Low Mass X-Ray Binaries and X-Ray Bursters NTHU Yi-Kuan Chiang 2007/12/13

Outline  Introduction  Eclipses and X-ray dippers  X-ray bursters  Summary

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 ≈ ~ 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 EXO

Eclipses and dippers

Accretion disc coronae

Disc-shadowing concept

Comparison of EXOSAT light curves of 2A and EXO  2A has a partial and broad X-ray eclipse  EXO exhibits narrow, but almost total eclipse  The erratic variations of EXO are due to structure on the edge of the disc obstructing the central source.  In 2A the central source is never visible at all. Extended X-ray region smooth the light curve.

X-ray bursters  Regular burst (4U )  Irregular burst ( )

Type I burst  X-ray outburst of an accreting NS  Energy release : ~ erg/s  Recurrence : hours~days  Regular or irregular

Profiles of MXB 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