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Register a new userA Major X-ray Outburst from an Ultraluminous X-Ray Source in M82
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We detected a major X-ray outburst from M82 with a duration of 79 days, an average flux of 5E-11 erg cm^-2 s^-1 in the 2-10 keV band, and strong variability. The X-ray spectrum remained hard throughout the outburst. We obtained a Chandra observation during the outburst that shows that the emission arises from the ultraluminous X-ray source X41.4+60. This source has an unabsorbed flux of (5.4 +/- 0.2)E-11 erg cm^-2 s^-1 in the 0.3-8 keV band, equivalent to an isotropic luminosity of 8.5E40 erg/s. The spectrum is adequately fitted with an absorbed power-law with a photon index of 1.55 +/- 0.05. This photon index is very similar to the value of 1.61 +/- 0.06 measured previously while the flux was (2.64 +/- 0.14)E-11 erg cm^-2 s^-1. Thus, the source appears to remain in the hard state even at the highest flux levels observed. The X-ray spectral and timing data available for X41.4+60 are consistent with the source being in a luminous hard state and a black hole mass in the range of one to a few thousand solar masses.
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The starburst galaxy M82 contains two ultraluminous X-ray sources (ULXs), CXOM82 J095550.2+694047 (=X41.4+60) and CXOM82 J095551.1+694045 (=X42.3+59), which are unresolved by XMM-Newton. We revisited the two XMM-Newton observations of M82 and analyzed the surface brightness profiles using the known Chandra source positions. We show that the quasi-periodic oscillations (QPOs) detected with XMM-Newton originate from X41.4+60, the brightest X-ray source in M82. Correcting for the contributions of the unresolved sources, the QPO at a frequency of 55.8+/-1.3 mHz on 2001 May 06 had a fractional rms amplitude of 32%, and the QPO at 112.9+/-1.3 mHz on 2004 April 21 had an amplitude of 21%. The QPO frequency may possibly be correlated with the source flux, similar to the type C QPOs in XTE 1550-564 and GRS 1915+105, but at luminosities two orders of magnitude higher. X42.3+59, the second brightest source in M82, displayed a strikingly high flux of 1.4E-11 ergs/cm^2/s in the 2-10 keV band on 2001 May 6. A seven-year light curve of X42.3+59 shows extreme variability over a factor of 1000; the source is not detected in several Chandra observations. This transient behavior suggests accretion from an unstable disk. If the companion star is massive, as might be expected in the young stellar environment, then the compact object would likely be an IMBH.
Using simultaneous optical (VLT/FORS2) and X-ray (XMM-Newton) data of NGC 5408, we present the first ever attempt to search for a reverberation signal in an ultraluminous X-ray source (NGC 5408 X-1). The idea is similar to AGN broad line reverberation mapping where a lag measurement between the X-ray and the optical flux combined with a Keplerian velocity estimate should enable us to weigh the central compact object. We find that although NGC 5408 X-1s X-rays are variable on a timescale of a few hundred seconds (RMS of 9.0$pm$0.5%), the optical emission does not show any statistically significant variations. We set a 3$sigma$ upper limit on the RMS optical variability of 3.3%. The ratio of the X-ray to the optical variability is an indicator of X-ray reprocessing efficiency. In X-ray binaries, this ratio is roughly 5. Assuming a similar ratio for NGC 5408 X-1, the expected RMS optical variability is $approx$2% which is still a factor of roughly two lower than what was possible with the VLT observations in this study. We find marginal evidence (3$sigma$) for optical variability on a $sim$ 24 hour timescale. Our results demonstrate that such measurements can be made, but photometric conditions, low sky background levels and longer simultaneous observations will be required to reach the optical variability levels similar to X-ray binaries.
We use XMM-Newton and Swift data to study spectral variability in the ultraluminous X-ray source (ULX), Holmberg IX X-1. The source luminosity varies by a factor 3-4, giving rise to corresponding spectral changes which are significant, but subtle, and not well tracked by a simple hardness ratio. Instead, we co-add the Swift data in intensity bins and do full spectral fitting with disc plus thermal Comptonisation models. All the data are well-fitted by a low temperature, optically thick Comptonising corona, and the variability can be roughly characterised by decreasing temperature and increasing optical depth as the source becomes brighter, as expected if the corona is becoming progressively mass loaded by material blown off the super-Eddington inner disc. This variability behaviour is seen in other ULX which have similar spectra, but is opposite to the trend seen in ULX with much softer spectra. This supports the idea that there are two distinct physical regimes in ULXs, where the spectra go from being dominated by a disc-corona to being dominated by a wind.
We present a multi-mission X-ray analysis of a bright (peak observed 0.3-10 keV luminosity of ~ 6x10^{40} erg s^{-1}), but relatively highly absorbed ULX in the edge-on spiral galaxy NGC 5907. The ULX is spectrally hard in X-rays (Gamma ~ 1.2-1.7, when fitted with an absorbed power-law), and has a previously-reported hard spectral break consistent with it being in the ultraluminous accretion state. It is also relatively highly absorbed for a ULX, with a column of ~ 0.4-0.9x10^{22} atom cm^{-2} in addition to the line-of-sight column in our Galaxy. Although its X-ray spectra are well represented by accretion disc models, its variability characteristics argue against this interpretation. The ULX spectra instead appear dominated by a cool, optically-thick Comptonising corona. We discuss how the measured 9 per cent rms variability and a hardening of the spectrum as its flux diminishes might be reconciled with the effects of a very massive, radiatively-driven wind, and subtle changes in the corona respectively. We speculate that the cool disc-like spectral component thought to be produced by the wind in other ULXs may be missing from the observed spectrum due to a combination of a low temperature (~ 0.1 keV), and the high column to the ULX. We find no evidence, other than its extreme X-ray luminosity, for the presence of an intermediate mass black hole (~ 10^2 - 10^4 Msun) in this object. Rather, the observations can be consistently explained by a massive (greater than ~ 20 Msun) stellar remnant black hole in a super-Eddington accretion state.
Ultraluminous X-ray sources (ULX) are off-nuclear point sources in nearby galaxies whose X-ray luminosity exceeds the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes. Their luminosity ranges from $10^{40}$ erg s$^{-1} < L_X$(0.5 - 10 keV) $<10^{40}$ erg s$^{-1}$. Since higher masses imply less extreme ratios of the luminosity to the isotropic Eddington limit theoretical models have focused on black hole rather than neutron star systems. The most challenging sources to explain are those at the luminous end ($L_X$ > $10^{40}$ erg s$^{-1}$), which require black hole masses MBH >50 solar masses and/or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries. Here we report broadband X-ray observations of the nuclear region of the galaxy M82, which contains two bright ULXs. The observations reveal pulsations of average period 1.37 s with a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to $L_X$(3 - 30 keV) = $4.9 times 10^{39}$ erg s$^{-1}$. The pulsating source is spatially coincident with a variable ULX which can reach $L_X$ (0.3 - 10 keV) = $1.8 times 10^{40}$ erg s$^{-1}$. This association implies a luminosity ~100 times the Eddington limit for a 1.4 solar mass object, or more than ten times brighter than any known accreting pulsar. This finding implies that neutron stars may not be rare in the ULX population, and it challenges physical models for the accretion of matter onto magnetized compact objects.
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