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

X-ray binaries www.ph.ed.ac.uk/nuclear/astro 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 ( 102-105 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!

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

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 J001529.6-391315

X-ray lightcurve 0.5 – 3.5 ct s-1 (I) – Initial low flux state 0.3 - 10.0 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

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 : 0.5-1.0 keV Med : 1.0-2.0 keV Hard : 2.0-4.5 keV Time (s)

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

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

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!

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

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