X-ray Flux Variability Research Articles

An X-ray bright nucleus in the low-surface-brightness galaxy UGC 6614

We report a study of the X-ray emission from the nuclear region of the low surface brightness (LSB) galaxy UGC 6614. Very little is known about the central objects in LSB galaxies especially their X-ray properties and X-ray spectra. In this study we have used XMM-Newton archival data to study the characteristics of the X-ray spectrum and the X-ray flux variability of the AGN in the LSB galaxy UGC 6614. The nucleus of UGC 6614 is very bright in X-ray emission with an absorption corrected 0.2-10.0 keV luminosity of ~1.1 x 10^{42} erg s^{-1}. The X-ray spectrum is found to be power-law type with a moderate column density. A short time scale of intensity variation and large X-ray flux is indicative of the presence of a black hole at the centre of this galaxy. Using the method of excess variance, we have determined the black hole mass to be ~0.12 x 10^{6} solar mass. The X-ray spectral properties are similar to that of the Seyfert I type AGNs. Our study thus demonstrates that although LSB galaxies are poor in star formation, they may harbour AGNs with X-ray properties comparable to that seen in more luminous spiral galaxies.

PROBING THE INNER JET OF THE QUASAR PKS 1510–089 WITH MULTI-WAVEBAND MONITORING DURING STRONG GAMMA-RAY ACTIVITY

We present results from monitoring the multi-waveband flux, linear polarization, and parsec-scale structure of the quasar PKS 1510-089, concentrating on eight major gamma-ray flares that occurred during the interval 2009.0-2009.5. The gamma-ray peaks were essentially simultaneous with maxima at optical wavelengths, although the flux ratio of the two wavebands varied by an order of magnitude. The optical polarization vector rotated by 720 degrees during a 5-day period encompassing six of these flares. This culminated in a very bright, roughly 1 day, optical and gamma-ray flare as a bright knot of emission passed through the highest-intensity, stationary feature (the "core") seen in 43 GHz Very Long Baseline Array images. The knot continued to propagate down the jet at an apparent speed of 22c and emit strongly at gamma-ray energies as a months-long X-ray/radio outburst intensified. We interpret these events as the result of the knot following a spiral path through a mainly toroidal magnetic field pattern in the acceleration and collimation zone of the jet, after which it passes through a standing shock in the 43 GHz core and then continues downstream. In this picture, the rapid gamma-ray flares result from scattering of infrared seed photons from a relatively slow sheath of the jet as well as from optical synchrotron radiation in the faster spine. The 2006-2009.7 radio and X-ray flux variations are correlated at very high significance; we conclude that the X-rays are mainly from inverse Compton scattering of infrared seed photons by 20-40 MeV electrons.

Multi-epoch X-ray observations of the narrow-line Seyfert 1 galaxy SDSS J160508.87+323921.4 with XMM-Newton

We present results of multi-epoch XMM-Newton observations separated by several years of the narrow-line Seyfert 1 (NLS1) galaxy, SDSS J160508.87+323921.4. The NLS1 showed X-ray flux variations on timescales of years as well as low amplitude variations on timescales as short as hours, while the X-ray spectral shape remained essentially unchanged. The spectrum shows a significant soft X-ray excess. The model of reflection from an ionized disc, that is dominant in the whole XMM-Newton bandpass, provides the best description of the observed spectra for all the observations, though alternatives such as Compton scattering (T ∼ 0.25 keV) cannot be ruled out in a statistical sense. For the disc reflection model, the observed variations may originate either from the change in the reflection fraction, or from the variability of the intrinsic X-ray luminosity.

SPIN-DOWN MEASUREMENT OF PSR J1852+0040 IN KESTEVEN 79: CENTRAL COMPACT OBJECTS AS ANTI-MAGNETARS

Using XMM-Newton and Chandra, we achieved phase-connected timing of the 105 ms X-ray pulsar PSR J1852+0040 that provides the first measurement of the spin-down rate of a member of the class of Central Compact Objects (CCOs) in supernova remnants. We measure P-dot = 8.68(9)E-18, and find no evidence for timing noise or variations in X-ray flux over 4.8 yr. In the dipole spin-down formalism, this implies a surface magnetic field strength B_s = 3.1E10 G, the smallest ever measured for a young neutron star, and consistent with being a fossil field. In combination with upper limits on B_s from other CCO pulsars, this is strong evidence in favor of the "anti-magnetar" explanation for their low luminosity and lack of magnetospheric activity or synchrotron nebulae. While this dipole field is small, it can prevent accretion of sufficient fall-back material so that the observed X-ray luminosity of L_x = 5.3E33(d/7.1 kpc)^2 erg/s must instead be residual cooling. The spin-down luminosity of PSR J1852+0040, E-dot = 3.0E32 erg/s, is an order-of-magnitude smaller than L_x. Fitting of the X-ray spectrum to two blackbodies finds small emitting radii, R_1 = 1.9 km and R_2 = 0.45 km, for components of kT_1 = 0.30 keV and kT_2 = 0.52 keV, respectively. Such small, hot regions are ubiquitous among CCOs, and are not yet understood in the context of the anti-magnetar picture because anisotropic surface temperature is usually attributed to the effects of strong magnetic fields.

Quenching of the strong aperiodic accretion disk variability at the magnetospheric boundary

We study power density spectra (PDS) of X-ray flux variability in binary systems where the accretion flow is truncated by the magnetosphere. PDS of accreting X-ray pulsars where the neutron star is close to the corotation with the accretion disk at the magnetospheric boundary, have a distinct break/cutoff at the neutron star spin frequency. This break can naturally be explained in the "perturbation propagation" model, which assumes that at any given radius in the accretion disk stochastic perturbations are introduced to the flow with frequencies characteristic for this radius. These perturbations are then advected to the region of main energy release leading to a self-similar variability of X-ray flux P~f^{-1...-1.5}. The break in the PDS is then a natural manifestation of the transition from the disk to magnetospheric flow at the frequency characteristic for the accretion disk truncation radius (magnetospheric radius). The proximity of the PDS break frequency to the spin frequency in corotating pulsars strongly suggests that the typical variability time scale in accretion disks is close to the Keplerian one. In transient accreting X-ray pulsars characterized by large variations of the mass accretion rate during outbursts, the PDS break frequency follows the variations of the X-ray flux, reflecting the change of the magnetosphere size with the accretion rate. Above the break frequency the PDS steepens to ~f^{-2} law which holds over a broad frequency range. These results suggest that strong f^{-1...-1.5} aperiodic variability which is ubiquitous in accretion disks is not characteristic for magnetospheric flows.

Simultaneous multiwavelength observations of the second exceptionalγ-ray flare of PKS 2155–304 in July 2006

Simultaneous HESS/CHANDRA/optical observations were performed on the BL Lac object PKS 2155-304 in the night of July 29-30 2006, when the source underwent its second major gamma-ray outburst in Summer 2006. This event took place about 44 hours after the July 28 outburst, known for its ultrafast variability. An unprecedented 6 to 8 hours of uninterrupted coverage was achieved, with spectra and light curves measured down to 7 and 2-minute timescales, respectively. The gamma-ray flux reached a maximum of 11x the Crab flux (>400 GeV), with rise/decay timescales of ~1 hour, plus a few smaller-amplitude flares superimposed on the decaying phase. The emission in the X-ray and VHE bands is strongly correlated, both in flux and spectrum, with no evidence of lags. The VHE spectrum shows a curvature that is variable with time and stronger at higher fluxes. The huge VHE variations (22x) are only accompanied by small-amplitude X-ray and optical variations (factor 2 and 15% respectively). The source has shown for the first time in an HBL a large Compton dominance (L_C/L_S ~10) -- rapidly evolving -- and a cubic relation between VHE and X-ray flux variations, during a decaying phase. These results challenge the common scenarios for the TeV-blazar emission.

From outburst to quiescence: the decay of the transient AXP XTE J1810-197

Aims. XTE J1810-197 is the first transient anomalous X-ray pulsar ever discovered. Its highly variable X-ray flux allowed us to study the timing and spectral emission properties of a magnetar candidate over a flux range of about two orders of magnitude. Methods. We analyzed nine XMM-Newton observations of XTE J1810-197 collected over a four year baseline (September 2003– September 2007). EPIC PN and MOS data were reduced and used for detailed timing and spectral analysis. Pulse-phase spectroscopic studies were also carried out for observations with a high enough signal-to-noise. Results. We find that (i) a three-blackbody model reproduces the spectral properties of XTE J1810-197 over the entire outburst statistically better than the two blackbodies model previously used in the literature, (ii) the coldest blackbody is consistent with the thermal emission from the whole surface and has temperature and radius similar to those inferred from ROSAT observations before the outburst onset, (iii) there is a spectral feature around 1.1 keV during six consecutive observations (since March 2005). If this stems from proton resonant cyclotron scattering, it would imply a magnetic field of ∼2 × 10 14 G. This closely agrees with the value from the spin period measurements.

SIMULTANEOUS X-RAY/TeV OBSERVATIONS OF AN EXCEPTIONAL FLARE OF PKS 2155-304

On the night of 29–30 July 2006, a major γ-ray outburst occurred from PKS 2155-304, which was observed simultaneously with H.E.S.S., Chandra and the Bronberg optical telescopes. An overall 6–8 hours of simultaneous uninterrupted coverage was obtained. Here we report the first preliminary results of this exceptional dataset. The source showed correlated variability between the X-ray and VHE bands, with no lags. The spectra evolve with similar patterns. Huge VHE variations (~ 22×) are accompanied only by small-amplitude X-ray and optical variations (factor 2 and 15% respectively). The source has shown for the first time in an HBL a large Compton dominance (LC/LS ~ 10), and a cubic relation between VHE and X-ray flux variations, during a decaying phase. These results challenge all known "standard" scenarios for the blazar emission.

SphinX: A fast solar Photometer in X-rays

The scientific goals and construction details of a new design, Polish X-ray spectrophotometer are given. It will be incorporated within the Russian TESIS X and EUV complex aboard the forthcoming CORO-NAS solar mission. SphinX (Solar Photometer in X-rays) will use PIN silicon detectors for high time resolution (0.01 s) measurements of the solar spectra of quiet and active corona in the range 0.5–15 keV. A new filter-fluorescence target concept will be employed to allow for a fast photometry of the solar X-ray flux variations in selected, well defined narrow spectral bands including the Fe XXVI and Fe XXV iron line groups.

The nature of the intranight variability of radio-quiet quasars

We select a sample of 10 radio-quiet quasars with confirmed intranight optical variability and with available X-ray data. We compare the variability properties and the broad-band spectral constraints to the predictions of intranight variability by three models: (i) irradiation of an accretion disc by a variable X-ray flux, (ii) an accretion disc instability, (iii) the presence of a weak blazar component. We concluded that the third model, e.g. the blazar component model, is the most promising if we adopt a cannonball model for the jet variable emission. In this case, the probability of detecting the intranight variability is within 20-80 per cent, depending on the ratio of the disc to the jet optical luminosity. Variable X-ray irradiation mechanism is also possible but only under additional requirement: either the source should have a very narrow Hβ line or occasional extremely strong flares should appear at very large disc radii.

Soft X-ray spectral variability of AM Herculis

Context. Polars (AM Herculis binaries) are a prominent class of bright soft X-ray sources, many of which were discovered with ROSAT. Aims. We present a homogenous analysis of all the pointed ROSAT PSPC observations of polars subdivided into two papers that discuss the prototype polar AM Her in detail and summarize the class properties of all other polars. Methods. We derive the high-state soft X-ray flux and short-term spectral variability of AM Her using a new detector response matrix and a confirmed flux calibration of the ROSAT PSPC below 0.28 keV. Results. The best-fit mean single-blackbody temperature and integrated bright-phase energy flux of AM Her in its April 1991 high state are 27.2 ± 1. 0e V and (2.6 ± 0.6) × 10 −9 erg cm −2 s −1 , respectively. The total blackbody flux of a multi-temperature model that fits both the soft X-ray and the fluctuating far-ultraviolet components is Fbb = (4.5 ± 1.5) × 10 −9 erg cm −2 s −1 . The total accretion luminosity at a distance of 80 pc, Lbb = (2.1 ± 0.7) × 10 33 erg s −1 , implies an accretion rate of u

Production of neutrinos and secondary electrons in cosmic sources

We study the individual contribution to secondary lepton production in hadronic interactions of cosmic rays (CRs) including resonances and heavier secondaries. For this purpose we use the same methodology discussed earlier [C.-Y. Huang, S.-E. Park, M. Pohl, C.D. Daniels, Astropart. Phys. 27 (2007) 429], namely the Monte-Carlo particle collision code DPMJET3.04 to determine the multiplicity spectra of various secondary particles with leptons as the final decay states, that result from inelastic collisions of cosmic-ray protons and Helium nuclei with the interstellar medium of standard composition. By combining the simulation results with parametric models for secondary particle (with resonances included) for incident cosmic-ray energies below a few GeV, where DPMJET appears unreliable, we thus derive production matrices for all stable secondary particles in cosmic-ray interactions with energies up to about 10 PeV. We apply the production matrices to calculate the radio synchrotron radiation of secondary electrons in a young shell-type SNR, RX J1713.7-3946, which is a measure of the age, the spectral index of hadronic cosmic rays, and most importantly the magnetic field strength. We find that the multi-mG fields recently invoked to explain the X-ray flux variations are unlikely to extend over a large fraction of the radio-emitting region, otherwise the spectrum of hadronic cosmic rays in the energy window 0.1–100 GeV must be unusually hard. We also use the production matrices to calculate the muon event rate in an IceCube-like detector that are induced by muon neutrinos from high-energy γ-ray sources such as RX J1713.7-3946, Vela Jr. and MGRO J2019+37. At muon energies of a few TeV, or in other word, about 10 TeV neutrino energy, an accumulation of data over about 5–10 years would allow testing the hadronic origin of TeV γ-rays.

Optical/Infrared Observations of the Anomalous X‐Ray Pulsar 1E 1048.1−5937 during Its 2007 X‐Ray Flare

We report on optical and infrared observations of the anomalous X-ray pulsar (AXP) 1E 1048.1–5937 made during its ongoing X-ray flare that started in 2007 March. We detected the source in the optical I and near-infrared Ks bands in two ground-based observations and obtained deep flux upper limits from four observations, including one with the Spitzer Space Telescope at 4.5 and 8.0 μm. The detections indicate that the source was approximately 1.3-1.6 mag brighter than in 2003-2006, when it was at the tail of a previous similar X-ray flare. Similar related flux variations have been seen in two other AXPs during their X-ray outbursts, suggesting common behavior for large X-ray flux variation events in AXPs. The Spitzer flux limits are sufficiently deep that we can exclude mid-infrared emission similar to that from the AXP 4U 0142+61, which has been interpreted as arising from a dust disk around the AXP. The optical/near-infrared emission from 1E 1048.1–5937 probably has a magnetospheric origin. The similarity in the flux spectra of 1E 1048.1–5937 and 4U 0142+61 challenges the dust disk model proposed for the latter.

XMM-Newtonobservations of the Lockman Hole

This paper presents the results of a study of X-ray spectral and flux variability on time scales from months to years, of the 123 brightest objects (including 46 type-1 AGN and 28 type-2 AGN) detected with XMM-Newton in the Lockman Hole field. We detected flux variability with a significance ≥3σ in ∼50% of the objects, including 68 ± 11% and 48 ± 15% among our samples of type-1 and type-2 AGN. However we found that the fraction of sources with best quality light curves that exhibit flux variability on the time scales sampled by our data is ≥80%, i.e the great majority of the AGN population may actually vary in flux on long time scales. The mean relative intrinsic amplitude of flux variability was found to be ∼0.15 although with a large dispersion in measured values, from ∼0.1 to ∼0.65. The flux variability properties of our samples of AGN (fraction of variable objects and amplitude of variability) do not significantly depend on the redshift or X-ray luminosity of the objects and seem to be similar for the two AGN types. Using a broad band X-ray colour we found that the fraction of sources showing spectral variability with a significance ≥3σ is ∼40% i.e. less common than flux variability. Spectral variability was found to be more common in type-2 AGN than in type-1 AGN with a significance of more than 99%. This result is consistent with the fact that part of the soft emission in type-2 AGN comes from scattered radiation, and this component is expected to be much less variable than the hard component. The observed flux and spectral variability properties of our objects and especially the lack of correlation between flux and spectral variability in most of them cannot be explained as being produced by variability of one spectral component alone, for example changes in Γ associated with changes in the mass accretion rate, or variability in the amount of X-ray absorption. At least two spectral components must vary in order to explain the X-ray variability of our objects.

Anisotropic X-ray emission in active galactic nucleus accretion discs

Abstract Straightforward models of X-ray reflection in the inner region of accretion discs predict that the primary X-ray flux and the flux reflected off the surface of the disc should vary together, albeit with a short light traveltime delay. Most of the observations, however, show that the X-ray flux can vary while the reflected features remain constant. Here we propose a simple explanation for this. In all likelihood, the emission of a moderately optically thick magnetic flare atop an accretion disc is anisotropic. A constant energy release rate in a flare will appear to produce a variable X-ray flux as the flare rotates with the accretion disc anchoring the magnetic tube. The reflector, on the other hand, receives a constant X-ray flux from the flare. Since the reflected emission is azimuthally symmetric, the observer will see a roughly constant reflected flux (neglecting relativistic effects). The model does not produce quasi-periodic oscillations if magnetic flux tubes are sheared out faster than they complete one orbit.

Long‐Term X‐Ray Spectral Variability of the Radio‐Loud Narrow‐Line Seyfert 1 Galaxy PKS 0558−504

We present X-ray observations of the narrow-line Seyfert 1 (NLS1) galaxy PKS 0558-504 obtained with RXTE during a 1 yr monitoring campaign. This source, which is one of the very few radio-loud NLS1 galaxies, shows strong X-ray flux variability on timescales of weeks to months, accompanied by spectral variability. The main goal of this study is to investigate the spectral variability with model-independent methods and time-resolved spectroscopy in order to shed light on the origin of the X-rays. The main results can be summarized as follows: (1) The flux typically changes by a factor of 1.5-2 on timescales of 10-30 days, with few extreme events where the flux increases by a factor of ~4 in 3 days. (2) We do not observe any large amplitude, flux-related spectral variations. During the flux variations, the spectrum varies mainly in normalization and not in shape. We do observe some small-amplitude spectral variations, which do not correlate with flux, although there is a hint of spectral hardening as the source brightens. (3) There is no evidence for reprocessing features, such as the Fe Kα line or a Compton hump. We argue that PKS 0558-504 is a peculiar object that appears to be different from most of the radio-quiet and radio-loud active galactic nuclei (AGNs). If a jet is responsible for the bulk of the X-rays, it must operate in an unusual way. If instead a corona is responsible for the X-rays, the system might be a large-scale analog of the Galactic black holes in the transient intermediate state.

High time resolution observations of solar Hα flares. I.

We present here the first results of a search for fast changes of solar flare Ha emission correlated in time with variations of hard X-ray fluxes recorded with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Using the Large Coronagraph (LC), Multi-Channel Subtractive Double Pass (MSDP) spectrograph and the Solar Eclipse Coronal Imaging System (SECIS) at the Bialkow Observatory, we collected a total of 23 sets of high time resolution (0.04-0.075 s) observations of Ha bright flare kernels during two observational seasons (2003 and 2004). For the first time, several-minute-long high-resolution observations of the H-alpha line profiles as well as simultaneous two-dimensional images have been obtained with time resolution much better than 1 s. Detailed observations of four flares recorded on 2003 July 16 and 2004 April 23 are described. We found examples of good time correlations between X-ray flux variations and variations of the Ha emission of the selected bright flaring kernels. In some events particularly, small but impulsive variations of the hard X-ray (20-50 keV) flux and of the Ha emission are well correlated. The Ha emission follows the 20-50 keV X-ray emission by similar to 2 s and similar to 3 s for two of the flares, longer for a third. The 3-10 keV and 10-20 keV X-ray emission, generally characterizing the thermal emission, follow these impulsive changes. There are, however, several examples of impulsive increases in 10-20 keV emission which appear to be of nonthermal origin. These results are consistent with previous observations and with theoretical studies in which the chromospheric response to energetic beamed electrons is calculated, provided that the chromospheric hydrogen densities are relatively high(similar to 10(14)-10(15) cm(-3)).

A Close Look at the Population of Supersoft and Quasi‐soft X‐Ray Sources Observed in M31 withXMM‐Newton

The deepest X-ray images of M31, obtained with XMM-Newton, are examined to derive spectral and statistical properties of the population of the very soft X-ray sources. When classifying supersoft X-ray sources (SSSs) with criteria based on the same hardness ratios defined for recent Chandra observations, one-quarter of the selected SSSs turn out to be supernova remnants (SNRs). Another quarter of the SSSs are spatially coincident with recent classical novae, although they are less than 10% of the nova population observed in the last 25 yr. Only 3 out of the 15 non-SNR SSSs show clear variability, with X-ray flux variation of more than 1 order of magnitude within a few months. Two of these sources display additional, smaller amplitude variability on timescales of several minutes. Their broadband spectra and those of the novae are approximately fitted with a blackbody or white dwarf atmospheric model at near-Eddington luminosity for the distance of M31. Two SSSs appear to reach very large, perhaps super-Eddington luminosities for part of the time and probably eject material in a wind until the luminosity decreases again after a few months. Most quasi-soft sources (QSSs) are repeatedly detected. I discuss the possibility that most QSSs in M31 may be SNRs or foreground neutron stars. Two X-ray sources with both a soft and hard component are in the positions of a recurrent nova and a possible symbiotic nova, but they are probably black hole transients.

Long‐Term X‐Ray and TeV Variability of Mrk 501

We present X-ray observations of the blazar Mrk501 obtained with RXTE during 1997, 1998, 1999, 2000, and 2004. The goal of this study is twofold: 1) characterize the long-term X-ray flux and spectral variability of the source with a model-independent analysis, and 2) investigate the X-ray and TeV correlation on long timescales. The spectral and temporal variability properties appear to be markedly different compared to those of non-jet-dominated radio-loud and radio-quiet AGN monitored with RXTE. To investigate the X-ray/TeV correlation on long timescales we compared RXTE data with HEGRA and Whipple historical light curves. We confirm the presence of a direct correlation, which appears to be stronger when the source is brighter. The analysis of individual flares shows that the X-ray\TeV correlated activity is heterogeneous. However, more sensitive TeV observations are necessary to confirm these findings, and to put tighter constraints on jet models (abridged).

XMM-NewtonSpectroscopy of the Highly Polarized and Luminous Broad Absorption Line Quasar CSO 755

We present the results from XMM-Newton observations of the highly optically polarized broad absorption line quasar (BALQSO) CSO 755. By analyzing its X-ray spectrum with a total of ∼3000 photons, we find that this source has the X-ray continuum of a typical radio-quiet quasar, with a photon index of Γ = 1.83 and a rather flat (X-ray bright) intrinsic optical–X-ray spectral slope of αox = -1.51. The source shows evidence for intrinsic absorption, and fitting the spectrum with a neutral-absorption model gives a column density of NH ∼ 1.2 × 1022 cm-2; this is among the lowest X-ray columns measured for BALQSOs. We do not detect, with high significance, any other absorption features in the X-ray spectrum. We place upper limits on the rest-frame equivalent width of a neutral (ionized) Fe Kα line, ≤180 eV (≤120 eV), and on the Compton-reflection component parameter, R ≤ 0.2, suggesting that most of the X-rays from the source are directly observed rather than being scattered or reflected; this is also supported by the relatively flat intrinsic αox we measure. The possibility that most of the X-ray flux is scattered due to the high level of UV-optical polarization is ruled out. Considering data for 46 BALQSOs from the literature, including CSO 755, we have found that the UV-optical continuum polarization level of BALQSOs is not correlated with any of their X-ray properties. A lack of significant short- and long-term X-ray flux variations in the source may be attributed to a large black hole mass in CSO 755. We note that another luminous BALQSO, PG 2112+059, has both similar shallow C IV BALs and moderate X-ray absorption.