously known object (Vanzella et al 2020), Godzilla was originally unjdiego@ifca.unican.es recognized as an extremely lensed star. The high brightness in Godzilla is interpreted in Diego et al (2022) as due to the fact that the star is being observed during an outburst phase (these outbursts can last decades in the observer frame). Due to its brightness (m AB ≈ 22), Godzilla represents the first star at z > 1 for which a spectrum was obtained (Vanzella et al 2020).…”

“…This is consistent with the magnification of the star being in the range of several thousands. In this situation, as discussed in Diego et al (2022) and Welch et al (2022), there is an effect known as the "more-is-less" effect, where increassing the magnification results in more overlapping microcaustics in the source plane. Crossing one of these microcaustics results in a relatively small change in flux, since the flux linked to other microcaustics is still significant, and remain more or less constant during the microcaustic crossing.…”

Fast algorithm for simulating light curves of stars at extreme magnification affected by microlensing

“…Although it was a previously known object generally (Vanzella et al 2020), Godzilla was not initially recognized as an extremely lensed star. The high degree of brightness in Godzilla was interpreted by Diego et al (2022) as due to the star's being observed during an outburst phase (these outbursts can last decades in the observer frame). Due to its brightness (m AB ≈ 22), Godzilla represents the first star at z > 1 for which a spectrum has been obtained (Vanzella et al 2020).…”