Equivalent Hydrogen Column Density Research Articles

A Study of Interstellar Medium in the Line of Sight of Transient Neutron Star Low-Mass X-ray Binary, MXB 1659-298, by Timing and Spectral Analysis

This work is dedicated to the study of interstellar medium (ISM) along the line of sight (LOS) of the transient low-mass X-ray binary, MXB 1659-298, capitalizing the high resolving power of XMM-Newton in the soft energy range. We emphasized the analysis of reflection grating spectrometer (RGS) data in the energy range 0.5–2.15 keV, suitable for the study of ISM. The paper includes an explanation of why, in the soft X-ray energy range, only two observations (out of seven) were deemed eligible for analysis. Three absorption lines associated with highly ionized Fe XX (1s22p2-2p2 (3p) 4d), Si XIV (1s2-1s2p), and Mg XI (1s2-1s6p) were identified in the observations, with IDs of 8620701(2001) and 748391601(2015). These new absorption lines and the absorption edge due to the neutral oxygen K edge seen in the spectra validate the multiphase structure of ISM. The predominance of interstellar medium over the ionized absorber is established along the direction of the source. The equivalent hydrogen column density measured is nearly equal to the galactic HI value derived previously. The small value of the ionic column density of Fe, Si, and Mg in the site of the high-temperature region resembles previous findings.

A Comprehensive Study of Soft X-ray Absorption Features in GX 13+1 Using XMM-Newton Observations

In this study, a comprehensive analysis of the reflection grating spectrometer (RGS) data (energy range 0.33 keV to 2.5 keV) of GX 13+1 from the XMM-Newton space observatory was conducted. Numerous absorption lines have been described in earlier publications, although the majority of these investigations focused on the energy range between 3 keV and 10 keV. We considered all ten on-axis observations for spectral analysis, but during timing analysis, off-axis observations were also analysed. A fresh dip in off-axis observation 0122340701(B) was observed together with the previously reported dip in on-axis observation 0505480501(F). No burst or eclipsing was observed in any of the observations. The spectral analysis revealed the presence of the highly ionized elements, Si XIII 1s2–1s2p and 1s2–1s3p transitions with energies of 2.02 keV and 2.24 keV, respectively, as well as Mg XII 1s–6p transitions with energies of 1.91 keV and Al XIII 1s–4p transitions with energies of 2.15 keV. Before this study, no analyses of XMM-Newton data reported Mg XII and Al XIII elements. Moreover, the 1s–6p transitions of Mg XII and 1s–4p transitions of Al XIII were not even reported in any Chandra data analysis. The equivalent hydrogen column densities obtained (2.35 × 1022 cm−2 to 4.18 × 1022 cm−2) are consistent with previously reported values. The equivalent widths of neighbouring observations are found to be nearly the same. This supports us in suggesting that the absorptions may be due to the presence of an interstellar medium (ISM) in the line of sight (LOS) of the source.

The extremely X-ray luminous radio-loud quasar CFHQS J142952 + 544717 at z = 6.18 under Chandra high-angular resolution lens

ABSTRACT We present the first X-ray observation at sub-arcsecond resolution of the high-redshift (z = 6.18) radio-loud quasar CFHQS J142952 + 544717 (J1429). The ∼100 net-count 0.3–7 keV spectrum obtained from ∼30 ks Chandra exposure is best fit by a single power-law model with a photon index Γ = 2.0 ± 0.2 and no indication of an intrinsic absorber, implying a 3.6–72 keV rest-frame luminosity $L_{\rm X}=(2.3^{+0.6}_{-0.5})\times 10^{46}$ erg s−1. We identify a second X-ray source at 30″ distance from J1429 position, with a soft (Γ ≃ 2.8) and absorbed (equivalent hydrogen column density NH < 13.4 × 1020 cm−2) spectrum, which likely contaminated J1429 spectra obtained in lower angular resolution observations. Based on the analysis of the Chandra image, the bulk of the X-ray luminosity is produced within the central ∼3 kpc region, either by the disc/corona system, or by a moderately aligned jet. In this context, we discuss the source properties in comparison with samples of low- and high-redshift quasars. We find indication of a possible excess of counts over the expectations for a point-like source in a 0.5″–1.5″ (∼3−8 kpc) annular region. The corresponding X-ray luminosity at J1429 redshift is 4 × 1045 erg s−1. If confirmed, this emission could be related to either a large-scale X-ray jet, or a separate X-ray source.

X-ray Energy Spectra Study and Black Hole Spin Estimate of GRS 1915+105

GRS 1915+105 is a galactic low-mass X-ray binary. Its energy spectra and black hole spin have been extensively studied. Since 2018 June, it has declined into a low-flux X-ray level, occasionally interrupted by multi-wavelength flares. Using the data of Insight-HXMT (Insight-Hard X-ray Modulation Telescope) satellite from August 30 to Oct 13, 2020, the energy spectra characteristics of GRS 1915+105 is investigated. The results show that the energy spectra can be well fitted with a Comptonized multi-temperature blackbody model. The evolution of the hardness-intensity diagram during the outburst remains in the soft state. Adopting new dynamical parameters of GRS 1915+105 (including the black hole mass M, the Sun mass M⊙, the inclination angle i, and the distance D): M=12.4−1.8+2.0M⊙, i=60∘±5∘, D=8.6−1.6+2.0 kpc, the group obtains a lower limit of the black hole spin, a*>0.9990, confirming that GRS 1915+105 is an extremely-spinning black hole. Considering the effects of a local absorber, the group adds the absorption component “tbpcf” into the model. Its equivalent hydrogen column density reaches 1023 cm−2, which is consistent with the characteristics of “Compton-thick”.

Changes in the distribution of circum-binary material around the HMXB GX 301-2 during a rapid spin-up episode of the neutron star

ABSTRACT Some accretion-powered X-ray pulsars with supergiant companion stars undergo occasional rapid spin-up episodes that last for weeks to a few months. We explore the changes in the accretion environment of the pulsar GX 301-2 during its latest 80 days long spin-up episode in 2019 when the spin frequency of the pulsar increased by ∼2 per cent over two orbits of the binary. By performing time-resolved spectroscopy with the MAXI/GSC spectra of the source, we estimated the equivalent hydrogen column density and equivalent width of the iron fluorescence line during the spin-up episode, and compared them with the long-term average values estimated by orbital-phase-resolved spectroscopy. The measured absorption column density during the spin-up episode is about twice that of an average orbit, while the equivalent width of the iron line is less than half of an average orbit. Though the spin-up episode started immediately after a pre-periastron flare and lasted for the two consecutive orbits of the binary, the associated enhancement in luminosity started a few days after the pre-periastron flare and lasted only during the first orbit, and some enhancement was seen again during the pre-periastron passage of the second orbit. The absorption column density and iron line equivalent width vary throughout the spin-up episode and are distinct from an average orbit. These observations indicate a significant change in the accretion and reprocessing environment in GX 301-2 during the spin-up episode and may hold important clues for the phenomenon in this source and several other sources with supergiant companions.

SWIFT J0503.7-2819: a nearly synchronous intermediate polar below the period gap?

ABSTRACT Based on the X-ray observations from XMM–Newton and Swift, and optical observations from Transiting Exoplanet Survey Satellite (TESS) and AAVSO, we present temporal and spectral properties of probable intermediate polar SWIFT J0503.7-2819. The X-ray light curve shows two distinctive features, where possibly the second pole seems to be active during the middle of the XMM–Newton observations. Present analysis confirms and also refines the previously reported orbital period of SWIFT J0503.7-2819 as 81.65 ± 0.04 min. The X-ray and optical variations of this target have been found to occur at the period of ∼65 min, which we propose as the spin period of the white dwarf. The energy-dependent modulation at this period, which are due to the photoelectric absorption in the accretion flow, also assures this conjecture. Two-temperature thermal plasma model well explains the X-ray spectra with temperatures of ∼150 eV and ∼18.5 keV, which is absorbed by a dense material with an average equivalent hydrogen column density of 3.8 × 1022 cm−2 that partially covers ∼27 per cent of the X-ray source. An attempt is made to understand the accretion flow in this system using the present data of SWIFT J0503.7-2819. If the proposed spin period is indeed the actual period, then SWIFT J0503.7-2819 could be the first nearly synchronous intermediate polar below the period gap.

Are Compton-thin AGNs Globally Compton Thin?

Abstract We select eight nearby active galactic nuclei (AGNs) which, based on previous work, appear to be Compton-thin in the line of sight. We model with mytorus their broadband X-ray spectra from 20 individual observations with Suzaku, accounting self-consistently for Fe Kα line emission, as well as direct and scattered continuum from matter with finite column density and solar Fe abundance. Our model configuration allows us to measure the global, out of the line of sight, equivalent hydrogen column density separately from that in the line of sight. For 5 out of 20 observations (in 3 AGNs) we find that the global column density is in fact ≳1.5 × 1024 cm−2, consistent with the distant scattering matter being Compton-thick. For a fourth AGN, two out of five observations are also consistent with being Compton-thick, although with large errors. Some of these AGNs have been reported to host relativistically broadened Fe Kα emission. Based on our modeling, the Fe Kα emission line is not resolved in all but two Suzaku observations, and the data can be fitted well with models that only include a narrow Fe Kα emission line.

Evidence of a non-conservative mass transfer in the ultra-compact X-ray source XB 1916-053

Context. The dipping source XB 1916-053 is a compact binary system with an orbital period of 50 min harboring a neutron star. It shows a positive and a negative superhump, which suggests the presence of a precessing elliptic accretion disk tilted with respect to the equatorial plane of the system. The orbital ephemeris indicates a large orbital period derivative, Ṗ/P = 1.53 × 10−7 yr−1, that can be explained assuming a high non-conservative mass transfer rate. Finally, the spectrum shows prominent absorption lines indicating the presence of an ionized absorber along the line of sight. Aims. Using ten new Chandra observations and one Swift/XRT observation, we are able to extend the baseline of the orbital ephemeris; this allows us to exclude some models that explain the dip arrival times. The Chandra observations provide a good plasma diagnostic of the ionized absorber and allow us to determine whether it is placed at the outer rim of the accretion disk or closer to the compact object. Methods. From the available observations we are able to obtain three new dip arrival times extending the baseline of the orbital ephemeris from 37 to 40 years. The Chandra spectra are fitted adopting a Comptonized continuum. To fit the absorption lines we adopt the ZXIPCF component obtaining information on the ionization parameter and the equivalent hydrogen column density of the ionized absorber. Results. From the analysis of the dip arrival times we confirm an orbital period derivative of Ṗ = 1.46(3) × 10−11 s s−1. Furthermore, the unabsorbed 0.1–100 keV luminosity observed from the Chandra spectra show a variation between 3 × 1036 and 1.4 × 1037 erg s−1. We show that the Ṗ value and the luminosity values are compatible with neutron star masses higher than 1.4 M⊙ with a mass accretion rate lower than 10% of the mass transfer rate. We show that the mass ratio q = m2/m1 of 0.048 explains the apsidal precession period of 3.9 d and the nodal precession period of 4.86 d deduced from the superhump and infrahump detected period. The observed absorption lines are associated with the presence of Ne X, Mg XII, Si XIV, SXVI, and Fe XXVI ions. We observe a redshift in the absorption lines between 1.1 × 10−3 and 1.3 × 10−3. By interpreting it as gravitational redshift, as recently discussed in the literature, we find that the ionized absorber is placed at a distance of 108 cm from the neutron star with a mass of 1.4 M⊙ and has a hydrogen atom density greater than 1015 cm−3. Instead, the absorber is more distant and could be placed at the outer rim of the accretion disk (1.7 × 1010 cm) during the dip activity. Conclusions. We show that the mass ratio of the source is 0.048; this value is obtained from the nodal precession period of the disk and from the apsidal precession period taking into account the pressure term due to the spiral wave present in the disk. From our analysis we estimate a pitch angle of the spiral wave smaller than 30°, in agreement with the values observed in several cataclysmic variables. We show that the outer radius of the disk is truncated at the radius in which a 3:1 resonance occurs, which is 1.7 × 1010 cm for a neutron star mass of 1.4 M⊙. The large orbital period derivative is likely due to a high non-conservative mass transfer with a mass transfer rate of 10−8 M⊙ yr−1. The variation in observed luminosity could be explained assuming that the ejection point from which the matter leaves the system moves close to the inner Lagrangian point.

Using realistic host galaxy metallicities to improve the GRB X-ray equivalent total hydrogen column density and constrain the intergalactic medium density

ABSTRACT It is known that the GRB equivalent hydrogen column density (NHX) changes with redshift and that, typically, NHX is greater than the GRB host neutral hydrogen column density. We have compiled a large sample of data for GRB NHX and metallicity [X/H]. The main aims of this paper are to generate improved NHX for our sample by using actual metallicities, dust corrected where available for detections, and for the remaining GRB, a more realistic average intrinsic metallicity using a standard adjustment from solar. Then, by approximating the GRB host intrinsic hydrogen column density using the measured neutral column (NHI, IC) adjusted for the ionization fraction, we isolate a more accurate estimate for the intergalactic medium (IGM) contribution. The GRB sample mean metallicity is = −1.17 ± 0.09 rms (or 0.07 ± 0.05 Z/Zsol) from a sample of 36 GRB with a redshift 1.76 ≤ z ≤ 5.91, substantially lower than the assumption of solar metallicity used as standard for many fitted NHX. Lower GRB host mean metallicity results in increased estimated NHX with the correction scaling with redshift as Δlog (NHX cm−2) = (0.59 ± 0.04)log(1 + z) + 0.18 ± 0.02. Of the 128 GRB with data for both NHX and NHI, IC in our sample, only six have NHI, IC > NHX when revised for realistic metallicity, compared to 32 when solar metallicity is assumed. The lower envelope of the revised NHX – NHI, IC, plotted against redshift can be fit by log(NHX – NHI, IC cm−2) = 20.3 + 2.4 log(1 + z). This is taken to be an estimate for the maximum IGM hydrogen column density as a function of redshift. Using this approach, we estimate an upper limit to the hydrogen density at redshift zero (n0) to be consistent with n0 = 0.17 × 10−7cm−3.

First Hard X-Ray Observation of a Compact Symmetric Object: A Broadband X-Ray Study of a Radio Galaxy OQ+208 with NuSTAR and Chandra

Abstract Compact symmetric objects (CSOs) have been observed with Chandra and XMM-Newton to gain insights into the initial stages of a radio source evolution and to probe the black hole activity at the time of relativistic outflow formation. However, there have been no CSO observations to date at the hard X-ray energies (>10 keV), impeding our ability to robustly constrain the properties of the intrinsic X-ray emission and of the medium surrounding the young expanding jets. We present the first hard X-ray observation of a CSO performed with the Nuclear Spectroscopic Telescope Array (NuSTAR). Our target, OQ +208, is detected up to 30 keV, and thus we establish CSOs as a new class of the NuSTAR sources. We analyze the NuSTAR data jointly with our new Chandra and archival XMM-Newton data and find that a young (∼250 yr old) radio jet spanning the length of 10 pc coexists with cold obscuring matter, consistent with a dusty torus, with an equivalent hydrogen column density of N H = 1023–1024 cm−2. The primary X-ray emission is characterized by a photon index of Γ ∼ 1.45 and an intrinsic 0.5–30 keV luminosity of L ≃ 1043 erg s−1. The results of our spectral modeling and broad-line optical classification of the source suggest a porous structure of the obscuring torus. Alternatively, the source may belong to the class of optically unobscured/X-ray-obscured active galactic nucleus. The observed X-ray emission is too weak compared to that predicted by the expanding radio lobes model, leaving an accretion disk corona or jets as the possible origins of the X-ray emission from this young radio galaxy.

Broadband spectral analysis of MXB 1659−298 in its soft and hard state

Context. The X-ray transient eclipsing source MXB 1659−298 went into outburst in 1999 and 2015. During these two outbursts the source was observed by XMM-Newton, NuSTAR, and Swift/XRT. Aims. Using these observations, we studied the broadband spectrum of the source to constrain the continuum components and to verify whether it had a reflection component, as is observed in other X-ray eclipsing transient sources. Methods. We combined the available spectra to study the soft and hard state of the source in the 0.45–55 keV energy range. Results. We report a reflection component in the soft and hard state. The direct emission in the soft state can be modeled with a thermal component originating from the inner accretion disk plus a Comptonized component associated with an optically thick corona surrounding the neutron star. On the other hand, the direct emission in the hard state is described only by a Comptonized component with a temperature higher than 130 keV; this component is associated with an optically thin corona. We observed narrow absorption lines from highly ionized ions of oxygen, neon, and iron in the soft spectral state. We investigated where the narrow absorption lines form in the ionized absorber. The equivalent hydrogen column density associated with the absorber is close to 6 × 1023 cm−2 and 1.3 × 1023 cm−2 in the soft and hard state, respectively.

X-Ray Observations of an Intermediate Polar V2400 Oph

Abstract Based on the X-ray observations from XMM-Newton and Suzaku satellites at four different epochs, we present temporal and spectral properties of an intermediate polar (IP) V2400 Oph (=RX J1712.6-2414). The X-ray variations are found to occur at the spin and synodic periods and are derived to be 929 s and 1003 s, respectively. The X-ray spectrum is strongly absorbed by a dense material with an average equivalent hydrogen column density of ∼5 × 1022 cm−2, which partially covers ∼46% of the X-ray source. Suzaku spectra in the 0.3–50 keV energy range are well explained by two temperature collisional equilibrium plasma emission models with its reflection from the cold matter. A soft X-ray emission is also seen in the X-ray spectrum of V2400 Oph and is well modeled by the blackbody with an average temperature of ∼98 eV. The partial covering absorbers and softness ratio are spin phase dependent indicating that the coverage of accretion curtains is variable. A strong emission line of fluorescent Fe Kα at 6.4 keV is also detected in the X-ray spectra. The central energy of Fe Kα appears to be redshifted and found to be modulated with the white dwarf (WD) rotation, where modulations are at minimum around the spin minimum, indicating that the redshifted line is originated from pre-shock accreting material via fluorescence. An attempt is also made to characterize the system and to understand the magnetic accretion flows using the present data of V2400 Oph.

Exploring the Interstellar Medium Using an Asymmetric X-Ray Dust Scattering Halo

Abstract SWIFT J1658.2−4242 is an X-ray transient discovered recently in the Galactic plane, with severe X-ray absorption corresponding to an equivalent hydrogen column density of ∼2 × 1023 cm−2. Using new Chandra and XMM-Newton data, we discover a strong X-ray dust scattering halo around it. The halo profile can be well fitted by the scattering from at least three separated dust layers. During the persistent emission phase of SWIFT J1658.2−4242, the best-fit dust scattering N H,sca based on the COMP-AC-S dust grain model is consistent with the X-ray absorbing N H,abs. The best-fit halo models show that 85%–90% of the intervening gas and dust along the line of sight of SWIFT J1658.2−4242 are located in the foreground interstellar medium in the Galactic disk. The dust scattering halo also shows significant azimuthal asymmetry, which appears consistent with the inhomogeneous distribution of foreground molecular clouds. By matching the different dust layers to the distribution of molecular clouds along the line of sight, we estimate the source distance to be ∼10 kpc, which is also consistent with the results given by several other independent methods of disance estimation. The dust scattering opacity and the existence of a halo can introduce a significant spectral bias, the level of which depends on the shape of the instrumental point-spread function and the source extraction region. We create the xspec dscor model to correct for this spectral bias for different X-ray instruments. Our study reinforces the importance of considering the spectral effects of dust scattering in other absorbed X-ray sources.

Systematic study for gas-to-dust ratio of short gamma-ray burst afterglows

Abstract Extragalactic X-ray absorption and optical extinction are often found in gamma-ray burst (GRB) afterglows, and they could be tracers of both circumburst and host galaxy environments. By performing spectral analyses of the spectral energy distribution of nine short GRB (SGRB) afterglows with a known redshift, we investigated the ratio of the equivalent hydrogen column density to the dust extinction, $N_{\rm H}^{\rm rest}/A_{V}^{\rm rest}$, in the rest frame of each SGRB. We found that the distribution of $N_{\rm H}^{\rm rest}/A_{V}^{\rm rest}$ is systematically smaller than for long GRBs, and is roughly consistent with the gas-to-dust ratio in the Milky Way. This result means that the measured gas-to-dust ratio of SGRBs would originate from the interstellar medium in each host galaxy. This scenario supports the prediction that SGRBs occur in non-star-forming regions in the host galaxies.

NuSTAR Detection of X-Rays Concurrent with Gamma-Rays in the Nova V5855 Sgr

Abstract We report the first detection of hard (>10 keV) X-ray emission simultaneous with gamma-rays in a nova eruption. Observations of the nova V5855 Sgr carried out with the NuSTAR satellite on Day 12 of the eruption revealed faint, highly absorbed thermal X-rays. The extreme equivalent hydrogen column density toward the X-ray emitting region (∼3 × 1024 cm−2) indicates that the shock producing the X-rays was deeply embedded within the nova ejecta. The slope of the X-ray spectrum favors a thermal origin for the bulk of the emission, and the constraints of the temperature in the shocked region suggest a shock velocity compatible with the ejecta velocities inferred from optical spectroscopy. While we do not claim the detection of nonthermal X-rays, the data do not allow us to rule out an additional, fainter component dominating at energies above 20 keV, for which we obtained upper limits. The inferred luminosity of the thermal X-rays is too low to be consistent with the gamma-ray luminosities if both are powered by the same shock under standard assumptions regarding the efficiency of nonthermal particle acceleration and the temperature distribution of the shocked gas.

Reanalysis of high-resolutionXMM-Newtondata of V2491 Cygni using models of collisionally ionized hot absorbers

We present a reanalysis of XMM-Newton Reflection Grating Spectrometer data of the classical nova V2491 Cyg obtained from two different pointings, 40 d and 50 d after outburst utilizing the SRON software SPEX version 2.05.04. We aim to model absorption components using hot collisionally ionized absorber models along with interstellar absorption (of gas and dust origin separately). We find blackbody temperatures in a range 61-91 eV for the continuum yielding a white dwarf mass of 1.15-1.3 M_sun. We derive two different hot absorber components with blueshifts yielding 2900-3800 km/s for the first (day 40) and 2600-3600 km/s for the second observation 50 days after outburst consistent with ejecta/wind speeds. The two collisionally ionized hot absorption components have temperatures kT_1 =1.0-3.6 keV and kT_2 =0.4-0.87 keV with rms velocities (sigma_v) 872 km/s and 56 km/s. These are consistent with shock temperatures in the X-ray wavelengths. V2491 Cyg shows signature of H-burning with underabundant carbon C/C_sun=0.3-0.5, and enhanced nitrogen N/N_sun=5-7 and oxygen O/O_sun=16-43. We find the equivalent hydrogen column density of the hot collisionally ionized absorbers in a range (0.6-18.0)x10^{23} cm^{-2} and (2.0-5.3)x10^{23} cm^{-2} on days 40 and 50 after outburst, respectively. Our fits yield the most adequate redchisq (range 1.8-2.9) up-to-date obtained for the modeling of high resolution X-ray data of V2491 Cyg. An additional photoionized absorber (third intrinsic absorber component) originating in the shell/ejecta improves the model fits with redchisq in a range 1.7-2.5, but shows only (1-0.1)% of the absorption by the collisionaly-ionized hot gas. Our analysis reveals a second blackbody component on day 50 with effective temperature 120-131 eV and effective radius about 10% of the WD which may indicate the onset of magnetic accretion.

Gas absorption and dust extinction towards the Orion Nebula Cluster

We characterise the relation between the gas and dust content of the interstellar medium towards young stellar objects in the Orion Nebula Cluster. X-ray observations provide estimates of the absorbing equivalent hydrogen column density N_H based on spectral fits. Near-infrared extinction values are calculated from intrinsic and observed colour magnitudes (J-H) and (H-K_s) as given by the VISTA Orion A survey. A linear fit of the correlation between column density and extinction values A_V yields an estimate of the N_H/A_V ratio. We investigate systematic uncertainties of the results by describing and (if possible) quantifying the influence of circumstellar material and the adopted extinction law, X-ray models, and elemental abundances on the N_H/A_V ratio. Assuming a Galactic extinction law with R_V=3.1 and solar abundances by Anders & Grevesse (1989), we deduce an N_H/A_V ratio of (1.39 +- 0.14) x 10^21 cm^-2 mag^-1 for Class III sources in the Orion Nebula Cluster where the given error does not include systematic uncertainties. This ratio is consistent with similar studies in other star-forming regions and approximately 31% lower than the Galactic value. We find no obvious trends in the spatial distribution of N_H/A_V ratios. Changes in the assumed extinction law and elemental abundances are demonstrated to have a relevant impact on deduced A_V and N_H values, respectively. Large systematic uncertainties associated with metal abundances in the Orion Nebula Cluster represent the primary limitation for the deduction of a definitive N_H/A_V ratio and the physical interpretation of these results.

Discovery of X-ray pulsations in the Be/X-ray binary IGR J21343+4738

We report on the discovery of X-ray pulsations in the Be/X-ray binary IGR J21343+4738 during an XMM-Newton observation. We obtained a barycentric corrected pulse period of 320.35+-0.06 seconds. The pulse profile displays a peak at low energy that flattens at high energy. The pulse fraction is 45+-3$% and independent of energy within the statistical uncertainties. The 0.2-12 keV spectrum is well fit by a two component model consisting of a blackbody with kT=0.11+-0.01 keV and a power law with photon index Gamma=1.02+-0.07. Both components are affected by photoelectric absorption with a equivalent hydrogen column density NH=(1.08+-0.15)x 10^{22} cm^{-2} The observed unabsorbed flux is 1.4x10^{-11} erg cm^{-2} s^{-1} in the 0.2-12 keV energy band. Despite the fact that the Be star's circumstellar disc has almost vanished, accretion continues to be the main source of high energy radiation. We argue that the observed X-ray luminosity (LX~10^{35} erg s^{-1}) may result from accretion via a low-velocity equatorial wind from the optical companion.

DISCOVERY OF X-RAY EMISSION IN THE OLD CLASSICAL NOVA DK LACERTAE

We report the discovery of X-ray emission at the position of the old classical nova DK Lacertae using the Swift satellite. Three observations were conducted using the X-ray telescope 62 years after the discovery of the nova, yielding 46 source signals in an exposure time of 4.8 ks. A background-subtracted count rate was 9+/-2x10^{-3} counts s^{-1}, corresponding to a detection significance level of 5-sigma. The X-ray spectrum was characterized by a continuum extending up to about 7 keV, which can be modeled by a power-law component with a photon index of 1.4--5.6, or by a thermal bremsstrahlung component with a temperature of 0.7--13.3 keV, convolved with interstellar absorption with an equivalent hydrogen column density of 0.3--2.4x10^{22} cm^{-2}. Assuming a distance of 3900 pc to the source, the luminosity was 10^{32}--10^{34} ergs s^{-1} in the 0.3--10 keV energy band. The origin of X-rays is considered to be either mass accretion on the white dwarf or adiabatic shocks in nova ejecta, with the former appearing much more likely. In either case, DK Lacertae represents a rare addition to the exclusive club of X-ray emitting old novae.

AN X-RAY WHIM METAL ABSORBER FROM A Mpc-SCALE EMPTY REGION OF SPACE

We report a detection of an absorption line at ~44.8 {\AA} in a > 500 ks Chandra HRC-S/LETG X-ray grating spectrum of the blazar H 2356-309. This line can be identified as intervening CV-K{\alpha} absorption, at z\approx0.112, produced by a warm (log T = 5.1 K) intergalactic absorber. The feature is significant at a 2.9{\sigma} level (accounting for the number of independent redshift trials). We estimate an equivalent hydrogen column density of log N_H=19.05 (Z/Zsun)^-1 cm^-2. Unlike other previously reported FUV/X-ray metal detections of warm-hot intergalactic medium (WHIM), this CV absorber lies in a region with locally low galaxy density, at ~2.2 Mpc from the closest galaxy at that redshift, and therefore is unlikely to be associated with an extended galactic halo. We instead tentatively identify this absorber with an intervening Warm-Hot Intergalactic Medium filament possibly permeating a large-scale, 30 Mpc extended, structure of galaxies whose redshift centroid, within a cylinder of 7.5 Mpc radius centered on the line of sight to H 2356-309, is marginally consistent (at a 1.8{\sigma} level) with the redshift of the absorber.