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1. On the progenitor of the Crab Pulsar

   https://doi.org/10.1093/mnras/staf333  

   Cruz-Cruz, Elvira; Kochanek, C_S (February 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We investigate the progenitor of the Crab supernova by examining the remnant’s surrounding stellar population. The Crab is interesting because of the apparently low energy and mass of the supernova remnant. We also know it was not a binary at death and that the explosion formed a neutron star. Using Gaia EDR3 parallaxes and photometry, we analyse stars inside a cylinder with a projected radius of 100 pc and spanning distances from $$\sim 1600$$ to 2300 pc set by the $$2\sigma$$ uncertainties in the Crab’s parallax. We also individually model the most luminous stars local to the Crab. The two most luminous stars are blue, roughly main sequence stars with masses of $$\sim 11\, {\rm M}_{\odot }$$. We estimate the stellar population’s age distribution using solar metallicity PARSEC isochrones. The estimated age distribution of the 205 $$M_{\mathrm{ G}} < 0$$ stars modestly favour lower mass stars, consistent with an AGB star or a lower mass binary merger as the progenitor, but statistically we cannot rule out higher masses. This may be driven by contamination due to the $$\sim 700$$ pc span of the cylinder in distance. 

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2. The Non-Uniform Expansion of the Crab Nebula

   Martin, T; Milisavljevic, D; Temim, T; Mandal, S; Duffell, P; Drissen, L; Ding, Z (February 2025, arXivorg)

   We present extensive proper motion measurements of the Crab Nebula made from Canada-France-Hawaii Telescope MegaPrime/MegaCam images taken in 2007, 2016, and 2019. A total of 19974 proper motion vectors with uncertainty < 10 mas yr−1 located over the majority of the Crab Nebula are used to map the supernova remnant’s two-dimensional expansion properties that reflect the dynamics of the original explosion, acceleration of ejecta imparted by spin-down energy from the pulsar, and interaction between the ejecta and surrounding circumstellar material (CSM). The average convergence date we derive is 1105.5 ± 0.5 CE, which is 15-35 years earlier compared to most previous estimates. We find that it varies as a function of position angle around the nebula, with the earliest date and smallest proper motions measured along the equator defined by the east and west bays. The lower acceleration of material along the equatorial plane may be indicative of the supernova’s interaction with a disk-like CSM geometry. Comparing our measurements to previous analytical solutions of the Crab’s expansion and our own numerical simulation using the moving mesh hydrodynamics code Sprout, we conclude that the ejecta have relaxed closer to homologous expansion than expected for the commonly adopted pulsar spindown age of τ ∼ 700 yr and a pulsar wind nebula (PWN) still evolving inside the flat part of the ejecta density profile. These findings provide further evidence that the PWN has broken out of the inner flat part of the supernova ejecta density profile and has experienced “blowout”. 

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3. The remarkable visual system of a Cretaceous crab

   https://doi.org/10.1016/j.isci.2021.103579  

   Jenkins, Kelsey M.; Briggs, Derek E.G.; Luque, Javier (January 2022, iScience)
4. Dense Molecular Clouds in the Crab Supernova Remnant

   https://doi.org/10.3847/1538-4357/ac391a  

   Wootten, Alwyn; Bentley, Rory O.; Baldwin, J.; Combes, F.; Fabian, A. C.; Ferland, G. J.; Loh, E.; Salome, P.; Shingledecker, C. N.; Castro-Carrizo, A. (January 2022, The Astrophysical Journal)

   Abstract Molecular emission was imaged with ALMA from numerous components near and within bright H₂-emitting knots and absorbing dust globules in the Crab Nebula. These observations provide a critical test of how energetic photons and particles produced in a young supernova remnant interact with gas, cleanly differentiating between competing models. The four fields targeted show contrasting properties but within them, seventeen distinct molecular clouds are identified with CO emission; a few also show emission from HCO⁺, SiO, and/or SO. These observations are compared with Cloudy models of these knots. It has been suggested that the Crab filaments present an exotic environment in which H₂emission comes from a mostly neutral zone probably heated by cosmic rays produced in the supernova surrounding a cool core of molecular gas. Our model is consistent with the observed COJ= 3 − 2 line strength. These molecular line emitting knots in the Crab Nebula present a novel phase of the ISM representative of many important astrophysical environments. 

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5. Single Pulse Dispersion Measure of the Crab Pulsar

   https://doi.org/10.3847/1538-4357/ac8055  

   Lewandowska, N.; Demorest, P. B.; McLaughlin, M. A.; Kilian, P.; Hankins, T. H. (August 2022, The Astrophysical Journal)

   Abstract We investigate the use of bright single pulses from the Crab pulsar to determine separately the dispersion measure (DM) for the Main Pulse and Interpulse components. We develop two approaches using cross-correlation functions (CCFs). The first method computes the CCF of the total intensity of each of the 64 frequency channels with a reference channel and converts the time lag of maximum correlation into a DM. The second method separately computes the CCF between every pair of channels for each individual bright pulse and extracts an average DM from the distribution of all channel-pair DMs. Both methods allow the determination of the DM with a relative uncertainty of better than 10⁻⁵and provide robust estimates for the uncertainty of the best-fit value. We find differences in DM between the Main Pulse, the Low Frequency Interpulse, and the High Frequency Interpulse using both methods in a frequency range from 4 to 6 GHz. Earlier observations of the High Frequency Interpulse carried out by Hankins et al. (2016) resulted in DM_HFIP–DM_MPof 0.010 ± 0.016 pc cm⁻³. Our results indicate a DM_HFIP–DM_MPof 0.0127 ± 0.0011 pc cm⁻³(with DM_compbeing the DM value of the respective emission component), confirming earlier results with an independent method. During our studies we also find a relation between the brightness of single pulses in the High Frequency Interpulse and their DM. We also discuss the application of the developed methods on the identification of substructures in the case of Fast Radio Bursts. 

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