Swift Multiwavelength Follow-up of LVC S200224ca and the Implications for Binary Black Hole Mergers

On 2020 February 24, during their third observing run ("O3"), the Laser Interferometer Gravitational-wave Observatory and Virgo Collaboration detected S200224ca: a candidate gravitational wave (GW) event produced by a binary black hole (BBH) merger. This event was one of the best-localized compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72 deg2), and so the Neil Gehrels Swift Observatory performed rapid near-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately 79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBH event most thoroughly followed-up in near-UV (u-band) and X-ray to date. No likely EM counterparts to the GW event were found by the Swift BAT, XRT, or UVOT, nor by other observatories. Here, we report on the results of our searches for an EM counterpart, both in the BAT data near the time of the merger, and in follow-up UVOT/XRT observations. We also discuss the upper limits we can place on EM radiation from S200224ca, as well as the implications these limits have on the physics of BBH mergers. Namely, we place a shallow upper limit on the dimensionless BH charge, $\hat{q}\lt 1.4\times {10}^{-4}$, and an upper limit on the isotropic-equivalent energy of a blast wave E < 4.1 × 1051 erg (assuming typical GRB parameters)

Swift-XRT follow-up of gravitational wave triggers during the third aLIGO/Virgo observing run

The Neil Gehrels Swift Observatory followed up 18 gravitational wave (GW) triggers from the LIGO/Virgo collaboration during the O3 observing run in 2019/2020, performing approximately 6500 pointings in total. Of these events, four were finally classified (if real) as binary black hole (BH) triggers, six as binary neutron star (NS) events, two each of NSBH and Mass Gap triggers, one an unmodelled (Burst) trigger, and the remaining three were subsequently retracted. Thus far, four of these O3 triggers have been formally confirmed as real gravitational wave events. While no likely electromagnetic counterparts to any of these GW events have been identified in the X-ray data (to an average upper limit of 3.60 x 10^{-12} erg cm^{-2} s^{-1} over 0.3-10 keV), or at other wavelengths, we present a summary of all the Swift-XRT observations performed during O3, together with typical upper limits for each trigger observed. The majority of X-ray sources detected during O3 were previously uncatalogued; while some of these will be new (transient) sources, others are simply too faint to have been detected by earlier survey missions such as ROSAT. The all-sky survey currently being performed by eROSITA will be a very useful comparison for future observing runs, reducing the number of apparent candidate X-ray counterparts by up to 95 per cent

Morphological evolution caused by many subtle-effect substitutions in regulatory DNA

Morphology evolves often through changes in developmental genes, but the causal mutations, and their effects, remain largely unknown. The evolution of naked cuticle—rather than trichomes—on larvae of Drosophila sechellia resulted from changes in five transcriptional enhancers of shavenbaby, a gene encoding a transcription factor that governs trichome morphogenesis. Here we show that the function of one of these enhancers evolved through multiple single nucleotide substitutions that altered both the timing and level of shavenbaby expression. The consequences of these nucleotide substitutions on larval morphology were quantified with a novel functional assay. We found that each substitution had a relatively small phenotypic effect, and that many nucleotide changes account for this large morphological difference. In addition, we observed that the substitutions displayed non-additive effects to generate a large phenotypic change. These data provide unprecedented resolution of the phenotypic effects of substitutions and show how individual nucleotide changes in a transcriptional enhancer have caused morphological evolution

Swift Multiwavelength Follow-up of LVC S200224ca and the Implications for Binary Black Hole Mergers

On 2020 February 24, during their third observing run ("O3"), the Laser Interferometer Gravitational-wave Observatory and Virgo Collaboration detected S200224ca: a candidate gravitational wave (GW) event produced by a binary black hole (BBH) merger. This event was one of the best-localized compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72 deg2), and so the Neil Gehrels Swift Observatory performed rapid near-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately 79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBH event most thoroughly followed-up in near-UV (u-band) and X-ray to date. No likely EM counterparts to the GW event were found by the Swift BAT, XRT, or UVOT, nor by other observatories. Here, we report on the results of our searches for an EM counterpart, both in the BAT data near the time of the merger, and in follow-up UVOT/XRT observations. We also discuss the upper limits we can place on EM radiation from S200224ca, as well as the implications these limits have on the physics of BBH mergers. Namely, we place a shallow upper limit on the dimensionless BH charge, $\hat{q}\lt 1.4\times {10}^{-4}$, and an upper limit on the isotropic-equivalent energy of a blast wave E < 4.1 × 1051 erg (assuming typical GRB parameters)