www.nustar.caltech.edu

Nuclear Spectroscopic Telescope Array

Probing the nature of Ultraluminous X-ray Sources using broad-band X-ray data

D. J. Walton1,2, M. Bachetti3, V. Rana1, F. A. Harrison1, D. Barret3, F. E. Bauer4, A. C. Fabian5, F. Fuerst1, B. W. Grefenstette1, A. Hornschmeier6, K.

K. Madsen1, M. J. Middleton5, J. M. Miller7, E. Mukherjee1, M. L. Parker5, A. Ptak6, D. Stern2, N. A.

Webb3, and the NuSTAR Science Team

1. Caltech – 2. JPL/Caltech – 3. IRAP Toulouse – 4. Pontificia – 5. Univ. Cambridge – 6. NASA/GSFC – 7. Univ. Michigan

Ultraluminous X-ray Sources: Ultraluminous X-ray sources (ULXs) are point sources in nearby galaxies with X-ray luminosities LX > 1039 erg/s, in excess of the Eddington luminosity for the typical Galactic stellar-mass black holes (StMBHs). Understanding the origin of these extreme luminosities remains one of the biggest mysteries in high-energy astrophysics. Possible explanations include the existence of a (poorly understood) regime of super-Eddington accretion onto StMBHs, or the presence of massive (>~100 M⊙) black holes.

With the launch of the first ever focusing hard X-ray optics aboard NuSTAR, it has finally become possible to observe ULXs at hard X-rays. We have undertaken a substantial program observing extreme (LX ~ 1040 erg/s), coordinated with soft X-ray coverage from XMM-Newton and/or Suzaku, in order to probe the broadband X-ray spectra of these enigmatic sources for the first time.

NuSTAR Targets:

Broadband X-ray Spectra

Key Results:

Spectral Variability:

Acknowledgements This work was supported under NASA No. NNG08FD60C, and

made use of data from the Nuclear Spectroscopic Telescope Array (NuSTAR) mission, a project led by Caltech, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with execution and analysis of these observations.

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Key NuSTAR targets include:

•  IC 342 X-1, X-2 (NuSTAR, XMM; below) •  Holmberg IX X-1 (NuSTAR, XMM, Suzaku) •  NGC 1313 X-1, X-2 (NuSTAR, XMM) •  Circinux ULX5 (ToO; NuSTAR, XMM) •  Holmberg II X-1 (NuSTAR, XMM, Suzaku)

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Optical image of IC 342, with NuSTAR hard X-rays (3-35 keV) overlayed in purple. The ULXs are detected above 10 keV for the first time.

•  NuSTAR has performed the first ever high energy observations of ULXs, helping provide high quality broadband (~0.3-30 keV) spectra.

•  All sources observed to date show curved high-energy continua, inconsistent with standard powerlaw-like Comptonised emission, as would have been expected if these sources host massive (~1,000-10.000 M⊙) black holes accreting at substantially sub-Eddington rates.

•  A number of ULXs also show strong spectral variability between observed epochs, with distinct differences to the sub-Eddington state transitions observed from Galactic BH binaries.

•  We caught an astonishing rise of ~2 orders of magnitude in only 4 days from NGC 5907 ULX1

•  It seems that we are witnessing an unusual, likely high-Eddington accretion regime. This is supported by the spectacular precedent set by the extreme ULX M82 X-2, LX,peak ~ 2x1040 erg/s, discovered by NuSTAR to be a highly super-Eddington accreting pulsar (Bachettl et al. 2014, Nature; see talk by Prof. Harrison).

Walton et al. 2014a

Walton et al. 2013

Over its first two years of operation, NuSTAR has undertaken targeted observations of ~10 well known ULXs, providing the first systematic high energy view of these sources. In all cases to date, we find that the hard X-ray continuum is very steep, robustly confirming the spectral curvature tentatively hinted at by observations with soft X-ray detectors alone.

NuSTAR (2013) NuSTAR (2013) XMM (2013) XMM (2001) Suzaku (2006)

NuSTAR 1 NuSTAR 2 XMM 1 XMM 2 Suzaku

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NuSTAR XMM

Bachetti et al. 2013 Rana et al. 2014

NGC 1313 X-1 IC 342 X-1

A Vanishing Act: NGC 5907 ULX1

The NuSTAR strategy has been to perform two observations of each target, separated by ~a week. While many sources display consistent spectra (e.g. above), a number show strong, yet unusual spectral variability between epochs (left). Although Circinus ULX5 was observed as a Target of Opportunity, the variability is marked when compared with archival data. We are now actively seeking to probe spectral variability in a larger sample of ULXs for comparison.

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Walton et al. 2013

Walton et al. 2014b (sub.)

Holmberg IX X-1 Circinus ULX5

(Images from XMM [top panels] and NuSTAR [bottom] from epoch 1 [left]

and epoch 2 [right])

One of the NuSTAR targets was NGC 5907 ULX1, known to reach LX,peak ~ 5x1040 erg/s. In the first observation this source was unexpectedly faint, barely detected by XMM. However, it was clearly present in the second, only ~4 days later, implying a brightening of ~2 orders of magnitude. This detection displayed a broadband spectrum similar to the other ULX targets.