1. XMM-NEWTON reveals a dipping black-hole X-ray binary in NGC 55
XMM-EPIC Consortium Meeting , Palermo Astronomical Observatory
Wednesday October 15th 2003
XMM-NEWTON reveals a dipping black-hole X-ray binary in NGC 55
Ann-Marie Stobbart
Robert Warwick, Tim Roberts

2. X-ray binaries
Important component of the X-ray emission from spiral galaxies
Lx ≲1038 erg s-1 – stellar mass accretors
BUT:
Einstein revealed discrete X-ray sources with Lx >1039 erg s-1
(e.g. Fabbiano & Trinchieri 1987)
 Ultraluminous X-ray sources
(ULXs)- New class of source
Suggestions
True super-Eddington luminosity
IMBH ( M⊙ )
Anisotropic / Beamed emission
ROSAT HRI Survey
(Roberts & Warwick 2000)
 1 in 5 galaxies have
at least one source with
Lx > 1039 erg s-1
 ULXs are rare!

3. NGC 55 Irregular Barred Spiral Galaxy
Morphologically similar to LMC but viewed edge on
Nearby (D ~1.8 Mpc)
 Excellent for point source studies
ROSAT Observations: revealed 25 point like X-ray sources and diffuse emission
(Roberts,T.P.,1997 PhD Thesis)
NOAO

4. X-ray data 2 observations, offset by 7’ in opposite directions
Net exposure time
1st Observation: 30.4 ks
2nd Observation: 21.5 ks
Gap of 6.2 ks
XMMU J

5. X-ray lightcurve 0.5 – 3.5 ct s-1 (I) – Initial low flux state
keV
(I)
(II)
(IId)
(IIId)
(III)
(I) – Initial low flux state
(II) – Intermediate flux state
(III) – Highest flux state
(IId) – 1st Dipping state
(IIId) – 2nd Dipping state
0.5 – 3.5 ct s-1
Upward drift
Pronounced dips
τdip as short ~100s
some reach ~ 100% flux diminution

6. X-ray lightcurves II
Initial count rate increase most significant > 1keV
Dips in all bands but deeper in the hard band
Soft
Medium
Counts s-1
Hard
Soft : keV
Med : keV
Hard : keV
Time (s)

7. Colour-correlation Red = Initial Low flux state
Hard counts
Observ. 1
Observ. 2
Medium counts
Red = Initial Low flux state
Green = Higher flux states of both observations
Blue = Dip regions of both observations
Spectral hardening after initial low state

8. X-ray spectra I Γ~4.2 Tin ~ 0.9 keV NH ~ 4.2 x 1021cm-2
Initial low flux state – Fitted with simple power-law, PL (constant soft component)
Higher flux states– Required 2nd component in the form of a disk black-body, DBB (hard variable component)
Results
Γ~4.2
Tin ~ 0.9 keV
NH ~ 4.2 x 1021cm-2
(II)
PL/(I) fit
DBB
(II) fit

9. X-ray spectra II Dips – fitted with the same model parameters
(IIId)
(III)
Dips – fitted with the
same model parameters
(Γ, Tin, NH) but with reduced contributions from the model components
During the dips : the DBB contribution is reduced more
than the PL  Consistent with evidence from the
lightcurves that the depth of the dips is greater at
higher energies
Lx (II) ~ 1.5 x1039 erg s-1 Lx (III) ~ 1.6x1039 erg s-1
 ULX regime!

10. What’s causing the dips?
Seeing the accretion disk edge on  Dips due to obscuring material
Spectrum in the dips is softer  Dips are not due to simple absorption / scattering process
During dips, hard component is blocked more than the soft component
Suggestion
Hard component comes from the inner accretion disk (DBB)
Soft component is from an extended corona (PL)
 Dips due to the orbital motion of the blocking / scattering material

11. Summary X-ray source exhibits prominent short term variability
XMM detection of a remarkably luminous X-ray source Lx ~ 1.5 x1039 erg s-1
Observed Lx just places the source in the ULX regime
Actually seeing the high luminosity end of the stellar mass BH population
X-ray source exhibits prominent short term variability
Particularly the dip structure
Spectral fits consistent to those found in some X-ray states of Galactic BHXBs