Vela X-1 is a pulsing, eclipsing high-mass X-ray binary (HMXB) system, associated with the Uhuru source 4U 0900-40 and the supergiant star HD 77581.

The X-ray emission of the neutron star is caused by the capture and accretion of matter from the stellar wind of the supergiant companion.

It has been given the variable star designation GP Velorum, and it varies from visual magnitude 6.76 to 6.99.

[5] The spin period of the neutron star is about 283 seconds, and gives rise to strong X-ray pulsations.

[7] The accreting matter causes the random spin period changes.