RXTE Studies of Accreting X-ray Pulsars via Cyclotron Lines

Rick Rothschild on behalf of the MAGNET collaboration

• 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 allowluminosity-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

The Numbers:

68 accreting X-ray pulsars25 Be binaries 6 Possible Be binaries14 Supergiants10 Symbiotics binaries3 Low mass X-ray Binaries1 High Mass X-ray binary 1 Intermediate mass (Her X-1)1 Possible white dwarf7 Unknown Companion

17 Sources exhibiting one or more cyclotron lines

9 Be binaries5 Supergiants1 Intermediate mass1 Symbiotic binary (New)1 Possible white dwarf

5 Sources possibly having a line3 Be binaries1 Supergiant1 Unknown

Caballero et al. 2010, 2011

A0535+26

RXTE Observations of Cyclotron Line Variability with Luminosity

Theory: Emission Geometry

Critical Luminosity for Bimodal Spectral Variabilityin Accretion-Powered X-Ray Pulsars

Theory: Variation of CRSF Energy

Critical Luminosity for Bimodal Spectral Variabilityin Accretion-Powered X-Ray Pulsars

• We see 4 distinct types of variation of Ecyc with variation of Lx

Supercritical,radiationdeceleration

Lx

Subcritical,Coulomb stopping

Subcritical,gas-shock

Subcritical,direct impact

Ecyc

Ecyc = E*

Lx

Region of Interest

Theory: Variation of Emission Height

Critical Luminosity for Bimodal Spectral Variabilityin Accretion-Powered X-Ray Pulsars

• The observations of Ecyc imply 4 distinct types of variation of the characteristic emission height h with variation of Lx

Supercritical,radiationdeceleration

Lx

h

Subcritical,Coulomb stopping

Subcritical,gas-shock

Subcritical,direct impact

h = 0

Lx

Advances in Understanding Cyclotron Lines from a Physics Point of View

Predicted cyclotron line shapes on top of a Fermi-Dirac continuum model with Ecut=10 keV and Efold=12 keV. Modeled is a cylindrical column with B/Bcrit = 0.05, kTe = 3 keV, τes = 10-3. Note Bcrit = 4.4 x 1013 Gauss

Combining the Continuum and Cyclotron Line Modeling

Her X-1, the Original Cyclotron Line Source

Trümper et al. 1977

35 day super modulation

1.24 s pulsed flux

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

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?

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.

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.

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

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

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.

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.

Thank you to everyone who made RXTE a big success.

Present List of Cyclotron Line Sources

Possible new cyclotron line source: IGR J18179-1621 with 11.8 s period and Ecyc~21 keV

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.