Evidence for an Intermediate Mass Black Hole in NGC 5408 X-1 Tod Strohmayer, NASA’s Goddard Space Flight Center

Black Hole Unification Substantial progress in demonstrating BH similarities across the mass scale Fundamental Plane and Variability Plane (Koerding et al. 2007). Now clear that timing properties do scale with mass. So let’s try and use it! McHardy 2009 Koerding et al. 2007 In the RXTE era particularly, tremendous growth in knowledge of stellar black hole timing and spectral properties. Can we leverage this knowledge and apply to ULX mass question? Should perhaps expect analogous behavior in ULXs

NGC 5408 X-1: Summary Bright, variable, non-nuclear point source: L ~ 2 x 1040 ergs/s (d=4.8 Mpc) X-ray spectrum; “cool” (0.15 keV) thermal (disk?) component, and steep (index ~ 2.5) power law (Kaaret et al. 2003; Soria et al. 2004; Strohmayer et al. 2007). “Steep Power-law State,” “Intermediate State.” Powerful radio nebula (Soria et al. 2006; Lang et al. 2007); steep spectrum, optically thin synchrotron from a jet-powered radio lobe? Optical counterpart with V ~ 22.4, continuum consistent with X-ray reprocessing in a standard disk, nebular emission lines detected, consistent with a photoionized nebula and ~ isotropic X-ray flux (Kaaret & Corbel 2009). 120 ksec XMM-Newton observation in January 2006 detected 20 mHz QPO and power spectral break, with QPO near the break, analogous to Galactic BH systems like XTE J1550-564 (Strohmayer et al. 2007). Second XMM-Newton pointing obtained in January 2008. Results reported here and in Strohmayer & Mushotzky (2009).

Timing -- Spectral Correlations in Stellar-mass Black Holes XTE J1550-564 Type C QPOs QPO frequency shows good correlation with PL index (Vignarca et al. 2003) Frequency also correlates with disk flux, and rms amplitude decreases as disk flux increases. Some sources show “saturation.” Shaposhnikov & Titarchuk (2009)

NGC 5408 X-1: New QPO Detections We identified ~72 ksec of useful exposure. Average count rate ~1 s-1 Significant variability apparent by “eye”. Strong QPO detection at 10 mHz Again evidence for pair of QPOs, consistent with 4/3 ratio. Qualitatively similar PDS form as first epoch. Similar to GBHs in SPL state.

Timing and Spectral Properties of X-1 Observation 2 Observation 1 Higher flux (mostly from disk component) corresponds to higher QPO frequency and lower QPO amplitude. Steeper power-law has higher QPO frequency.

Timing -- Spectral Correlations for X-1: Mass Estimates X-1 x 300 Type C QPOs Scale factors can be derived for each comparison system with overlapping spectral index measurements. Mass estimate for X-1 is then the mass of the comparison system x scale factor. Shaposhnikov & Titarchuk (2009)

Mass Estimates Using Galactic Systems With Mass Measurements. Derived scale factors for six Galactic systems. Use QPO frequency measurements when power-law index is between 2.4 and 2.6. We scale to align the centers of the two frequency ranges. Derive conservative bounds by scaling lower frequency to the higher, and vice versa. Conservative mass range from ~1000 - 9000 Msolar

NGC 5408 X-1: Mass from High Frequency Variability Recent study by Gierlinski et al. (2008) suggests that the amplitude of high frequency variability in BH scales inversely with mass. Demonstrate mass scaling using Galactic BH and AGN, most applicable to “hard state” measurements. Scaling parameter is the integrated rms variability in the power law above the break frequency. NGC 5408 X-1 appears to fit within the relationship, suggests mass of ~3,500 Msun NGC 5408 X-1  Gierlinski et al. (2008)

Long-term Modulation in M82 X-1 Kaaret et al. found 62 day modulation in data obtained with RXTE/PCA from M82. X-1 is so luminous It dominates the flux from the galaxy. Spectrum is hard enough that PCA can detect the source. No other ULX can be monitored in this way. Consistent with period modulation, likely orbital. Roche lobe overflow implies donor is a giant or supergiant. Motivated more recent Swift campaigns (5408 X-1).

NGC 5408 X-1: The Binary Period Kaaret et al. found 62 day modulation in M82 X-1 with RXTE. Likely orbital period. Few ULXs have been monitored in such a way as to maximize orbital period searches. Swift campaign has now achieved ~700 day baseline. Now compelling evidence for 113 day periodic modulation! Very likely the orbital period of the black hole binary.

Interpretation: Super-orbital Periods? Some binaries (NS an BH) show “super-orbital” periods. Disk precession modulations have been suspected. Known BH systems have long periods, only NS systems in the range observed from M82 and NGC 5408 X-1. Observed fluxes appear much to super-eddington for NS accretion. Appears inconsistent with super-orbital. Kaaret & Feng (2007)

Orbital Modulation: Energy Dependence Amplitude is 18% in 0.2 - 8 keV band Increases to ~25% below 1 keV. The modulation is stronger and “sharper” at low energies. Peak is broader and arrives later in the > 1 keV band. 113 day orbital period, and assuming Roche lobe overflow, implies mean density of donor is ~1.5 x 10-5 g cm-3 . Donor has to be a giant or supergiant. Deeper optical spectroscopy could yield mass function.

Future Possibilities: Higher Frequencies? Galactic systems show high frequency QPOs (100s of Hz). 3:2 ratios in several systems, suggested as perhaps due to GR resonances (Abramowicz et al.). GRO J1655-40 Scaling with 1/M suggested by results in three systems (McClintock & Remillard (2006). If NGC 5408 X-1 is an IMBH, then expect analogs would be in ~1 Hz range. One goal of longer exposures. Strohmayer (2001)

Conclusions The strong QPOs detected in NGC 5408 X-1 have properties consistent with the ubiquitous “Type C” QPOs observed in Galactic systems, but with systematically lower frequency. NGC 5408 X-1 appears to behave like a stellar mass black hole in the SPL or “intermediate” state, but with QPO frequencies, X-ray luminosity, and disk temperature more indicative of a ~ few 1000 solar mass black hole. Detection of the optical counterpart and photoionized nebula suggests optical continuum is due to reprocessing of X-rays in a “standard” disk, and that X-ray emission is largely isotropic (Kaaret & Corbel 2009). Swift monitoring has revealed a 113 day periodicity that is plausibly the orbital period of a Roche lobe overflow binary with a giant or supergiant donor. The properties of NGC 5408 X-1 appear consistent with it being an intermediate mass black hole. Additional XMM-Newton observations will further map out the Timing - Spectral correlations. A radial velocity curve measured over the 113 day period would be decisive.

Power Spectrum of ULX in M82: 54 mHz QPO

New QPO Detections Strong QPO detected at 10 mHz. 2nd QPO evident in higher energy band. QPOs in 4/3 ratio, similar to 2006 observation. 0.5 - 6 keV 1.3 - 6 keV Power spectral form very similar to earlier (2006) observation, but with lower frequency QPO, higher rms variability, weaker disk flux component than in 2006.