Simultaneous X-ray Observations Research Articles

Constraining the geometry of the dipping atoll 4U 1624–49 with X-ray spectroscopy and polarimetry

We present the spectro-polarimetric results obtained from simultaneous X-ray observations with IXPE, NuSTAR, and NICER of the dipping neutron star X-ray binary 4U 1624–49. This source is the most polarized Atoll source so far observed with IXPE, with a polarization degree of 2.7%±0.9% in the 2–8 keV band during the nondip phase and marginal evidence of an increasing trend with energy. The higher polarization degree compared to other Atolls can be explained by the high inclination of the system (i ≈ 60°). The spectra are well described by the combination of soft thermal emission, a Comptonized component, and reflection of soft photons off the accretion disk. During the dips, the hydrogen column density of the highly ionized absorber increases while the ionization state decreases. The Comptonized radiation seems to be the dominant contribution to the polarized signal, with additional reflected photons that contribute significantly even though their fraction in the total flux is not high.

A puzzling 2-hour X-ray periodicity in the 1.5-hour orbital period black widow PSR J1311−3430

Time-domain analysis of an archival XMM-Newton observation unveiled a very unusual variability pattern in the soft X-ray emission of PSR J1311−3430, a black widow millisecond pulsar in a tight binary (PB = 93.8 min) with a very low-mass (M ∼ 0.01 M⊙) He companion star, known to show flaring emission in the optical and in the X-rays. A series of six pulses with a regular recurrence time of ∼124 min is apparent in the 0.2−10 keV light curve of the system, also featuring an initial, bright flare and a quiescent phase lasting several hours. The X-ray spectrum does not change when the pulses are seen and it is consistent with a power law with photon index Γ ∼ 1.5, also describing the quiescent emission. The peak luminosity of the pulses is of several 1032 erg s−1. Simultaneous observations in the U band with the Optical Monitor onboard XMM and in the g′ band from the Las Cumbres Observatory do not show any apparent counterpart of the pulses and only display the well-known orbital modulation of the system. We consider different hypotheses to explain the recurrent pulses: we investigate their possible analogy with other phenomena already observed in this pulsar and in similar systems and we also study possible explanations related to the interaction of the energetic pulsar wind with intra-binary material, but we found none of these pictures to be convincing. We identify simultaneous X-ray observations and optical spectroscopy as a possible way to constrain the nature of the phenomenon.

XMM and NuSTAR Observations of an Optically Quiescent Quasar

Optically quiescent quasars (OQQs) represent a recently systematized class of infrared-luminous active galactic nuclei (AGNs) that have galaxy-like optical continua. They may represent an interesting, brief phase in the AGN life cycle, e.g., either cocooned within high-covering-factor media or indicative of recent triggering, though their nature remains unclear. Here, we present the first targeted simultaneous X-ray observations of an OQQ, our previously identified prototype, SDSS J075139.06+402811.2 at z = 0.587. The source is significantly detected over 0.5–16 keV with XMM-Newton and NuSTAR, unambiguously confirming the presence of current accretion activity. Spectral modeling yields an intrinsic luminosity L 2–10 keV ≈ 4.4 × 1043 erg s−1, well within the AGN regime, but underluminous relative to its infrared power. It is lightly obscured, with log N H [cm−2] ≈ 22.

Multiwavelength campaign on the Super-Eddington NLS1 RX J0134.2-4258 – I. Peculiar X-ray spectra and variability

ABSTRACTWe have conducted a new long-term multiwavelength campaign on one of the most super-Eddington narrow-line Seyfert 1s (NLS1s) known, namely RX J0134.2-4258. In this first paper, we report deep simultaneous X-ray observations performed by XMM–Newton and NuSTAR on 2019 December 19, during which RX J0134.2-4258 was fortuitously at one of its lowest X-ray flux states. However, there is a clear rise above 4 keV which implies that the intrinsic source flux may be higher. The X-ray spectra observed between 1996 and 2019 show drastic variability, probably due to complex, variable absorption along the line of sight. Unusually, the soft X-ray excess appears extremely weak in all these spectra, even when the hard X-ray spectrum has a steep slope of Γ ≃ 2.2. We explore the spectral-timing properties of the new (low X-ray flux) and archival (high X-ray flux) XMM–Newton data, fitting their time-average, rms, and lag spectra simultaneously. The variability spectra indicate the presence of a very weak soft X-ray Comptonization component, whose shape is similar to the soft excess in normal super-Eddington NLS1s, but with flux relative to the power law which is lower by more than one order of magnitude. Above 4 keV the low-flux data are dominated by a different component, which lags with respect to the lower energy emission. This is consistent with an origin of reflection or partial covering absorption from low ionization material located within 100 Rg. We interpret this as further indication of the presence of a clumpy disc wind.

The Origin of The Soft X-Ray Excess in the Seyfert 1.5 Galaxy ESO 362-G18

We review the Seyfert 1.5 Galaxy ESO 362-G18 for exploring the origin of the soft X-ray excess. The Warm Corona and Relativistic Reflection models are two main scenarios to interpret the soft X-ray excess in active galactic nuclei at present. We use the simultaneous X-ray observation data of XMM-Newton and NuSTAR on 2016 September 24 to perform spectral analysis in two steps. First, we analyze the time-average spectra by using Warm Corona and Relativistic Reflection models. Moreover, we also explore the Hybrid model, Double Reflection model and Double Warm Corona model. We find that both the Warm Corona and Relativistic Reflection models can interpret the time-average spectra well but cannot be distinguished easily based on the time-averaged spectra fit statistics. Second, we add the rms and covariance spectra to perform the spectral analysis with time-average spectra. The result shows that the warm corona could reproduce all of these spectra well. The hot, optical thin corona and neutral distant reflection will increase their contribution with the temporal frequency, meaning that the corona responsible for X-ray continuum comes from the inner compact X-ray region and the neutral distant reflection is made of some moderate scale neutral clumps.

A connection between accretion states and the formation of ultrarelativistic outflows in a neutron star X-ray binary

The nearby accreting neutron star binary Sco X-1 is the closest example of ongoing relativistic jet production at high Eddington ratios. Previous radio studies have revealed that alongside mildly relativistic, radio-emitting ejecta, there is at times a much faster transfer of energy from the region of the accretion flow along the jet. The nature of this ultrarelativistic flow remains unclear and while there is some evidence for a similar phenomenon in other systems which might contain neutron stars, it has never been observed in a confirmed black hole system. We have compared these previous radio observations with a new analysis of simultaneous X-ray observations which were performed with the RXTE mission. We find that the ejection of the ultra-relativistic flow seems to be associated with the simultaneous appearance of two particular types of quasi-periodic oscillations in the X-ray power spectrum. In contrast, the mildly relativistic, radio-emitting outflows may be associated with flat-topped broad band noise in the X-ray power spectrum. This is the first time a link, albeit tentative, has been found between these mysterious unseen flows and the accretion flow from which they are launched.

Optical/X-ray correlations during the V404 Cygni June 2015 outburst

Context. We present a multiwavelength analysis of the simultaneous optical and X-ray light curves of the microquasar V404 Cyg during the June 2015 outburst. Aims. We have performed a comprehensive analysis of all the INTEGRAL/IBIS, JEM–X, and OMC observations during the brightest epoch of the outburst, along with complementary NuSTAR, AAVSO, and VSNET data, to examine the timing relationship between the simultaneous optical and X-ray light curves, in order to understand the emission mechanisms and physical locations. Methods. We have identified all optical flares that have simultaneous X-ray observations, and performed a cross-correlation analysis to estimate the time delays between the optical and soft and hard X-ray emission. We also compared the evolution of the optical and X-ray emission with the hardness ratios. Results. We have identified several types of behaviour during the outburst. On many occasions, the optical flares occur simultaneously with X-ray flares, but at other times, positive and negative time delays between the optical and X-ray emission are measured. Conclusions. We conclude that the observed optical variability is driven by different physical mechanisms, including reprocessing of X-rays in the accretion disc and/or the companion star, interaction of the jet ejections with surrounding material or with previously ejected blobs, and synchrotron emission from the jet.

Discrete knot ejection from the jet in a nearby low-luminosity active galactic nucleus, M81∗

Observational constraints of relativistic jets from black holes has largely come from the most powerful and extended jets\cite{Jorstad05,Asada14}, leaving the nature of the low luminosity jets a mystery\cite{Falcke04}. M81* is one of the nearest low-luminosity jets, which underwent an extremely large radio flare in 2011, allowing us to study compact core emission with unprecedented sensitivity and linear resolution. Utilizing a multi-wavelength campaign, we were able to track the flare as it re-brightened and became optically thick. Simultaneous X-ray observations indicated the radio re-brightening was preceded by a low energy X-ray flare at least $t_{\rm delay}>12\ {\rm days}$ prior. Associating the time delay between the two bands as the cooling time in a synchrotron flare\cite{Urry97,Bai03}, we find the magnetic field strength was $1.9<B<9.2\ {\rm G}$, which is consistent with magnetic field estimate from spectral-energy distribution modeling\cite{Kellerman81}, $B<10.2\ {\rm G}$. In addition, VLBA observations at 23 GHz clearly illustrate a discrete knot moving mildly relativistically at $v_{\rm app}/c=0.51\pm0.17$ associated with the initial radio flare. The observations indicate radial jet motions for the first time in M81*. This has profound implications for jet production, as it means radial motion can be observed in even the lowest-luminosity AGN, but at slower velocities and smaller radial extents ($\approx10^4\ R_{\rm G}$).

On proton synchrotron blazar models: the case of quasar 3C 279

In the present work we propose an innovative estimation method for the minimum Doppler factor and energy content of the gamma-ray emitting region of quasar 3C 279, using a standard proton synchrotron blazar model and the principles of automatic photon quenching. The latter becomes relevant for high enough magnetic fields and results in spontaneous annihilation of gamma-rays. The absorbed energy is then redistributed into electron-positron pairs and soft radiation. We show that as quenching sets an upper value for the source rest-frame gamma-ray luminosity, one has, by neccessity, to resort to Doppler factors that lie above a certain value in order to explain the TeV observations. The existence of this lower limit for the Doppler factor has also implications on the energetics of the emitting region. In this aspect, the proposed method can be regarded as an extension of the widely used one for estimating the equipartition magnetic field using radio observations. In our case, the leptonic synchrotron component is replaced by the proton synchrotron emission and the radio by the VHE gamma-ray observations. We show specifically that one can model the TeV observations by using parameter values that minimize both the energy density and the jet power at the cost of high-values of the Doppler factor. On the other hand, the modelling can also be done by using the minimum possible Doppler factor; this, however, leads to a particle dominated region and high jet power for a wide range of magnetic field values. Despite the fact that we have focused on the case of 3C 279, our analysis can be of relevance to all TeV blazars favoring hadronic modelling that have, moreover, simultaneous X-ray observations.

XMM-Newton and INTEGRAL observations of the very faint X-ray transient IGR J17285−2922/XTE J1728−295 during the 2010 outburst

We report the first broad-band (0.5–150 keV) simultaneous X-ray observations of the very faint X-ray transient IGR J17285−2922/XTE J1728−295 performed with the XMM–Newton and INTEGRAL satellites during its last outburst, started on 2010 August 28. XMM–Newton observed the source on 2010 September 9–10 for 22 ks. INTEGRAL observations were part of the publicly available Galactic bulge program, and overlapped with the times covered by XMM–Newton. The broad-band spectroscopy resulted in a best fit with an absorbed power law displaying a photon index Γ of 1.61 ± 0.01, an absorbing column density NH of (5.10 ± 0.05) × 1021 cm−2 and a flux of 2.4 × 10−10 erg cm−2 s−1 (1–100 keV), corrected for the absorption. The data did not require either a spectral cut-off (Ec > 50 keV) or an additional soft component. The slopes of the power law fitting the XMM–Newton and INTEGRAL separate spectra were compatible, within the uncertainties. The timing analysis does not show evidence either for X-ray pulsations or for type I X-ray bursts. The broad-band X-ray spectrum as well as the power-density spectrum is indicative of a low-hard state in a low-mass X-ray binary, although nothing conclusive can be said about the nature of the compact object (neutron star or black hole). The results we report here allow us to conclude that IGR J17285−2922 is a low-mass X-ray binary, located at a distance greater than 4 kpc.

EXTREME ULTRAVIOLET EXPLOREROBSERVATIONS OF HERCULES X-1 OVER A 35 DAY CYCLE

Observations of Hercules X-1 by the Extreme Ultraviolet Explorer covering most of the 35 day cycle are reported here. This is the only long extreme ultraviolet (EUV) observation of Her X-1. Simultaneous X-ray observations with the Rossi X-ray Timing Explorer All-Sky Monitor (RXTE/ASM) X-ray show that Her X-1 is in an X-ray anomalous low state. The first 4 days are also observed with the RXTE proportional counter array (PCA), which shows that the X-ray properties are nearly the same as for normal low states in Her X-1 with flux reduced by a factor of 2. In contrast, the EUV emission from Her X-1 is reduced by a factor of ∼4 compared to normal low states. The twisted-tilted accretion disk responsible for the normal 35 day X-ray cycle can be modified to explain this behavior. An increased disk twist reduces the X-ray illumination of HZ Her by a factor of ∼2 and of the disk surface by a somewhat larger factor, leading to a larger reduction in EUV flux compared to X-ray flux.

Dynamics of shear-induced orientation transitions in block copolymers

This research investigates the dynamics of mechanical and structural transformations of styrene–butadiene–styrene block copolymer, known as RCM1, which has cylindrical mesophase morphology. The polymer was processed using a Multipass Rheometer (MPR) with simultaneous X-ray observation. The method allowed investigation of the dynamics with 1 second time resolution. Samples were processed at 160 °C and an apparent wall shear rate in the range ∼1.4 to 1125 s−1. Microstructural alignment was quantified using a second order X-ray orientation function. The polymer showed self-organization in the melt, aligning along the capillary walls. Shear flow disrupted the organization, which then rebuilt during the post-shearing pause, as the residual pressure relaxed across the capillary. The dynamics of structure ordering and disordering was a function of shear rate. The transition was faster for high shear rate cycles.

Further Evidence for Variable Synchrotron Emission in XTE J1118+480 in Outburst

We present simultaneous multicolor infrared and optical photometry of the black hole X-ray transient XTE J1118+480 during its short 2005 January outburst, supported by simultaneous X-ray observations. The variability is dominated by short timescales, ~10 s, although a weak superhump also appears to be present in the optical. The optical rapid variations, at least, are well correlated with those in X-rays. Infrared JHK_s photometry, as in the previous outburst, exhibits especially large-amplitude variability. The spectral energy distribution (SED) of the variable infrared component can be fitted with a power law of slope α = -0.78 ± 0.07, where F_ν ∝ ν_α. There is no compelling evidence for evolution in the slope over five nights, during which time the source brightness decayed along almost the same track as seen in variations within the nights. We conclude that both short-term variability and longer timescale fading are dominated by a single component of constant spectral shape. We cannot fit the SED of the IR variability with a credible thermal component, either optically thick or thin. This IR SED is, however, approximately consistent with optically thin synchrotron emission from a jet. These observations therefore provide indirect evidence to support jet-dominated models for XTE J1118+480 and also provide a direct measurement of the slope of the optically thin emission, which is impossible, based on the average spectral energy distribution alone.

Simultaneous optical and X-ray observations of flares and rotational modulation on the RS CVn binary HR 1099 (V711 Tau) from the MUSICOS 1998 campaign

We present simultaneous and continuous observations of the Hα, Hβ, He I D_3, Na I D_1,D_2 doublet and the Ca II H&K lines for the RS CVn system HR 1099. The spectroscopic observations were obtained during the MUSICOS 1998 campaign involving several observatories and instruments, both echelle and long-slit spectrographs. During this campaign, HR 1099 was observed almost continuously for more than 8 orbits of 2^d.8. Two large optical flares were observed, both showing an increase in the emission of Hα, Ca II H K, Hβ and He I D_3 and a strong filling-in of the Na I D_1, D_2 doublet. Contemporary photometric observations were carried out with the robotic telescopes APT-80 of Catania and Phoenix-25 of Fairborn Observatories. Maps of the distribution of the spotted regions on the photosphere of the binary components were derived using the Maximum Entropy and Tikhonov photometric regularization criteria. Rotational modulation was observed in Hα and He I D_3 in anti-correlation with the photometric light curves. Both flares occurred at the same binary phase (0.85), suggesting that these events took place in the same active region. Simultaneous X-ray observations, performed by ASM on board RXTE, show several flare-like events, some of which correlate well with the observed optical flares. Rotational modulation in the X-ray light curve has been detected with minimum flux when the less active G5 V star was in front. A possible periodicity in the X-ray flare-like events was also found.

Far-Ultraviolet Observations of NGC 3516 Using the Hopkins Ultraviolet Telescope

We observed the Seyfert 1 galaxy NGC 3516 twice during the flight of Astro-2 using the Hopkins Ultraviolet Telescope in March 1995. Simultaneous X-ray observations were performed with ASCA. Our far-ultraviolet spectra cover the spectral range 820-1840 A with a resolution of 2-4 A. No significant variations were found between the two observations. The total spectrum shows a red continuum, $f_\nu \sim \nu^{-1.89}$, with an observed flux of $\rm 2.2 \times 10^{-14}~erg~cm^{-2}~s^{-1}~\AA^{-1}$ at 1450 A, slightly above the historical mean. Intrinsic absorption in Lyman $\beta$ is visible as well as absorption from O~vi 1032,1038, N~v 1239,1243, Si~iv 1394,1403, and C~iv 1548,1551. The UV absorption lines are far weaker than is usual for NGC~3516, and also lie closer to the emission line redshift rather than showing the blueshift typical of these lines when they are strong. The neutral hydrogen absorption, however, is blueshifted by $400~\rm km~s^{-1}$ relative to the systemic velocity, and it is opaque at the Lyman limit. The sharpness of the cutoff indicates a low effective Doppler parameter, $b < \rm 20~km~s^{-1}$. For $b = \rm 10~km~s^{-1}$ the derived intrinsic column is $\rm 3.5 \times 10^{17}~cm^{-2}$. As in NGC~4151, a single warm absorber cannot produce the strong absorption visible over the wide range of observed ionization states. Matching both the UV and X-ray absorption simultaneously requires absorbers spanning a range of $10^3$ in both ionization parameter and column density.

Photometry of slow X-ray pulsars. II - The 13.9 minute period of X Persei

Results are presented for time-resolved narrow-band photometry and spectrophotometry of X Per performed in an unsuccessful effort to confirm previously reported observations of 13.9-min pulsations in the intensity of the He II line at 4686 A. No features that are synchronous with a 13.9-min period are found in the optical data, and simultaneous X-ray observations of 3U 0352+30 are reported which show that a strong 13.9-min X-ray modulation was present during the optical photometry. Some implications of the X-ray periodicities observed for X Per are considered.