A new candidate pulsating ULX in NGC 7793 Quintin, E; Webb, N A; Gúrpide, A ...
Monthly notices of the Royal Astronomical Society,
06/2021, Volume:
503, Issue:
4
Journal Article
Peer reviewed
Open access
ABSTRACT
We report here the discovery of NGC 7793 ULX-4, a new transient ultraluminous X-ray source (ULX) in NGC 7793, a spiral galaxy already well known for harbouring several ULXs. This new source ...underwent an outburst in 2012, when it was detected by XMM–Newton and the Swift X-ray telescope. The outburst reached a peak luminosity of 3.4 × 1039 erg s−1 and lasted for about eight months, after which the source went below a luminosity of 1037 erg s−1; previous Chandra observations constrain the low-state luminosity below ∼2 × 1036 erg s−1, implying a variability of at least a factor 1000. We propose four possible optical counterparts, found in archival HST observations of the galaxy. A pulsation in the XMM–Newton signal was found at 2.52 Hz, with a significance of $\sim 3.4\, \sigma$, and an associated spin-up of $\dot{f} = 3.5\times 10^{-8}$ Hz s−1. NGC 7793 is therefore the first galaxy to host more than one pulsating ULX.
We discovered 2.8 s pulsations in the X-ray emission of the ultraluminous X-ray source (ULX) M51 ULX-7 within the UNSEeN project, which was designed to hunt for new pulsating ULXs (PULXs) with ...XMM-Newton. The pulse shape is sinusoidal, and large variations of its amplitude were observed even within single exposures (pulsed fraction from less than 5% to 20%). Source M51 ULX-7 is variable, generally observed at an X-ray luminosity between 1039 and 1040 erg s−1, located in the outskirts of the spiral galaxy M51a at a distance of 8.6 Mpc. According to our analysis, the X-ray pulsar orbits in a 2 day binary with a projected semimajor axis 28 lt-s. For a neutron star (NS) of 1.4 M , this implies a lower limit on the companion mass of 8 M , placing the system hosting M51 ULX-7 in the high-mass X-ray binary class. The barycentric pulse period decreased by 0.4 ms in the 31 days spanned by our 2018 May-June observations, corresponding to a spin-up rate . In an archival 2005 XMM-Newton exposure, we measured a spin period of ∼3.3 s, indicating a secular spin-up of , a value in the range of other known PULXs. Our findings suggest that the system consists of a massive donor, possibly an OB giant or supergiant, and a moderately magnetic (dipole field component in the range 1012 G G) accreting NS with weakly beamed emission ( ).
ABSTRACT We report the detection of a 78.1 0.5 day period in the X-ray light curve of the extreme ultraluminous X-ray source NGC 5907 ULX1 ( L X,peak ∼ 5 × 10 40 erg s−1), discovered during an ...extensive monitoring program with Swift. These periodic variations are strong, with the observed flux changing by a factor of ∼3-4 between the peaks and the troughs of the cycle; our simulations suggest that the observed periodicity is detected comfortably in excess of 3 significance. We discuss possible origins for this X-ray period, but conclude that at the current time we cannot robustly distinguish between orbital and super-orbital variations.
On the magnetic field in M51 ULX-8 Middleton, M J; Brightman, M; Pintore, F ...
Monthly notices of the Royal Astronomical Society,
06/2019, Volume:
486, Issue:
1
Journal Article
Peer reviewed
Abstract
The reported discovery of a cyclotron resonance scattering feature (CRSF) in the spectrum of M51 ULX-8 may provide an important clue as to the nature of the magnetic field in those ...ultraluminous X-ray sources hosting neutron stars. In this paper, we present the covariance (linearly correlated variability) spectrum of M51 ULX-8 on long (>2000 s) time-scales. This allows us to unambiguously decompose the spectrum which requires multiple components in order to explain the broadband emission. Having a clearer picture of the spectral decomposition leads to various tests for the dipole field strength of the neutron star which can be extended to other ULXs when certain criteria are met. In the case of M51 ULX-8, we rule out a very strong (1015 G) dipole solution with either a sub- or supercritical disc. Instead, our tests indicate an upper limit on the dipole field of 1012 G and a classical supercritical inflow, similar to that inferred in other ULXs found to harbour neutron stars, although we do not rule out the presence of an additional, strong (1015 G) multipole field falling off steeply with distance from the neutron star.
The majority of ultraluminous X-ray sources are point sources that are spatially offset from the nuclei of nearby galaxies and whose X-ray luminosities exceed the theoretical maximum for spherical ...infall (the Eddington limit) onto stellar-mass black holes. Their X-ray luminosities in the 0.5-10 kiloelectronvolt energy band range from 10(39) to 10(41) ergs per second. Because 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 of the range (more than 10(40) ergs per second), which require black hole masses of 50-100 times the solar value or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries, or both. Here we report broadband X-ray observations of the nuclear region of the galaxy M82 that reveal pulsations with an average period of 1.37 seconds and 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 an X-ray luminosity in the 3-30 kiloelectronvolt range of 4.9 × 10(39) ergs per second. The pulsating source is spatially coincident with a variable source that can reach an X-ray luminosity in the 0.3-10 kiloelectronvolt range of 1.8 × 10(40) ergs per second. This association implies a luminosity of about 100 times the Eddington limit for a 1.4-solar-mass object, or more than ten times brighter than any known accreting pulsar. This implies that neutron stars may not be rare in the ultraluminous X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects.
We present NuSTAR observations of neutron star (NS) low-mass X-ray binaries: 4U 1636-53, GX 17+2, and 4U 1705-44. We observed 4U 1636-53 in the hard state, with an Eddington fraction, , of 0.01; GX ...17+2 and 4U 1705-44 were in the soft state with fractions of 0.57 and 0.10, respectively. Each spectrum shows evidence for a relativistically broadened Fe K line. Through accretion disk reflection modeling, we constrain the radius of the inner disk in 4U 1636-53 to be ISCO (innermost stable circular orbit), assuming a dimensionless spin parameter , and ISCO for (errors quoted at 1 ). This value proves to be model independent. For and , for example, 1.08 0.06 ISCO translates to a physical radius of km, and the NS would have to be smaller than this radius (other outcomes are possible for allowed spin parameters and masses). For GX 17+2, ISCO for and ISCO for . For and , ISCO translates to km. The inner accretion disk in 4U 1705-44 may be truncated just above the stellar surface, perhaps by a boundary layer or magnetosphere; reflection models give a radius of 1.46-1.64 ISCO for and 1.69-1.93 ISCO for . We discuss the implications our results may have on the equation of state of ultradense, cold matter and our understanding of the innermost accretion flow onto NSs with low surface magnetic fields, and systematic errors related to the reflection models and spacetime metric around less idealized NSs.
ABSTRACT
We present results from the major coordinated X-ray observing programme on the ULX NGC 1313 X-1 performed in 2017, combining XMM–Newton, Chandra, and NuSTAR, focusing on the evolution of the ...broad-band (∼0.3–30.0 keV) continuum emission. Clear and unusual spectral variability is observed, but this is markedly suppressed above ∼10–15 keV, qualitatively similar to the ULX Holmberg IX X-1. We model the multi-epoch data with two-component accretion disc models designed to approximate super-Eddington accretion, allowing for both a black hole and a neutron star accretor. With regards to the hotter disc component, the data trace out two distinct tracks in the luminosity–temperature plane, with larger emitting radii and lower temperatures seen at higher observed fluxes. Despite this apparent anticorrelation, each of these tracks individually shows a positive luminosity–temperature relation. Both are broadly consistent with L ∝ T4, as expected for blackbody emission with a constant area, and also with L ∝ T2, as may be expected for an advection-dominated disc around a black hole. We consider a variety of possibilities for this unusual behaviour. Scenarios in which the innermost flow is suddenly blocked from view by outer regions of the super-Eddington disc/wind can explain the luminosity–temperature behaviour, but are difficult to reconcile with the lack of strong variability at higher energies, assuming this emission arises from the most compact regions. Instead, we may be seeing evidence for further radial stratification of the accretion flow than is included in the simple models considered, with a combination of winds and advection resulting in the suppressed high-energy variability.
Abstract
The presence of neutron stars in at least three ultraluminous X-ray sources is now firmly established and offers an unambiguous view of super-critical accretion. All three systems show ...long-time-scale periods (60-80 d) in the X-rays and/or optical, two of which are known to be super-orbital in nature. Should the flow be classically super critical, i.e. the Eddington limit is reached locally in the disc (implying surface dipole fields that are sub-magnetar in strength), then the large scale-height flow can precess through the Lense-Thirring effect which could provide an explanation for the observed super-orbital periods. By connecting the details of the Lense-Thirring effect with the observed pulsar spin period, we are able to infer the moment of inertia and therefore equation of state of the neutron star without relying on the inclination of or distance to the system. We apply our technique to the case of NGC 7793 P13 and demonstrate that stronger magnetic fields imply stiffer equations of state. We discuss the caveats and uncertainties, many of which can be addressed through forthcoming radiative magnetohydrodynamic (RMHD) simulations and their connection to observation.
We present results from the coordinated broadband X-ray observations of the extreme ultraluminous X-ray source Holmberg IX X-1 performed by NuSTAR, XMM-Newton, and Suzaku in late 2012. These ...observations provide the first high-quality spectra of Holmberg IX X-1 above 10 keV to date, extending the X-ray coverage of this remarkable source up to 30 keV. Broadband observations were undertaken at two epochs, between which Holmberg IX X-1 exhibited both flux and strong spectral variability, increasing in luminosity from L(sub x) = (1.90 +/- 0.03) x 10(exp 40) erg s(exp -1) to L(sub x) = (3.35 +/- 0.03) x 10(exp 40) erg s(exp -1) . Neither epoch exhibits a spectrum consistent with emission from the standard lowhard accretion state seen in Galactic black hole binaries, which would have been expected if Holmberg IX X-1 harbors a truly massive black hole accreting at substantially sub-Eddington accretion rates. The NuSTAR data confirm that the curvature observed previously in the 3-10 keV bandpass does represent a true spectral cutoff. During each epoch, the spectrum appears to be dominated by two optically thick thermal components, likely associated with an accretion disk. The spectrum also shows some evidence for a nonthermal tail at the highest energies, which may further support this scenario. The available data allow for either of the two thermal components to dominate the spectral evolution, although both scenarios require highly nonstandard behavior for thermal accretion disk emission.
ABSTRACT In 2015 June, the black hole X-ray binary (BHXRB) V404 Cygni went into outburst for the first time since 1989. Here, we present a comprehensive search for quasi-periodic oscillations (QPOs) ...of V404 Cygni during its recent outburst, utilizing data from six instruments on board five different X-ray missions: Swift/XRT, Fermi/GBM, Chandra/ACIS, INTEGRAL's IBIS/ISGRI and JEM-X, and NuSTAR. We report the detection of a QPO at 18 mHz simultaneously with both Fermi/GBM and Swift/XRT, another example of a rare but slowly growing new class of mHz-QPOs in BHXRBs linked to sources with a high orbital inclination. Additionally, we find a duo of QPOs in a Chandra/ACIS observation at 73 mHz and 1.03 Hz, as well as a QPO at 136 mHz in a single Swift/XRT observation that can be interpreted as standard Type-C QPOs. Aside from the detected QPOs, there is significant structure in the broadband power, with a strong feature observable in the Chandra observations between 0.1 and 1 Hz. We discuss our results in the context of current models for QPO formation.
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