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Register a new userX-ray variability of NGC 2516 stars in the XMM-Newton observations
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We present the characteristics of the X-ray variability of stars in the cluster NGC2516 as derived from XMM-Newton/EPIC/pn data. The X-ray variations on short (hours), medium (months), and long (years) time scales have been explored. We detected 303 distinct X-ray sources by analysing six EPIC/pn observations; 194 of them are members of the cluster. Stars of all spectral types, from the early-types to the late-M dwarfs, were detected. The Kolmogorov-Smirnov test applied to the X-ray photon time series shows that, on short time scales, only a relatively small fraction (ranging from 6% to 31% for dG and dF, respectively) of the members of NGC2516 are variable with a confidence level $geq$99%; however, it is possible that the fraction is small only because of the poor statistics. The time X-ray amplitude distribution functions (XAD) of a set of dF7-dK2 stars, derived on short (hours) and medium (months) time scales, seem to suggest that medium-term variations, if present, have a much smaller amplitude than those on short time scales; a similar result is also obtained for dK3-dM stars. The amplitude variations of late-type stars in NGC2516 are consistent with those of the coeval Pleiades stars. Comparing these data with those of ROSAT/PSPC, collected 7-8 years earlier, and of ROSAT/HRI, just 4-5 years earlier, we find no evidence of significant variability on the related time scales, suggesting that long-term variations due to activity cycles similar to the solar cycle are not common among young stars. Indications of spectral variability was found in one star whose spectra at three epochs were available.
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We present a series of X-ray variability results from a long XMM-Newton + NuSTAR campaign on the bright, variable AGN NGC 3227. We present an analysis of the lightcurves, showing that the source displays typically softer-when-brighter behaviour, although also undergoes significant spectral hardening during one observation which we interpret as due to an occultation event by a cloud of absorbing gas. We spectrally decompose the data and show that the bulk of the variability is continuum-driven and, through rms variability analysis, strongly enhanced in the soft band. We show that the source largely conforms to linear rms-flux behaviour and we compute X-ray power spectra, detecting moderate evidence for a bend in the power spectrum, consistent with existing scaling relations. Additionally, we compute X-ray Fourier time lags using both the XMM-Newton and - through maximum-likelihood methods - NuSTAR data, revealing a strong low-frequency hard lag and evidence for a soft lag at higher frequencies, which we discuss in terms of reverberation models.
We present the results of two XMM-Newton observations of the ultraluminous X-ray source (ULX) NGC 5204 X-1. The EPIC spectra are well-fit by the standard spectral model of a black-hole X-ray binary, comprising a soft multi-colour disc blackbody component plus a harder power-law continuum. The cool (kT_in ~ 0.2 keV) inner-disc temperature required by this model favours the presence of an intermediate-mass black hole (IMBH) in this system, though we highlight a possible anomaly in the slope of the power-law continuum in such fits. We discuss the interpretation of this and other, non-standard spectral modelling of the data.
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We present a high-quality hard X-ray spectrum of the ultraluminous X-ray source (ULX) NGC 5643 X-1 measured with NuSTAR in May-June 2014. We have obtained this spectrum by carefully separating the signals from the ULX and from the active nucleus of its host galaxy NGC 5643 located 0.8 arcmin away. Together with long XMM-Newton observations performed in July 2009 and August 2014, the NuSTAR data confidently reveal a high-energy cutoff in the spectrum of NGC 5643 X-1 above ~10 keV, which is a characteristic signature of ULXs. The NuSTAR and XMM-Newton data are consistent with the source having a constant luminosity ~1.5E40 erg/s (0.2-12 keV) in all but the latest observation (August 2014) when it brightened to ~3E40 erg/s. This increase is associated with the dominant, hard spectral component (presumably collimated emission from the inner regions of a supercritical accretion disc), while an additional, soft component (with a temperature ~0.3 keV if described by multicolor disk emission), possibly associated with a massive wind outflowing from the disk, is also evident in the spectrum but does not exhibit significant variability.
We analyzed two XMM-Newton observations in the direction of the high density, high latitude, neutral hydrogen cloud MBM20 and of a nearby low density region that we called the Eridanus hole. The cloud MBM20 is at a distance evaluated between 100 and 200 pc from the Sun and its density is sufficiently high to shield about 75% of the foreground emission in the 3/4 keV energy band.The combination of the two observations makes possible an evaluation of the OVII and OVIII emission both for the foreground component due to the Local Bubble,and the background one, due primary to the galactic halo.The two observations are in good agreement with each other and with ROSAT observations of the same part of the sky and the OVII and OVIII fluxes are OVII=3.89+/-0.56 photons cm^-2 s^-1 sr^-1, OVIII=0.68+/-0.24 photons cm^-2 s^-1 sr^-1 for MBM20 and OVII=7.26+/-0.34 photons cm^-2 s^-1 sr^-1,OVIII=1.63+/-0.17 photons cm^-2 s^-1 sr^-1 for the Eridanus hole. The spectra are in agreement with a simple three component model, one unabsorbed and one absorbed plasma component, and a power law, without evidence for any strong contamination from ion exchange in the solar system. Assuming that the two plasma components are in thermal equilibrium we obtain a temperature of 0.096 keV for the foreground component and 0.197 keV for the background one. Assuming the foreground component is due solely to Local Bubble emission we obtain a lower and upper limit for the plasma density of 0.0079 cm^-3 and 0.0095 cm^-3 and limits of 16,200 cm^-3 K and 19,500 cm^-3 K for the plasma pressure, in good agreement with theoretical predictions. Similarly, assuming that the absorbed plasma component is due to Galactic halo emission, we obtain a plasma density ranging from 0.0009 cm^-3 to 0.0016 cm^-3, and a pressure ranging from 3.0*10^3 to 6.7*10^3 cm^-3 K.
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