The Nature of the Doppler Lines in the Ultra-compact Binary 4U1626-67

Doppler Lines and Fe Fluorescence in the Ultra-compact Binary 4U1626-67

Previous HETG GTO observations: - Schulz et al. 2001: Double Peaked X-ray Lines from the O/Ne-rich Accretion Disk in 4U 1626-67: OBSIDs 104, 39 ksec - Krauss et al. 2007: High Resolution X-ray Spectroscopy of the Ultra-compact LMXB Pulsar 4U 1626-67: OBSIDs 104, 39 ksec 3504, 97 ksec

Doppler Lines and Fe Fluorescence in the Ultra-compact Binary 4U1626-67

New HETG GTO observations:-Chakrabarty & Schulz 2009 Cycle 11 GO time: OBSIDs 11058, 80 ksec, Jan 14. 2010

Doppler Lines and Fe Fluorescence in the Ultra-compact Binary 4U1626-67

Tbnew (Powerlaw + Bbodyrad):NH = 1.2x1021 cm-1 A = 0.0084 ph cm-2 s-1

= 0.80Abb = 593 (R2

km / D2kpc)

kTbb = 0.20 keV

Tbnew (Powerlaw + Bbodyrad):NH = 1.2x1021 cm-1 A = 0.0384 ph cm-2 s-1

= 1.19Abb = 83 (R2

km / D2kpc)

kTbb = 0.48 keV

Flares and Dips in the New Light Curve of 4U1626-67

Obsid 11058:

Obsid 3504:

Doppler Lines in the Ultra-compact Binary 4U1626-67

Doppler Lines in the Ultra-compact Binary 4U1626-67

Ionization Model Fits to the X-ray Spectrum of 4U1626-67

Photo-ionized modeling:

Collisional-ionized modeling:

Aped_density = 13:

Collisional Ionization Model Fits to the X-ray Spectrum of 4U1626-67

Pure C/O/Ne disk?:

1) Cannot maintain C I/Ne I/O I : log > 2 @ r~109 cm

2) Pure C/O disk model predicts T = 28000 K @ 20000 km (Werner et al. 2006)

Doppler Lines and Fe Fluorescence in the Ultra-compact Binary 4U1626-67

Magnetospheric Accretion Shocks in the Ultra-compact Binary 4U1626-67

Emission Volumes:

In conclusion we propose the following:

Doppler Lines in the Ultra-compact Binary 4U1626-67

Magnetospheric Accretion Shocks

Rco = 8.5x108 cm

Rco = Rmag

Vshift ~ 2000 km/s

~ Vco

Vco ~ Vshock

Tjump < 60 MK

CO plasma< 20 deg impact

Vshift = Vion

Tshock < 10 K

In conclusion we propose the following:

Doppler Lines in the Ultra-compact Binary 4U1626-67

• The light curve before torque reversal is featureless. The light curve after torque reversal shows enhance variability which includes type II flaring, intensity dips, and periods of quiescence.

• The X-ray flux at the time of the observation in 2009 is at about the same level as it was in 1994.

• The X-ray continuum after torque reversal is fit by the same spectrum as before, however with a higher blackbody temperature and a smaller emission radius.• The spectrum shows a narrow Fe K fluorescence line, which was not observed before torque reversal.

• A photo-ionized plasma cannot fit the Ne and O Doppler line emissions.

• The large ratio between the Ne X Lα line and the upper limit to the Ne X Lβ line rules out significant contributions due to resonance scattering.• A collisions ionized plasma fits both Ne and O line ratios very well with enhanced plasma densities and plasma temperature between 1 MK and 10 MK. Magnetospheric Accretion Shocks