arXiv subjects

J. Poutanen

Publications and source records attributed to J. Poutanen.

At least 55 records · Page 3Linked to original sources

arXiv2017

The 2015 outburst of the accretion-powered pulsar IGR J00291+5934: INTEGRAL and Swift observations

IGR J00291+5934 is the fastest-known accretion-powered X-ray pulsar, discovered during a transient outburst in 2004. In this paper, we report on Integral and Swift observations during the 2015 outburst, which lasts for $\sim25$ d. The source has not been observed in outburst since 2008, suggesting that the long-term accretion rate has decreased by a factor of two since discovery. The averaged broad-band (0.1 - 250 keV) persistent spectrum in 2015 is well described by a thermal Comptonization model with a column density of $N_{\rm H} \approx4\times10^{21}$ cm$^{-2}$, a plasma temperature of $kT_{\rm e} \approx50$ keV, and a Thomson optical depth of $τ_{\rm T}\approx1$. Pulsations at the known spin period of the source are detected in the Integral data up to the $\sim150$ keV energy band. We also report on the discovery of the first thermonuclear burst observed from IGR J00291+5934, which lasts around 7 min and occurs at a persistent emission level corresponding to roughly $1.6\%$ of the Eddington accretion rate. The properties of the burst suggest it is powered primarily by helium ignited at a depth of $y_{\rm ign}\approx1.5\times10^9$ g cm$^{-2}$ following the exhaustion by steady burning of the accreted hydrogen. The Swift/BAT data from the first $\sim20$ s of the burst provide indications of a photospheric radius expansion phase. Assuming this is the case, we infer a source distance of $d = 4.2 \pm 0.3$ kpc.

astro-ph.HE↗

arXiv2016

Propeller effect in the transient X-ray pulsar SMC X-2

We report the results of the monitoring campaign of the transient X-ray pulsar SMC X-2 performed with the Swift/XRT telescope over the period of 2015 September - 2016 January during the Type II outburst. During this event, the bolometric luminosity of the source ranged from $\simeq10^{39}$ down to several$\times10^{34}$ erg/s. Moreover, we discovered its dramatic drop by a factor of more than 100 below the limiting value of $L_{\rm lim}\simeq4\times10^{36}$ erg/s, which can be interpreted as a transition to the propeller regime. These measurements make SMC X-2 the sixth pulsating X-ray source where such a transition is observed and allow us to estimate the magnetic field of the neutron star in the system $B\simeq3\times10^{12}$ G, which is in agreement with independent results of the spectral analysis.

astro-ph.HE↗

arXiv2016

Expanding hot flow in the black hole binary SWIFT J1753.5-0127: evidence from optical timing

We describe the evolution of optical and X-ray temporal characteristics during the outburst decline of the black hole X-ray binary SWIFT J1753.5-0127. The optical/X-ray cross-correlation function demonstrates a single positive correlation at the outburst peak, then it has multiple dips and peaks during the decline stage, which are then replaced by the precognition dip plus peak structure in the outburst tail. Power spectral densities and phase lags show a complex evolution, revealing the presence of intrinsically connected optical and X-ray quasi-periodic oscillations. For the first time, we quantitatively explain the evolution of these timing properties during the entire outburst within one model, the essence of which is the expansion of the hot accretion flow towards the tail of the outburst. The pivoting of the spectrum produced by synchrotron Comptonization in the hot flow is responsible for the appearance of the anti-correlation with the X-rays and for the optical quasi-periodic oscillations. Our model reproduces well the cross-correlation and phase lag spectrum during the decline stage, which could not be understood with any model proposed before.

astro-ph.HE↗

arXiv2016

X-ray burst induced spectral variability in 4U 1728-34

Aims. INTEGRAL has been monitoring the Galactic center region for more than a decade. Over this time INTEGRAL has detected hundreds of type-I X-ray bursts from the neutron star low-mass X-ray binary 4U 1728-34, a.k.a. "the slow burster". Our aim is to study the connection between the persistent X-ray spectra and the X-ray burst spectra in a broad spectral range. Methods. We performed spectral modeling of the persistent emission and the X-ray burst emission of 4U 1728-34 using data from the INTEGRAL JEM-X and IBIS/ISGRI instruments. Results. We constructed a hardness intensity diagram to track spectral state variations. In the soft state the energy spectra are characterized by two thermal components - likely from the accretion disc and the boundary/spreading layer - together with a weak hard X-ray tail that we detect in 4U 1728-34 for the first time in the 40 to 80 keV range. In the hard state the source is detected up to 200 keV and the spectrum can be described by a thermal Comptonization model plus an additional component: either a powerlaw tail or reflection. By stacking 123 X-ray bursts in the hard state, we detect emission up to 80 keV during the X-ray bursts. We find that during the bursts the emission above 40 keV decreases by a factor of about three with respect to the persistent emission level. Conclusions. Our results suggest that the enhanced X-ray burst emission changes the spectral properties of the accretion disc in the hard state. The likely cause is an X-ray burst induced cooling of the electrons in the inner hot flow near the neutron star.

astro-ph.HE↗

arXiv2016

Detection of burning ashes from thermonuclear X-ray bursts

When neutron stars (NS) accrete gas from low-mass binary companions, explosive nuclear burning reactions in the NS envelope fuse hydrogen and helium into heavier elements. The resulting thermonuclear (type-I) X-ray bursts produce energy spectra that are fit well with black bodies, but a significant number of burst observations show deviations from Planck spectra. Here we present our analysis of RXTE/PCA observations of X-ray bursts from the NS low-mass X-ray binary HETE J1900.1-2455. We have discovered that the non-Planckian spectra are caused by photo-ionization edges. The anti-correlation between the strength of the edges and the colour temperature suggests that the edges are produced by the nuclear burning ashes that have been transported upwards by convection and become exposed at the photosphere. The atmosphere model fits show that occasionally the photosphere can consist entirely of metals, and that the peculiar changes in black body temperature and radius can be attributed to the emergence and disappearance of metals in the photosphere. As the metals are detected already in the Eddington-limited phase, it is possible that a radiatively driven wind ejects some of the burning ashes into the interstellar space.

astro-ph.HE↗

arXiv2016

eXTP -- enhanced X-ray Timing and Polarimetry Mission

eXTP is a science mission designed to study the state of matter under extreme conditions of density, gravity and magnetism. Primary targets include isolated and binary neutron stars, strong magnetic field systems like magnetars, and stellar-mass and supermassive black holes. The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV (and beyond). Key elements of the payload are: the Spectroscopic Focusing Array (SFA) - a set of 11 X-ray optics for a total effective area of about 0.9 m^2 and 0.6 m^2 at 2 keV and 6 keV respectively, equipped with Silicon Drift Detectors offering <180 eV spectral resolution; the Large Area Detector (LAD) - a deployable set of 640 Silicon Drift Detectors, for a total effective area of about 3.4 m^2, between 6 and 10 keV, and spectral resolution <250 eV; the Polarimetry Focusing Array (PFA) - a set of 2 X-ray telescope, for a total effective area of 250 cm^2 at 2 keV, equipped with imaging gas pixel photoelectric polarimeters; the Wide Field Monitor (WFM) - a set of 3 coded mask wide field units, equipped with position-sensitive Silicon Drift Detectors, each covering a 90 degrees x 90 degrees FoV. The eXTP international consortium includes mostly major institutions of the Chinese Academy of Sciences and Universities in China, as well as major institutions in several European countries and the United States. The predecessor of eXTP, the XTP mission concept, has been selected and funded as one of the so-called background missions in the Strategic Priority Space Science Program of the Chinese Academy of Sciences since 2011. The strong European participation has significantly enhanced the scientific capabilities of eXTP. The planned launch date of the mission is earlier than 2025.

astro-ph.IM↗

arXiv2016

Propeller effect in two brightest transient X-ray pulsars: 4U 0115+63 and V 0332+53

We present the results of the monitoring programmes performed with the Swift/XRT telescope and aimed specifically to detect an abrupt decrease of the observed flux associated with a transition to the propeller regime in two well known X-ray pulsars 4U 0115+63 and V 0332+53 during their giant outbursts in 2015. Such transitions were detected at the threshold luminosities of $(1.4\pm0.4)\times10^{36}$ erg s$^{-1}$ and $(2.0\pm0.4)\times10^{36}$ erg s$^{-1}$ for 4U 0115+63 and V 0332+53, respectively. Spectra of the sources are shown to be significantly softer during the low state. In both sources, the accretion at rates close to the aforementioned threshold values briefly resumes during the periastron passage following the transition into propeller regime. The strength of the dipole component of the magnetic field required to inhibit the accretion agrees well with estimates based on the position of the cyclotron lines in their spectra, thus excluding presence of a strong multipole component of the magnetic field in the vicinity of the neutron star.

astro-ph.HE↗

arXiv2016

High-precision broad-band linear polarimetry of early-type binaries I. Discovery of variable, phase-locked polarization in HD 48099

We investigate the structure of the O-type binary system HD 48099 by measuring linear polarization that arises due to light scattering process. Linear polarization measurements of HD 48099 in the B, V and R passbands with the high-precision Dipol-2 polarimeter have been carried out. The data have been obtained with the 60 cm KVA (Observatory Roque de los Muchachos, La Palma, Spain) and T60 (Haleakala, Hawaii, USA) remotely controlled telescopes during 31 observing nights. Polarimetry in the optical wavelengths has been complemented by observations in the X-rays with the SWIFT space observatory. Optical polarimetry revealed small intrinsic polarization in HD 48099 with 0.1% peak to peak variation over the orbital period of 3.08 days. The variability pattern is typical for binary systems, showing strong second harmonic of the orbital period. We apply our model code for the electron scattering in the circumstellar matter to put constraints on the system geometry. A good model fit is obtained for scattering of light on a cloud produced by the colliding stellar winds. The geometry of the cloud, with a broad distribution of scattering particles away from the orbital plane, helps in constraining the (low) orbital inclination. We derive from the polarization data the inclination i = 17 deg +/- 2deg and the longitude of the ascending node Omega = 82 deg +/- 1 deg of the binary orbit. The available X-ray data provide additional evidence for the existence of the colliding stellar winds in the system. Another possible source of the polarized light could be scattering from the stellar photospheres. The models with circumstellar envelopes, or matter confined to the orbital plane, do not provide good constraints on the low inclination, better than i < 27 deg, as is already suggested by the absence of eclipses.

astro-ph.SR↗

arXiv2016

Equation of state constraints for the cold dense matter inside neutron stars using the cooling tail method

The cooling phase of thermonuclear (type-I) X-ray bursts can be used to constrain the neutron star (NS) compactness by comparing the observed cooling tracks of bursts to accurate theoretical atmosphere model calculations. By applying the so-called cooling tail method, where the information from the whole cooling track is used, we constrain the mass, radius, and distance for three different NSs in low-mass X-ray binaries 4U 1702-429, 4U 1724-307, and SAX J1810.8-260. Care is taken to only use the hard state bursts where it is thought that only the NS surface alone is emitting. We then utilize a Markov chain Monte Carlo algorithm within a Bayesian framework to obtain a parameterized equation of state (EoS) of cold dense matter from our initial mass and radius constraints. This allows us to set limits on various nuclear parameters and to constrain an empirical pressure-density relation for the dense matter. Our predicted EoS results in NS radius between 10.5-12.8 km (95% confidence limits) for a mass of 1.4 $M_{\odot}$. Due to systematic errors and uncertainty in the composition these results should be interpreted as lower limits for the radius.

astro-ph.HE↗

arXiv2016

Progenitor constraints for core-collapse supernovae from Chandra X-ray observations

The progenitors of hydrogen-poor core-collapse supernovae (SNe) of types Ib, Ic and IIb are believed to have shed their outer hydrogen envelopes either by extremely strong stellar winds, characteristic of classical Wolf-Rayet stars, or by binary interaction with a close companion star. The exact nature of the progenitors and the relative importance of these processes are still open questions. One relatively unexplored method to constrain the progenitors is to search for high-mass X-ray binaries (HMXB) at SN locations in pre-explosion X-ray observations. In a HMXB, one star has already exploded as a core-collapse SN, producing a neutron star or a stellar-mass black hole. It is likely that the second star in the system will also explode as a supernova, which should cause a detectable long-term change in the system's X-ray luminosity. In particular, a pre-explosion detection of a HMXB coincident with a SN could be informative about the progenitor's nature. In this paper we analyze pre-explosion ACIS observations of 18 nearby type Ib, Ic and IIb supernovae from the Chandra X-ray observatory public archive. Two sources that could potentially be associated with the supernova are identified in the sample. Additionally we make similar post-explosion measurements for 46 SNe. Although our modelling indicates that progenitor systems with compact binary companions are probably quite rare, studies of this type can in the future provide more stringent constraints as the number of discovered nearby SNe and suitable pre-explosion X-ray data are both increasing.

astro-ph.HE↗

arXiv2015

Searching for X-ray sources in nearby late-type galaxies with low star formation rates

Late type non-starburst galaxies have been shown to contain X-ray emitting objects, some being ultraluminous X-ray sources. We report on XMM-Newton observations of 11 nearby, late-type galaxies previously observed with the Hubble Space Telescope (HST) in order to find such objects. We found 18 X-ray sources in or near the optical extent of the galaxies, most being point-like. If associated with the corresponding galaxies, the source luminosities range from $2 \times 10^{37}$ erg s$^{-1}$ to $6 \times 10^{39}$ erg s$^{-1}$. We found one ultraluminous X-ray source, which is in the galaxy IC 5052, and one source coincident with the galaxy IC 4662 with a blackbody temperature of $0.166 \pm 0.015$ keV that could be a quasi-soft source or a quiescent neutron star X-ray binary in the Milky Way. One X-ray source, XMMU J205206.0$-$691316, is extended and coincident with a galaxy cluster visible on an HST image. The X-ray spectrum of the cluster reveals a redshift of $z = 0.25 \pm 0.02$ and a temperature of 3.6$\pm$0.4 keV. The redshift was mainly determined by a cluster of Fe XXIV lines between the observed energy range $0.8-1.0$ keV.

astro-ph.HE↗

arXiv2015

Measuring the basic parameters of neutron stars using model atmospheres

Model spectra of neutron star atmospheres are nowadays widely used to fit the observed thermal X-ray spectra of neutron stars. This fitting is the key element in the method of the neutronstar radius determination. Here, we present the basic assumptions used for the neutron star atmosphere modeling as well as the main qualitative features of the stellar atmospheres leading to the deviations of the emergent model spectrum from blackbody. We describe the properties of two of our model atmosphere grids: (i) pure carbon atmospheres for relatively cool neutron stars (1--4 MK) and (ii) hot atmospheres with Compton scattering taken into account. The results obtained by applying these grids to model the X-ray spectra of the central compact object in supernova remnant HESS 1731-347, and two X-ray bursting neutron stars in low-mass X-ray binaries, 4U 1724-307 and 4U 1608-52, are presented. Possible systematic uncertainties associated with the obtained neutron star radii are discussed.

astro-ph.HE↗

arXiv2015

Transient X-ray pulsar V0332+53: pulse phase-resolved spectroscopy and the reflection model

We present the results of the pulse phase- and luminosity-resolved spectroscopy of the transient X-ray pulsar V0332+53, performed for the first time in a wide luminosity range (1-40)x10^{37} erg/s during a giant outburst observed by the RXTE observatory in Dec 2004 - Feb 2005. We characterize the spectra quantitatively and built the detailed "three-dimensional" picture of spectral variations with pulse phase and throughout the outburst. We show that all spectral parameters are strongly variable with the pulse phase, and the pattern of this variability significantly changes with luminosity directly reflecting the associated changes in the structure of emission regions and their beam patterns. Obtained results are qualitatively discussed in terms of the recently developed reflection model for the formation of cyclotron lines in the spectra of X-ray pulsars.

astro-ph.HE↗

arXiv2015

The physics of accretion-ejection with LOFT

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of the physics of accretion and ejection around compact objects. For a summary, we refer to the paper.

astro-ph.HE↗

arXiv2015

The LOFT perspective on neutron star thermonuclear bursts

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of thermonuclear X-ray bursts on accreting neutron stars. For a summary, we refer to the paper.

astro-ph.HE↗

arXiv2014

Simulations of gamma-ray burst afterglows with a relativistic kinetic code

This paper introduces a kinetic code that simulates gamma-ray burst (GRB) afterglow emission from the external forward shock and presents examples of some of its applications. One interesting research topic discussed in the paper is the high-energy radiation produced by Compton scattering of the prompt GRB photons against the shock-accelerated electrons. The difference between the forward shock emission in a wind-type and a constant-density medium is also studied, and the emission due to Maxwellian electron injection is compared to the case with pure power-law electrons. The code calculates the time-evolving photon and electron distributions in the emission region by solving the relativistic kinetic equations for each particle species. For the first time, the full relativistic equations for synchrotron emission/absorption, Compton scattering, and pair production/annihilation were applied to model the forward shock emission. The synchrotron self-absorption thermalization mechanism, which shapes the low-energy end of the electron distribution, was also included in the electron equation. The simulation results indicate that inverse Compton scattering of the prompt GRB photons can produce a luminous TeV emission component, even when pair production in the emission region is taken into account. This very high-energy radiation may be observable in low-redshift GRBs. The test simulations also show that the low-energy end of a pure power-law distribution of electrons can thermalize owing to synchrotron self-absorption in a wind-type environment, but without an observable impact on the radiation spectrum. Moreover, a flattening in the forward shock X-ray light curve may be expected when the electron injection function is assumed to be purely Maxwellian instead of a power law.

astro-ph.HE↗

arXiv2013

Active Galactic Nuclei under the scrutiny of CTA

Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer excellent conditions for efficient particle acceleration in internal and external shocks, turbulence, and magnetic reconnection events. The jets as well as particle accelerating regions close to the supermassive black holes (hereafter SMBH) at the intersection of plasma inflows and outflows, can produce readily detectable very high energy gamma-ray emission. As of now, more than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the present ground-based gamma-ray telescopes, which represents more than one third of the cosmic sources detected so far in the VHE gamma-ray regime. The future Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the VHE range by about one order of magnitude, shedding new light on AGN population studies, and AGN classification and unification schemes. CTA will be a unique tool to scrutinize the extreme high-energy tail of accelerated particles in SMBH environments, to revisit the central engines and their associated relativistic jets, and to study the particle acceleration and emission mechanisms, particularly exploring the missing link between accretion physics, SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an extremely rewarding observing program which will inform us about the inner workings and evolution of AGN. Furthermore these AGN are bright beacons of gamma-rays which will allow us to constrain the extragalactic infrared and optical backgrounds as well as the intergalactic magnetic field, and will enable tests of quantum gravity and other "exotic" phenomena.

astro-ph.HE↗

arXiv2013

Accreting millisecond X-ray pulsars: 10 years of INTEGRAL observations

During the last 10 years, INTEGRAL made a unique contribution to the study of accreting millisecond X-ray pulsars (AMXPs), discovering three of the 14 sources now known of this class. Besides increasing the number of known AMXPs, INTEGRAL also carried out observations of these objects above 20 keV, substantially advancing our understanding of their behaviour. We present here a review of all the AMXPs observed with INTEGRAL and discuss the physical interpretation of their behaviour in the X-ray domain. We focus in particular on the lightcurve profile during outburst, as well as the timing, spectral, and thermonuclear type-I X-ray bursts properties.

astro-ph.HE↗