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The majority of ultraluminous X-ray sources are point sources that are spatially offset from the nuclei of nearby galaxies and whose X-ray luminosities exceed the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes (1,2). Their X-ray luminosities in the 0.5-10 kiloelectronvolt energy band range from [10.sup.39] to [10.sup.41] ergs per second (3). Because higher masses imply less extreme ratios of the luminosity to the isotropic Eddington limit, theoretical models have focused on black hole rather than neutron star systems (1,2). The most challenging sources to explain are those at the luminous end of the range (more than [10.sub.40] ergs per second), which require black hole masses of 50-100 times the solar value or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries, or both. Here we report broadband X-ray observations of the nuclear region of the galaxy M82 that reveal pulsations with an average period of 1.37 seconds and a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to an X-ray luminosity in the 3-30 kiloelectronvolt range of 4.9 x [10.sup.39] ergs per second. The pulsating source is spatially coincident with a variable source (4) that can reach an X-ray luminosity in the 0.3-10 kiloelectronvolt range of 1.8 x [10.sup.40] ergs per second (1). This association implies a luminosity of about 100 times the Eddington limit for a 1.4-solar-mass object, or more than ten times brighter than any known accreting pulsar. This implies that neutron stars may not be rare in the ultraluminous X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects., The brightest accretion-powered X-ray pulsars, A0538-66 (5), SMCX-1 (6) and GRO J1744-28 (7), are variable, with reported X-ray luminosities up to [L.sub.X] (2-20 keV) ≅ [10.sup.39] erg [s.sup.-1], which are [...]

Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive [sup.44]Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium (1), directly probes the explosion asymmetries. Cassiopeia A is a young (2), nearby (3), core-collapse (4) remnant from which [sup.44]Ti emission has previously been detected (5-8) but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed [sup.44]Ti emission to estimated [sup.56]Ni emission (9), from optical light echoes (10), and from jet-like features seen in the X-ray (11) and optical (12) ejecta. Here we report spatial maps and spectral properties of the [sup.44]Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the [sup.44]Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae., Titanium-44 is produced in Si burning in the innermost regions of the material ejected in core-collapse supernovae, in the same processes that produce Fe and [56.sup]Ni (ref. 13). The decay [...]

Broad X-ray emission lines from neutral and partially ionized iron observed in active galaxies have been interpreted as fluorescence produced by the reflection of hard X-rays off the inner edge [...]