Long-Term Spectral Variations of Ultraluminous X-Ray Sources in the Interacting Galaxy Systems M 51 and NGC 4490/85

Abstract

Variable ultraluminous X-ray sources (ULXs), which are considered to be black hole binaries (BHBs), are known to show state transitions similarly to Galactic BHBs. However, the relation between the ULX states and the Galactic BHB states is still unclear primarily due to less well-understood behaviors of ULXs in contrast to the Galactic BHBs. Here, we report a statistical X-ray spectral study of 34 energy spectra from seven bright ULXs in the interacting galaxy systems M 51 and NGC 4490/85, using archive data from multiple Chandra and XMM-Newton observations spanning for a few years. In order to compare with Galactic BHB states, we applied representative spectral models of BHBs; a power-law (PL), a multi-color disk black body (MCD), and a slim disk model to all the ULX spectra. We found a hint of a bimodal structure in the luminosity distribution of the samples, suggesting that ULXs have two states with typical luminosities of 3-6×10 39 and 1.5-3×10 39 ergs s −1 . Most spectra in the brighter state are explained by the MCD or the slim disk model, whereas those in the fainter state are explained by the PL model. In particular, the slim disk model successfully explains the observed spectral variations of NGC 4490/85 ULX-6 and ULX-8 by changes of the mass accretion rate to a black hole of an estimated mass of <40 M ⊙ . From the best-fit model parameters of each state, we speculate that the brighter state in these two ULXs corresponds to the brightest state of Galactic BHBs, which is often called the "apparently standard state". The fainter state of the ULXs has a PL shaped spectrum, but the photon index range is much wider than that seen in any single state of Galactic BHBs. We thus speculate that it is a state unique to ULXs. Some sources show much fainter and steeper spectra than the faint state, which we identified as another state.

The most extreme ultraluminous X-ray sources: evidence for intermediate-mass black holes?

“…4.6 of the Chandra Proposer’s Observatory Guide; also the Chandra High‐Resolution Mirror Array calibration documentation 11 ). We have verified that the off‐axis extents are consistent with point‐like objects by comparison to the population of point‐like ULXs in the galaxy NGC 4490 (see Roberts et al 2002; Fridriksson et al 2008; Gladstone & Roberts 2009; Yoshida et al 2010), eight objects that lie up to 3 arcmin off‐axis in three separate Chandra ACIS‐S observations. Our extents are consistent with the srcextent ‐measured extents for these ULXs over the full range of off‐axis angles seen.…”

“…One further mass limit could be placed assuming that the sample contains high accretion rate objects. Both Gladstone & Roberts (2009) and Yoshida et al (2010) note that the low‐luminosity ULXs in NGC 4490 and (in the latter paper) M51 appear to transit between a power‐law spectral state at ∼10 39 erg s −1 to an MCD‐like model at slightly higher luminosities. Gladstone & Roberts suggest that this could be a transition from a sub‐Eddington steep power‐law state to a ∼Eddington rate state; if so, the highest luminosity in which a power‐law spectrum is observed provides an estimate of the black hole mass from the requirement L X / L Edd < 1.…”

A deficit of ultraluminous X-ray sources in luminous infrared galaxies

“…We do note, however, that previous analyses have suggested transitions in ULX spectra at similar luminosities. Both and Yoshida et al (2010) examine ULXs in the interacting galaxies NGC 4485/90 (and also M51 in the latter paper), and note a change from power-law-like spectra dominating, to more disc-like spectra at ∼2-3 × 10 39 erg s −1 , that they also reconcile with the objects becoming substantially super-Eddington beyond that luminosity. Furthermore, the detailed ULX spectroscopy of and follow-up work by Sutton et al (2013b) show that below ∼3 × 10 39 erg s −1 most ULXs have disc-like spectra in the wider XMM-Newton bandpass; above this luminosity their spectra resolve into two components, with roughly equal numbers dominated by either the hard or soft component.…”

XMM-Newtonobservations of the hot spot galaxy NGC 2903

“…These models have been proven to successfully provide a phenomenological description of the XMM-Newton and CHANDRA spectra of a sample of ULXs (e.g. in the galaxies M 51 and NGC 4490/85; Yoshida et al 2010;Makishima et al 2000) . Furthermore, we applied the thermal bremsstrahlung bremss model.…”