mass=0.5 (primary) / 0.5 (b) | radius= | temperature=

RX J0806.3+1527 or HM Cancri (sometimes shortened to HM Cnc or J0806 after establishing identity) is an X-ray binary star system about {{convert|1600|ly|pc+km}} away.^[1] It comprises two dense white dwarfs orbiting each other once every 321.5 seconds (in this system the "year" duration is of only 5.4 minutes), at an estimated distance of only {{convert|80000|km|miles}} apart (about 1/5 the distance between the Earth and the Moon). The two stars orbit each other at speeds in excess of {{convert|400|km/s|mph}}. The stars are estimated to be about half as massive as the Sun. Like typical white dwarfs, they are extremely dense, being composed of degenerate matter, and so have radii of order the radius of Earth. Astronomers believe that the two stars will eventually merge, based on data from many X-ray satellites, such as Chandra X-Ray Observatory, XMM-Newton and the Swift Gamma-Ray Burst Mission. These data show that the orbital period of the two stars is steadily decreasing at a rate of 1.2 milliseconds per year as they thus are getting closer by approximately {{convert|60|cm|ft}} per day. At this rate, they can be expected to merge in approximately 340,000 years. With a revolution period of 5.4 minutes RX J0806 is the shortest orbital period binary system currently known.

Observations

As RX J0806.3+1527 is a pair of white dwarfs, it has a relatively low optical luminosity. Scientists discovered periodic spikes occurring every 321.5 s by looking at the source in the X-rays. In fact, the 321.5 s modulation of RX J0806.3+1527 was discovered serendipitously in 1999 thanks to the ROSAT mission working in the X-ray band. Optical follow-up observations with the ESO Very Large Telescope (VLT), Telescopio Nazionale Galileo (TNG) and Nordic Optical Telescope (NOT) allowed the counterpart to be identified, a relatively dim (20.7 magnitude in the B filter) object which shows an optical modulation at the same period detected in the X-ray band. The optical monitoring of the counterpart of RX J0806.3+1527 during 2001-2004 clearly shows that the period is decreasing at a rate of about 1/1000 s each year. This result was confirmed by monitoring the source in the X-rays for several years.

Relation to general relativity

The decreasing separation of the components of the system mean that the system is losing orbital energy. Albert Einstein's theory of General Relativity predicts such a system will lose orbital energy through the generation of gravitational waves. Scientists believe that RX J0806.3+1527 may be one of the strongest sources of gravitational waves in the Milky Way galaxy.{{Citation needed|date=December 2013}}

Sources

• {{APOD |date=1 June 2005 |title=White Dwarf Star Spiral}}
• NASA Sees Orbiting Stars Flooding Space With Gravitational Waves
• RX J0806.3+1527: Orbiting Stars Flooding Space with Gravitational Waves

Further reading

• Science Journal: News of the Week
• European southern Observatory Press Release
• Simultaneous X-ray UV observations of HM Cnc
• Doppler tomography of RXJ0806
• [https://arxiv.org/abs/1003.1986 Phase Resolved Optical spectroscopy of RXJ0806]
• RXJ0806 and the unipolar inductor model
• Coherent timing solution for RXJ0806
• Discovery of the 321.5s modulation in RXJ0806
• Spectroscopic optical study of RXJ0806
• Monitoring the spin up in RX J0806+15
• Phase Coherent Timing of RX J0806.3+1527 with ROSAT and Chandra
• RX J0806
• [https://www.youtube.com/watch?v=ryqN6dyUmJg RX J0806.3+1527 Gravitational Wave Merger]
• [https://arxiv.org/abs/1003.0658v1 SPECTROSCOPIC EVIDENCE FOR A 5.4-MINUTE ORBITAL PERIOD IN HM CANCRI]