The Moving Hotspots model for kHz QPOs in accreting neutron stars

3D MHD simulation of accretion onto neutron stars have shown in the last few years that the footprint (hotspot) of the accretion flow changes with time. Two different kinds of accretion, namely the funnel flow and the equatorial accretion produced by instabilities at the inner disk, produce different kinds of motion of the hotspot. The funnel flow produces hotspots that move around the magnetic pole, while instabilities produce other hotspots that appear randomly and move along the equator or slightly above. The angular velocities of the two hotspots are different, the equatorial one being higher and both close to the Keplerian velocity in the inner region. Modeling of the lightcurves of these hotspots with Monte Carlo simulations show that the signatures produced in power specra by them, if observed, are QPOs plus low frequency components. Their frequencies, general behavior and features describe correctly most of the properties of kHz QPOs, if we assume the funnel flow hotspots as the origin of the lower kHz QPO and instabilities as the origin of the upper kHz QPO.Comment: 4 pages, 2 figures, to appear in the Proceedings of the ASTRONS 2010 conferenc

No Time for Dead Time: Use the Fourier Amplitude Differences to Normalize Dead-time-affected Periodograms

Dead time affects many of the instruments used in X-ray astronomy, by producing a strong distortion in power density spectra. This can make it difficult to model the aperiodic variability of the source or look for quasi-periodic oscillations. Whereas in some instruments a simple a priori correction for dead-time-affected power spectra is possible, this is not the case for others such as NuSTAR, where the dead time is non-constant and long (~2.5 ms). Bachetti et al. 2015 suggested the cospectrum obtained from light curves of independent detectors within the same instrument as a possible way out, but this solution has always only been a partial one: the measured rms was still affected by dead time, because the width of the power distribution of the cospectrum was modulated by dead time in a frequency-dependent way. In this Letter we suggest a new, powerful method to normalize cospectra and, with some caveats, even power density spectra. Our approach uses the difference of the Fourier amplitudes from two independent detectors to characterize and filter out the effect of dead time. This method is crucially important for the accurate modelling of periodograms derived from instruments affected by dead time on board current missions like NuSTAR and ASTROSAT, but also future missions such as IXPEComment: 8 pages, 5 figures, Published on ApJL on 2018 January 3

Fourier Domain

The changes in brightness of an astronomical source as a function of time are key probes into that source's physics. Periodic and quasi-periodic signals are indicators of fundamental time (and length) scales in the system, while stochastic processes help uncover the nature of turbulent accretion processes. A key method of studying time variability is through Fourier methods, the decomposition of the signal into sine waves, which yields a representation of the data in frequency space. With the extension into \textit{spectral timing} the methods built on the Fourier transform can not only help us characterize (quasi-)periodicities and stochastic processes, but also uncover the complex relationships between time, photon energy and flux in order to help build better models of accretion processes and other high-energy dynamical physics. In this Chapter, we provide a broad, but practical overview of the most important relevant methods.Comment: 50 pages, 13 figures. This Chapter will appear in the Section "Timing Analysis" of the "Handbook of X-ray and Gamma-ray Astrophysics" (Editors in chief: C. Bambi and A. Santangelo

3D MHD Simulations of accreting neutron stars: evidence of QPO emission from the surface

3D Magnetohydrodynamic simulations show that when matter accretes onto neutron stars, in particular if the misalignment angle is small, it does not constantly fall at a fixed spot. Instead, the location at which matter reaches the star moves. These moving hot spots can be produced both during stable accretion, where matter falls near the magnetic poles of the star, and unstable accretion, characterized by the presence of several tongues of matter which fall on the star near the equator, due to Rayleigh-Taylor instabilities. Precise modeling with Monte Carlo simulations shows that those movements could be observed as high frequency Quasi Periodic Oscillations. We performed a number of new simulation runs with a much wider set of parameters, focusing on neutron stars with a small misalignment angle. In most cases we observe oscillations whose frequency is correlated with the mass accretion rate $\dot{M}$. Moreover, in some cases double QPOs appear, each of them showing the same correlation with $\dot{M}$.Comment: 2 pages, 1 figure, to appear in the Proceedings of the Bologna x-ray conference 2009, uses aipproc.cls, aip-6s.clo,

A search for hyperluminous X-ray sources in the XMM-Newton source catalog

We present a new method to identify luminous off-nuclear X-ray sources in the outskirts of galaxies from large public redshift surveys, distinguishing them from foreground and background interlopers. Using the 3XMM-DR5 catalog of X-ray sources and the SDSS DR12 spectroscopic sample of galaxies, with the help of this off-nuclear cross-matching technique, we selected 98 sources with inferred X-ray luminosities in the range $10^{41} < L_{\rm X} < 10^{44}\,{\rm erg\,s}^{-1}$, compatible with hyperluminous X-ray objects (HLX). To validate the method, we verify that it allowed us to recover known HLX candidates such as ESO 243$-$49 HLX$-$1 and M82 X$-$1. From a statistical study, we conservatively estimate that up to $71 \pm 11$ of these sources may be fore- or background sources, statistically leaving at least 16 that are likely to be HLXs, thus providing support for the existence of the HLX population. We identify two good HLX candidates and using other publicly available datasets, in particular the VLA FIRST in radio, UKIDSS in the near-infrared, GALEX in the ultra-violet and CFHT Megacam archive in the optical, we present evidence that these objects are unlikely to be foreground or background X-ray objects of conventional types, e.g. active galactic nuclei, BL Lac objects, Galactic X-ray binaries or nearby stars. However, additional dedicated X-ray and optical observations are needed to confirm their association with the assumed host galaxies and thus secure their HLX classification.Comment: 20 pages, 6 figures, accepted to Ap

Accurate X-ray Timing in the Presence of Systematic Biases With Simulation-Based Inference

Because many of our X-ray telescopes are optimized towards observing faint sources, observations of bright sources like X-ray binaries in outburst are often affected by instrumental biases. These effects include dead time and photon pile-up, which can dramatically change the statistical inference of physical parameters from these observations. While dead time is difficult to take into account in a statistically consistent manner, simulating dead time-affected data is often straightforward. This structure makes the issue of inferring physical properties from dead time-affected observations fall into a class of problems common across many scientific disciplines. There is a growing number of methods to address them under the name of Simulation-Based Inference (SBI), aided by new developments in density estimation and statistical machine learning. In this paper, we introduce SBI as a principled way to infer variability properties from dead time-affected light curves. We use Sequential Neural Posterior Estimation to estimate the posterior probability for variability properties. We show that this method can recover variability parameters on simulated data even when dead time is variable, and present results of an application of this approach to NuSTAR observations of the galactic black hole X-ray binary GRS 1915+105

Magnetar-like activity from the central compact object in the SNR RCW103

The 6.67 hr periodicity and the variable X-ray flux of the central compact object (CCO) at the center of the SNR RCW 103, named 1E 161348-5055, have been always difficult to interpret within the standard scenarios of an isolated neutron star or a binary system. On 2016 June 22, the Burst Alert Telescope (BAT) onboard Swift detected a magnetar-like short X-ray burst from the direction of 1E 161348-5055, also coincident with a large long-term X-ray outburst. Here we report on Chandra, NuSTAR, and Swift (BAT and XRT) observations of this peculiar source during its 2016 outburst peak. In particular, we study the properties of this magnetar-like burst, we discover a hard X-ray tail in the CCO spectrum during outburst, and we study its long-term outburst history (from 1999 to July 2016). We find the emission properties of 1E 161348-5055 consistent with it being a magnetar. However in this scenario, the 6.67 hr periodicity can only be interpreted as the rotation period of this strongly magnetized neutron star, which therefore represents the slowest pulsar ever detected, by orders of magnitude. We briefly discuss the viable slow-down scenarios, favoring a picture involving a period of fall-back accretion after the supernova explosion, similarly to what is invoked (although in a different regime) to explain the "anti-magnetar" scenario for other CCOs.Comment: 6 pages, 3 figures. To be published in the Astrophysical Journal Letters; replaced to match the version accepted for publication on 2016 August 1

An XMM-Newton and NuSTAR study of IGR J18214-1318: a non-pulsating high-mass X-ray binary with a neutron star

IGR J18214-1318, a Galactic source discovered by the International Gamma-Ray Astrophysics Laboratory, is a high-mass X-ray binary (HMXB) with a supergiant O-type stellar donor. We report on the XMM-Newton and NuSTAR observations that were undertaken to determine the nature of the compact object in this system. This source exhibits high levels of aperiodic variability, but no periodic pulsations are detected with a 90% confidence upper limit of 2% fractional rms between 0.00003-88 Hz, a frequency range that includes the typical pulse periods of neutron stars (NSs) in HMXBs (0.1-10$^3$ s). Although the lack of pulsations prevents us from definitively identifying the compact object in IGR J18214-1318, the presence of an exponential cutoff with e-folding energy $\lesssim30$ keV in its 0.3-79 keV spectrum strongly suggests that the compact object is an NS. The X-ray spectrum also shows a Fe K$\alpha$ emission line and a soft excess, which can be accounted for by either a partial-covering absorber with $N_{\mathrm{H}}\approx10^{23}$ cm$^{-2}$ which could be due to the inhomogeneous supergiant wind, or a blackbody component with $kT=1.74^{+0.04}_{-0.05}$ keV and $R_{BB}\approx0.3$ km, which may originate from NS hot spots. Although neither explanation for the soft excess can be excluded, the former is more consistent with the properties observed in other supergiant HMXBs. We compare IGR J18214-1318 to other HMXBs that lack pulsations or have long pulsation periods beyond the range covered by our observations.Comment: 15 pages, 12 figures, 4 table

A multi-wavelength pipeline for pulsar searches

Pulsar studies in the recent years have shown, more than others, to have benefited from a multi-wavelength approach. The INAF - Astronomical Observatory in Cagliari (INAF-OAC) is a growing facility with a young group devoted to pulsar and fast transients studies across the electromagnetic spectrum. Taking advantage of this expertise we have worked to provide a suite of multi-wavelength software and databases for the observations of pulsars and compact Galactic objects at the Sardinia Radio Telescope (SRT). In turn, radio pulsar observations at SRT will be made available, in a processed format, to gamma-ray searches using AGILE and Fermi gamma-ray satellite and, in a near future, they will be complementary to polarimetric X-ray observations with IXPE.Comment: Accepted for publications in Rendiconti Lincei as Proceedings of "A Decade of AGILE: Results, Challenges and Prospects of Gamma-Ray Astrophysics

Spectral and temporal properties of the ultra-luminous X-ray pulsar in M82 from 15 years of Chandra observations and analysis of the pulsed emission using NuSTAR

The recent discovery by Bachetti et al. (2014) of a pulsar in M82 that can reach luminosities of up to 10^40 ergs s^-1, a factor of ~100 the Eddington luminosity for a 1.4 Msol compact object, poses a challenge for accretion physics. In order to better understand the nature of this source and its duty cycle, and in the light of several physical models that have been subsequently published, we conduct a spectral and temporal analysis of the 0.5-8 keV X-ray emission from this source from 15 years of Chandra observations. We fit the Chandra spectra of the pulsar with a power-law model and a disk black body model, subjected to interstellar absorption in M82. We carefully assess for the effect of pile-up in our observations, where 4/19 observations have a pile-up fraction >10%, which we account for during spectral modeling with a convolution model. When fitted with a power-law model, the average photon index when the source is at high luminosity (L_X>10^39 ergs s^-1) is Gamma=1.33+/-0.15. For the disk black body model, the average temperature is T=3.24+/-0.65 keV, consistent with other luminous X-ray pulsars. We also investigated the inclusion of a soft excess component and spectral break, finding that the spectra are also consistent with these features common to luminous X-ray pulsars. In addition, we present spectral analysis from NuSTAR over the 3-50 keV range where we have isolated the pulsed component. We find that the pulsed emission in this band is best fit by a power-law with a high-energy cut-off, where Gamma=0.6+/-0.3 and E_C=14^{+5}_{-3} keV. While the pulsar has previously been identified as a transient, we find from our longer-baseline study that it has been remarkably active over the 15-year period, where for 9/19 (47%) observations that we analyzed, the pulsar appears to be emitting at a luminosity in excess of 10^39 ergs s^-1, greater than 10 times its Eddington limit.Comment: Accepted for publication by Ap