Low Mass XRay Binaries and XRay Bursters NTHU

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) ³ ³ � � 130 in Milky Way , 13 in globular cluster , 2 in LMC 63 are X-ray bursters Typical Luminosity : Lx ≈ 1033 ~ 1038 erg/s X-ray spectra : soft (< 10 ke. V) 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 0748 -676

Eclipses and dippers

Accretion disc coronae

Disc-shadowing concept

Comparison of EXOSAT light curves of 2 A 1822 -371 and EXO 0748 -676 � � 2 A 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 2 A 1822 -371 the central source is never visible at all. Extended X-ray region smooth the light curve.

X-ray bursters � Regular burst (4 U 1820 -303) � Irregular burst (1636 -536)

Type I burst � � X-ray outburst of an accreting NS Energy release : 1036 ~ 1038 erg/s Recurrence : hours~days Regular or irregular

Profiles of MXB 1728 -34 observed by SAS-3 � � � Fast rise ( typically<2 s ) Exponential-like decay ( typically 10 s~20 s) 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