1. RXTE has doubled the number of known cyclotron lines sources RXTE has observed multiple line sources in outburst RXTE has provided multiple visits to both persistent and transient sources to allow luminosity-dependent effects to be investigated Spin phase resolved spectroscopy has allowed for probing different portions of the scattering region as the neutron star rotates. RXTE has revealed variations with pulse shape. Heindl et al. 2003

2. The Numbers: 68 accreting X-ray pulsars 25 Be binaries 6 Possible Be binaries 14 Supergiants 10 Symbiotics binaries 3 Low mass X-ray Binaries 1 High Mass X-ray binary 1 Intermediate mass (Her X-1) 1 Possible white dwarf 7 Unknown Companion 17 Sources exhibiting one or more cyclotron lines 9 Be binaries 5 Supergiants 1 Intermediate mass 1 Symbiotic binary (New) 1 Possible white dwarf 5 Sources possibly having a line 3 Be binaries 1 Supergiant 1 Unknown Caballero et al. 2010, 2011 A0535+26

3. RXTE Observations of Cyclotron Line Variability with Luminosity

4. Theory: Emission Geometry Critical Luminosity for Bimodal Spectral Variability in Accretion-Powered X-Ray Pulsars

5. Theory: Variation of CRSF Energy Critical Luminosity for Bimodal Spectral Variability in Accretion-Powered X-Ray Pulsars We see 4 distinct types of variation of E cyc with variation of L x Supercritical, radiation deceleration LxLx Subcritical, Coulomb stopping Subcritical, gas-shock Subcritical, direct impact E cyc E cyc = E * LxLx Region of Interest

6. Theory: Variation of Emission Height Critical Luminosity for Bimodal Spectral Variability in Accretion-Powered X-Ray Pulsars The observations of E cyc imply 4 distinct types of variation of the characteristic emission height h with variation of L x Supercritical, radiation deceleration LxLx h Subcritical, Coulomb stopping Subcritical, gas-shock Subcritical, direct impact h = 0 LxLx

7. Advances in Understanding Cyclotron Lines from a Physics Point of View Predicted cyclotron line shapes on top of a Fermi-Dirac continuum model with E cut =10 keV and E fold =12 keV. Modeled is a cylindrical column with B/B crit = 0.05, kT e = 3 keV, τ es = 10 -3. Note Bcrit = 4.4 x 10 13 Gauss

8. Combining the Continuum and Cyclotron Line Modeling

9. Her X-1, the Original Cyclotron Line Source Trümper et al. 1977 35 day super modulation 1.24 s pulsed flux

10. The evolution of the pulse profile is intimately linked to the evolution of the 35 day Main-On. The left shoulder goes away as the Main- On progresses. Klochkov & Staubert

11. Variation of Power Law Photon Index with Pulse Phase and Main-On Interval The power law photon index softens slightly with pulse phase as the pulse profile varies throughout the 35 day Main-ON, with two EXCEPTIONS: 1) Phase ~0.6: Abrupt change in index lessens as the left shoulder of the profile diminishes. 2) Phase ~0.7-0.9: The index flattens from 0.7-0.8 and steepens again from 0.8-0.9 Is this Steve Pravdo’s viewing down the pole towards us and the gravitationally beamed pole on the back side?

12. While the power law photon index varies with pulse shape in the Main-On, the cyclotron line variation is essentially the same throughout the Main-On Thus our view of the cyclotron resonant scattering region is unchanged throughout the 35-day Main-On, but the continuum emission region is affected.

13. Variation of the Cyclotron Line with 35 Day Phase Behavior is the same for different 35 Day Main-On’s: Apparent slow rise in centroid energy and then abrupt decrease.

14. What is the long term cyclotron line variation in Her X-1 telling us? The abrupt change in the 1980’s The slow decrease since the 1990’s

15. Variation of Her X-1 Cyclotron Line Energy with Mean Main-On Flux Staubert & Klochkov Red Points are Post-2006 Black Points are Pre-2006

16. Variation of Cyclotron Energy with Average Main-On Flux This works well until about 2007, then another possible abrupt change occurs or the relationship no longer holds.

17. Luminosity (i.e. ASM Main-On Maximum) Corrected Cyclotron Line Energies up to ~2007, and uncorrected after that. Yet again, Her X-1 is presenting us with surprises and challenges to our understanding of magnetic poles of neutron stars.

18. Thank you to everyone who made RXTE a big success.

19. Present List of Cyclotron Line Sources Possible new cyclotron line source: IGR J18179-1621 with 11.8 s period and E cyc ~21 keV

20. Becker & Wolff (2007) and subsequent work by Becker and his students are developing a bulk Comptonization model of accretion onto the magnetic poles. Eventually this physics-based treatment will replace empirical modeling of accretion powered pulsar spectra.