A First Attempt to Detect Extragalactic Antimatter: Positronium Lyman alpha toward M31*

0 Assigned ID: 5737 ----- Attempting Submission 1 (Tue Mar 26 22:26:44 GMT 2024) ----- A target acquisition should probably be performed before doing spectroscopy or coronography with STIS or COS. A target acquisition should probably be performed before doing spectroscopy or coronography with STIS or COS. ORBITAL VISIBILITY OVERRUN A target acquisition should probably be performed before doing spectroscopy or coronography with STIS or COS. ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN The plan is to obtain a spectrum of M31* in the Ps Lyman alpha transition at 243 nm. We split the observations into two visits of 3 orbits each, and spread the four FP-POS settings over the two visits (six orbits). For each visit and orbit, we do two COS exposures with G230L, TIME-TAG mode, wavelength 3360 A, placing Segment A on the 243 nm Ps transition. The M31* nuclear environment is extended in the NUV and will not reward an aquisition obervation, even for peak-finding. We use the radio coordinates for M31*, which are good to less than 0.25". The default pointing by HST will be perfectly adequate to obtain a spectrum of the nuclear region to search for antimatter. **This resubmission adds a Visit 03 because the last orbit on Visit 01 failed. The new Visit replicates the third orbit in Visit 01 and leaves the unexecuted Visit 02 unchanged. 1864363989 ----- Attempting Submission 1 (Fri Aug 16 21:23:58 GMT 2024) ----- ----- Attempting Submission 2 (Tue Jul 22 04:31:35 GMT 2025) ----- As explained in the submission comments and the Proposal Description, we intentionally omit aquisition observations due to the bright and very extended nature of the nuclear NUV light in M31. The default pointing accuracy and error is fine to search for antimatter in the nuclear region of M31*. A First Attempt to Detect Extragalactic Antimatter: Positronium Lyman alpha toward M31* We propose to attempt the first direct detection of antimatter outside the Galaxy by observing the positronium Lyman-alpha transition toward M31*. The existence of antimatter in the form of positrons has been observed in the Galaxy via 511 keV annihilation photons for half a century, but the astrophysical engines producing the positrons have yet to be identified. Numerous plausible sources of positrons have been proposed, some of which are distributed and some of which are pointlike such as massive black holes, stellar-mass black holes, and compact X-ray binaries. Studies of the annihilation gamma-ray spectrum show that nearly all annihilation events are preceded by the formation of an electron-positron bound state called Positronium (Ps), a hydrogen-like "atom" where the proton is replaced by the positron. Positronium has quantum transitions with half the energy of hydrogen: its Lyman alpha line has a wavelength 243 nm. There will therefore be Ps "recombination" emission lines that precede annihilation. These are faint but within the reach of HST, and ground-based observations have thus far been unable to adequately remove terrestrial atmospheric spectral features, despite custom-built spectrometers. We propose to expand the study of the sources of antimatter beyond the Galaxy by observing M31* in the Ps Lyman-alpha transition. Successful detection will provide important insight into positron production mechanisms and the general physics of high-energy phenomena. Theory alone has so far been unable to identify the dominant mechanism for antimatter production. The plan is to obtain a spectrum of M31* in the Ps Lyman alpha transition at 243 nm. We split the observations into two visits of 3 orbits each, and spread the four FP-POS settings over the two visits (six orbits). For each visit and orbit, we do two COS exposures with G230L, TIME-TAG mode, wavelength 3360 A, placing Segment A on the 243 nm Ps transition. The M31* nuclear environment is extended in the NUV and will not reward an aquisition obervation, even for peak-finding. We use the radio coordinates for M31*, which are good to less than 0.25". The default pointing by HST will be perfectly adequate to obtain a spectrum of the nuclear region to search for antimatter. Dr. Jeremy Darling is an expert in spectroscopy of AGN and star-forming galaxies, has publications focused on M31, and has extensive experience with probing strange and interesting physics using telescopes, particularly spectroscopy, such as measuring physical constants and physical conditions of interstellar gas. He has recently won JWST time to study positronium and is completing an extensive UV spectroscopic study of quasars. Galaxies Supermassive Black Holes and Active Galaxies 6 false false false false false false SUPPORTED GALAXY NUCLEUS NO J2000 J2000 The HST-measured FUV flux at 2000 A is 1e-15 erg/s/cm^2/A; we use this to estimate the 2430 A continuum. COS/NUV Spectroscopic No No G230L 3360 1723842352183 The scheduling data was generated by CASM 33.1. 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