X-ray sky in galactic coordinates Faint sources are uniformly distributed (extragalatic)
Classification of LMXB’s A compact object (neutron star or black hole) and a companion (a star or white dwarf) with lower mass
Properties of LMXB’s 150 known LMXB’s (2001) 130 in Milky Way, 13 in globular cluster, 2 in LMC 63 are X-ray bursters Typical Luminosity : L x ≈ 10 33 ~ 10 38 erg/s X-ray spectra : soft (< 10keV) Accretion process : Roche-lobe overflow Orbital periods : from 11 minutes to 17 days
Basic model of LMXB The angular momentum of the material make it form an accretion disc The accretion stream impact the disc forming a bulge
20 hours EXOSAT observation of EXO0748-676
Eclipses and dippers
Accretion disc coronae
Comparison of EXOSAT light curves of 2A 1822-371 and EXO 0748-676 2A 1822-371 has a partial and broad X-ray eclipse EXO 0748-676 exhibits narrow, but almost total eclipse The erratic variations of EXO 0748-676 are due to structure on the edge of the disc obstructing the central source. In 2A 1822-371 the central source is never visible at all. Extended X-ray region smooth the light curve.
Type I burst X-ray outburst of an accreting NS Energy release : 10 36 ~ 10 38 erg/s Recurrence : hours~days Regular or irregular
Profiles of MXB1728-34 observed by SAS-3 Fast rise ( typically<2s ) Exponential-like decay ( typically 10s~20s) Profiles shorter at higher energies (cooling, spectrum becomes softer during decay)
Thermonuclear flash model Hydrogen burns steadily into helium The conditions in helium layer go critical, a thermonuclear flash take place
Relation between total burst energy and the burst recurrence time
Type II burst (rapid burster) Recurrence : about 10 seconds No evidence of cooling in the tail eight orbits of SAS-3 observations of a Rapid Burster
The time to the following burst depends on the energy in previous burst
Magnetospheric gate model Material accreting from the disc is held back by the magnetosphere. When enough material has built up outside the gate,the magnetosphere can no longer hold it.
Summary Type I nuclear flashes in surface layers nuclear energy Energy proportional to preceding inactive period Burning accumulated material Spectral softening during decay Type II Accreting material force the magnetospheric gate to open again Gravitational energy Energy proportional to following inactive period