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Register a new userThe rare X-ray flaring activity of the Ultraluminous X-ray source NGC 4559 X7
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Ultraluminous X-ray sources are considered amongst the most extremely accreting objects in the local Universe. The recent discoveries of pulsating neutron stars in ULXs strengthened the scenario of highly super-Eddington accretion mechanisms on stellar mass compact objects. In this work, we present the first long-term light curve of the source NGC 4559 X7 using all the available Swift, XMM-Newton, Chandra and NuSTAR data. Thanks to the high quality 2019 XMM-Newton and NuSTAR observations, we investigated in an unprecedented way the spectral and temporal properties of NGC 4559 X7. The source displayed flux variations of up to an order of magnitude and an unusual flaring activity. We modelled the spectra from NGC 4559 X7 with a combination of two thermal components, testing also the addition of a further high energy cut-off powerlaw. We observed a spectral hardening associated with a luminosity increase during the flares, and a spectral softening in the epochs far from the flares. Narrow absorption and emission lines were also found in the RGS spectra, suggesting the presence of an outflow. Furthermore, we measured hard and (weak) soft lags with magnitudes of a few hundreds of seconds whose origin is possibly be due to the accretion flow. We interpret the source properties in terms of a super-Eddington accretion scenario assuming the compact object is either a light stellar mass black hole or a neutron star.
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Some ultraluminous X-ray sources (ULXs) are surrounded by collisionally ionized bubbles, larger and more energetic than supernova remnants: they are evidence of the powerful outflows associated with super-Eddington X-ray sources. We illustrate the most recent addition to this class: a huge (350 pc x 220 pc in diameter) bubble around a ULX in NGC 5585. We modelled the X-ray properties of the ULX (a broadened-disc source with L_X ~ 2-4 x 10^{39} erg/s) from Chandra and XMM-Newton, and identified its likely optical counterpart in Hubble Space Telescope images. We used the Large Binocular Telescope to study the optical emission from the ionized bubble. We show that the line emission spectrum is indicative of collisional ionization. We refine the method for inferring the shock velocity from the width of the optical lines. We derive an average shock velocity ~125 km/s, which corresponds to a dynamical age of ~600,000 years for the bubble, and an average mechanical power P_w ~ 10^{40} erg/s; thus, the mechanical power is a few times higher than the current photon luminosity. With Very Large Array observations, we discovered and resolved a powerful radio bubble with the same size as the optical bubble, and a 1.4-GHz luminosity ~10^{35} erg/s, at the upper end of the luminosity range for this type of source. We explain why ULX bubbles tend to become more radio luminous as they expand while radio supernova remnants tend to fade.
Ultra-Luminous X-ray sources are accreting black holes that might represent strong evidence of the Intermediate Mass Black Holes (IMBH), proposed to exist by theoretical studies but with no firm detection (as a class) so far. We analyze the best X-ray timing and spectral data from the ULX in NGC 5408 provided by XMM-Newton. The main goal is to study the broad-band noise variability of the source. We found an anti-correlation of the fractional root-mean square variability versus the intensity of the source, similar to black-hole binaries during hard states.
We report on the serendipitous discovery of a new transient in NGC 5907, at a peak luminosity of 6.4x10^{39} erg/s. The source was undetected in previous 2012 Chandra observations with a 3 sigma upper limit on the luminosity of 1.5x10^{38} erg/s, implying a flux increase of a factor of >35. We analyzed three recent 60ks/50ks Chandra and 50ks XMM-Newton observations, as well as all the available Swift observations performed between August 2017/March 2018. Until the first half of October 2017, Swift observations do not show any emission from the source. The transient entered the ULX regime in less than two weeks and its outburst was still on-going at the end of February 2018. The 0.3-10 keV spectrum is consistent with a single multicolour blackbody disc (kT~1.5 keV). The source might be a ~30 solar mass black hole accreting at the Eddington limit. However, although we did not find evidence of pulsations, we cannot rule-out the possibility that this ULX hosts an accreting neutron star.
We have studied a highly variable ultraluminous X-ray source (ULX) in the Fornax galaxy NGC 1365, with a series of 12 Chandra and XMM-Newton observations between 2002 and 2006. In 2006 April, the source peaked at a luminosity ~ 3 x 10^{40} erg/s in the 0.3-10 keV band (similar to the maximum luminosity found by ASCA in 1995), and declined on an e-folding timescale ~ 3 days. The X-ray spectrum is always dominated by a broad power-law-like component. When the source is seen at X-ray luminosities ~ 10^{40} erg/s, an additional soft thermal component (which we interpret as emission from the accretion disk) contributes ~ 1/4 of the X-ray flux; when the luminosity is higher, ~ 3 x 10^{40} erg/s, the thermal component is not detected and must contribute < 10% of the flux. At the beginning of the decline, ionized absorption is detected around 0.5-2 keV; it is a possible signature of a massive outflow. The power-law is always hard, with a photon index Gamma ~ 1.7 (and even flatter at times), as is generally the case with bright ULXs. We speculate that this source and perhaps most other bright ULXs are in a high/hard state: as the accretion rate increases well above the Eddington limit, more and more power is extracted from the inner region of the inflow through non-radiative channels, and is used to power a Comptonizing corona, jet or wind. The observed thermal component comes from the standard outer disk; the transition radius between outer standard disk and Comptonizing inner region moves further out and to lower disk temperatures as the accretion rate increases. This produces the observed appearance of a large, cool disk. Based on X-ray luminosity and spectral arguments, we suggest that this accreting black hole has a likely mass ~ 50-150 Msun (even without accounting for possible beaming).
Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 1e39 erg/s. The ULX population counts several hundreds objects but only a minor fraction is well studied. Here we present a detailed analysis of all ULXs hosted in the galaxy NGC 7456. It was observed in X-rays only once in the past (in 2005) by XMM-Newton, but the observation was short and strongly affected by high background. In 2018, we obtained a new, deeper (~90 ks) XMM-Newton observation that allowed us to perform a detailed characterization of the ULXs hosted in the galaxy. ULX-1 and ULX-2, the two brightest objects (Lx~(6-10)e39 erg/s), have spectra that can be described by a two-thermal component model as often found in ULXs. ULX-1 shows also one order of magnitude in flux variability on short-term timescales (hundreds to thousand ks). The other sources (ULX-3 and ULX-4) show flux changes of at least an order of magnitude, and these objects may be candidate transient ULXs although longer X-ray monitoring or further studies are required to ascribe them to the ULX population. In addition, we found a previously undetected source that might be a new candidate ULX (labelled as ULX-5) with a luminosity of ~1e39 erg/s and hard power-law spectral shape, whose nature is still unclear and for which a background Active Galactic Nucleus cannot be excluded. We discuss the properties of all the ULXs in NGC 7456 within the framework of super-Eddington accretion onto stellar mass compact objects. Although no pulsations were detected, we cannot exclude that the sources host neutron stars.
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