Luminosity-dependent spectral and timing properties of the accreting pulsar GX 304-1 measured with INTEGRAL

Context: Be/X-ray binaries show outbursts with peak luminosities up to a few times $10^{37}\,$erg/s, during which they can be observed and studied in detail. Most (if not all) Be/X-ray binaries harbour accreting pulsars, whose X-ray spectra in many cases contain cyclotron resonant scattering features related to the magnetic field of the sources. Spectral variations as a function of luminosity and of the rotational phase of the neutron star are observed in many accreting pulsars. Aims: We explore X-ray spectral and timing properties of the Be/X-ray binary GX 304-1 during an outburst episode. Specifically, we investigate the behavior of the cyclotron resonant scattering feature, the continuum spectral parameters, the pulse period, and the energy- and luminosity-resolved pulse profiles. We combine the luminosity-resolved spectral and timing analysis to probe the accretion geometry and the beaming patterns of the rotating neutron star. Methods: We analyze the INTEGRAL data from the two JEM-X modules, ISGRI and SPI, covering the January-February 2012 outburst, divided in six observations. We obtain pulse profiles in two energy bands, phase-averaged and phase-resolved spectra for each observation. Results: We confirm the positive luminosity-dependence of the cyclotron line energy in GX 304-1, and report a dependence of the photon index on luminosity. Using a pulse-phase connection technique, we find a pulse period solution valid for the entire outburst. Our pulse-phase resolved analysis shows, that the centroid energy of the cyclotron line is varying only slightly with pulse phase, while other spectral parameters show more pronounced variations. Our results are consistent with a scenario in which, as the pulsar rotates, we are exploring only a small portion of its beam pattern.Comment: 12 pages, 12 figures, Accepted for publication in A&

X-ray spectral and timing properties of the High Mass X-ray Binaries GX 304-1 and Vela X-1

Accreting X-ray pulsars are binary systems hosting a rotating, highly magnetized neutron star, and an optical companion star whose expelled plasma is accreted onto the compact object, a process that eventually leads to pulsed X-ray emission. Discovered more than forty years ago, substantial progress has been reached in the comprehension of these objects, in terms of both their individual binary components and of the X-ray emission properties. However, further efforts (both observational and theoretical) are needed to constrain the many unsolved aspects. Among the key open issues there are the details of the emission mechanism, the geometry and the radiation beaming pattern of the accretion structure, as well as the influence of the optical companion on the observed X-ray properties. This thesis is focused on the spectroscopical and timing analysis of two accreting X-ray pulsars: the Be/X-ray Binary GX 304-1, and the wind-fed binary Vela X-1. INTEGRAL data (5 − 100 keV) have been used for the analysis of GX 304-1, which was observed during a bright outburst in 2012. MAXI data (2 − 20 keV), on the other hand, provide continuous observations of Vela X-1 since 2009 and therefore offer a valuable opportunity for studying its orbital variability. The INTEGRAL analysis of GX 304-1 allowed to characterize the spectral and timing behaviour of the source as a function of the luminosity. A timing solution valid throughout the outburst has been found, which allowed to construct pulse profiles at different luminosities and for different energy bands. The pulse profiles appear strongly luminosity-dependent, thus suggesting a change of the radiation beam pattern with luminosity. Pulse profiles also appear strongly energy-dependent, possibly due to a geometrical effect arising from the rotation of the neutron star or to a distorted magnetic dipole field. Also, contrary to other accreting pulsars, GX 304-1 pulse profiles show only a small pulsed fraction that does not correlate with energy nor luminosity. The pulse period has negative derivative during the accretion phase, implying a spin up episode, and likely indicating the presence of an accretion disk. The pulse-phase averaged spectroscopy confirms the correlation between the cyclotron line energy and the luminosity, with a more precise detector photon energy calibration, and therefore a better estimation of the neutron star magnetic field and of its critical luminosity. The spectral photon index and folding energy have been found to be negatively correlated with luminosity (as expected for a Comptonization spectrum), thus suggesting that Compton cooling becomes more efficient at higher luminosities. The timing solution has also been used to identify pulse-phases of the rotating neutron star, which allowed to perform pulse-phase resolved spectroscopy. Pulse-phase resolved spectra of individual INTEGRAL observations show a variation of the cyclotron line energy up to ~16% with pulse phase. Furthermore, pulse-phase resolved spectra of stacked observations have been analyzed, again favoring a small variation of the cyclotron line energy with pulse phase. These results can be interpreted in terms of a geometrical configuration such that the observer is exploring only a small part of the radiation beam pattern emerging from the accretion structure, thus causing the spectral parameters to result mostly insensitive to the pulsar rotation, a scenario that is also supported by the observed small pulsed fraction. Concerning Vela X-1, spectral analysis on the sub-orbital timescale has been performed, to explore the light curve features and the effects of large scale (i.e. of the order of the optical companion size) structures that are known to affect the binary system. Such studies are important to constrain the stellar wind properties and to study the mechanisms behind the high X-ray variability typical of such sources. First, a sample of double-peaked orbital light curves has been extracted from the entire population of orbital profiles. The double-peaked sample shows a dip around the inferior conjunction that is difficult to explain invoking absorption by neutral matter alone. Instead, Thomson scattering from an extended and ionized accretion wake is shown to be a possible reason for the observed dip. Orbital-phase resolved spectra show that a phenomenological cutoff power-law model, commonly used in literature for accreting pulsars, leads to orbital variation of the photon index, thus it is likely inadequate to describe the Vela X-1 spectral properties. The addition of a partial covering component to model certain orbital phase-bins spectra avoids the photon index modulation and offers a physical interpretation: a highly structured ambient wind that affects only a part of the original X-ray emission, leaving the other part unaffected. The partial covering component is usually attributed to the clumpy nature of the stellar wind, but the observed orbital modulation of such a component does not favor this interpretation. The necessity of such a component in Vela X-1 seems to be more likely due to either a wobbling, or to an intrinsically structured accretion wake. Due to the ability of the neutron star traveling in the ambient wind to develop density inhomogeneities (i.e. the structured accretion wake), the compact object itself can be considered responsible for the formation of clumps which are then accreted, thus feeding the observed high X-ray variability typical of this system

Timing the X-ray pulsating companion of the hot-subdwarf HD 49798 with NICER

HD 49798 is a hot subdwarf of O spectral type in a 1.55 day orbit with the X-ray source RX J0648.0-4418, a compact object with spin period of 13.2 s. We use recent data from the NICER instrument, joined with archival data from XMM-Newton and ROSAT, to obtain a phase-connected timing solution spanning ~30 years. Contrary to previous works, that relied on parameters determined through optical observations, the new timing solution could be derived using only X-ray data. We confirm that the compact object is steadily spinning up with Pdot = -2.28(2)x10^-15 s/s and obtain a refined measure of the projected semi-major axis of the compact object aX sini = 9.60(5) lightsec. This allows us to determine the inclination and masses of the system as i = 84.5(7) deg, MX = 1.220(8) Msun and Mopt = 1.41(2) Msun. We also study possible long term (~year) and orbital variations of the soft X-ray pulsed flux, without finding evidence for variability. In the light of the new findings, we discuss the nature of the compact object, concluding that the possibility of a neutron star in the subsonic propeller regime is unlikely, while accretion of the subdwarf wind onto a massive white dwarf can explain the observed luminosity and spin-up rate for a wind velocity of ~800 km/s.Comment: Accepted for publication in MNRAS, 7 pages, 4 figures, 2 table

10 Years of Accreting Pulsars with Fermi GBM

No abstract availabl

Discovery of spin-phase dependent QPOs in the super-critical accretion regime from the X-ray pulsar RX J0440.9+4431

RX J0440.9+4431 is an accreting X-ray pulsar (XRP) that remained relatively unexplored until recently, when major X-ray outburst activity enabled more in-depth studies. Here, we report on the discovery of ${\sim}0.2$ Hz quasi-periodic oscillations (QPOs) from this source observed with $Fermi$-GBM. The appearance of QPOs in RX J0440.9+4431 is thricely transient, that is, QPOs appear only above a certain luminosity, only at certain pulse phases (namely corresponding to the peak of its sine-like pulse profile), and only for a few oscillations at time. We argue that this newly discovered phenomenon (appearance of thricely transient QPOs -- or ATTO) occurs if QPOs are fed through an accretion disk whose inner region viscosity is unstable to mass accretion rate and temperature variations. Such variations are triggered when the source switches to the super-critical accretion regime and the emission pattern changes. We also argue that the emission region configuration is likely responsible for the observed QPOs spin-phase dependence.Comment: 5 + 2 appendix pages. Accepted on A&A. Proofs versio

X-ray pulsar XTE J1858+034: discovery of the cyclotron line and the revised optical identification

We present results of a detailed investigation of the poorly studied X-ray pulsar XTE J1858+034 based on the data obtained with the NuSTAR observatory during the outburst of the source in 2019. The spectral analysis resulted in the discovery of a cyclotron absorption feature in the source spectrum at ~48 keV both in the pulse phase averaged and resolved spectra. Accurate X-ray localization of the source using the NuSTAR and Chandra observatories allowed us to accurately determine the position of the X-ray source and identify the optical companion of the pulsar. The analysis of the counterpart properties suggested that the system is likely a symbiotic binary hosting an X-ray pulsar and a late type companion star of K-M classes rather than Be X-ray binary as previously suggested.Comment: 12 pages, 12 figures, accepted by Ap

Probing spectral and timing properties of the X-ray pulsar RX J0440.9+4431 in the giant outburst of 2022-2023

The X-ray pulsar RX J0440.9+4431 went through a giant outburst in 2022 and reached a record-high flux of 2.3 Crab, as observed by Swift/BAT. We study the evolution of different spectral and timing properties of the source using NICER observations. The pulse period is found to decrease from 208 s to 205 s, and the pulse profile evolves significantly with energy and luminosity. The hardness ratio and hardness intensity diagram (HID) show remarkable evolution during the outburst. The HID turns towards the diagonal branch from the horizontal branch above a transition (critical) luminosity, suggesting the presence of two accretion modes. Each NICER spectrum can be described using a cutoff power law with a blackbody component and a Gaussian at 6.4 keV. At higher luminosities, an additional Gaussian at 6.67 keV is used. The observed photon index shows negative and positive correlations with X-ray flux below and above the critical luminosity, respectively. The evolution of spectral and timing parameters suggests a possible change in the emission mechanism and beaming pattern of the pulsar depending on the spectral transition to sub- and super-critical accretion regimes. Based on the critical luminosity, the magnetic field of the neutron star can be estimated in the order of 10$^{12}$ or 10$^{13}$ G, assuming different theoretical models. Moreover, the observed iron emission line evolves from a narrow to a broad feature with luminosity. Two emission lines originating from neutral and highly ionized Fe atoms were evident in the spectra around 6.4 keV and 6.67 keV (higher luminosities).Comment: Published in Monthly Notices of the Royal Astronomical Societ

Thermonuclear X-ray Bursts with late secondary peaks observed from 4U 1608-52

We report the temporal and spectral analysis of three thermonuclear X-ray bursts from 4U 1608-52, observed by the Neutron Star Interior Composition Explorer (NICER) during and just after the outburst observed from the source in 2020. In two of the X-ray bursts, we detect secondary peaks, 30 and 18 seconds after the initial peaks. The secondary peaks show a fast rise exponential decay-like shape resembling a thermonuclear X-ray burst. Time-resolved X-ray spectral analysis reveals that the peak flux, blackbody temperature, and apparent emitting radius values of the initial peaks are in agreement with X-ray bursts previously observed from 4U 1608-52, while the same values for the secondary peaks tend toward the lower end of the distribution of bursts observed from this source. The third X-ray burst, which happened during much lower accretion rates did not show any evidence for a deviation from an exponential decay and was significantly brighter than the previous bursts. We present the properties of the secondary peaks and discuss the events within the framework of short recurrence time bursts or bursts with secondary peaks. We find that the current observations do not fit in standard scenarios and challenge our understanding of flame spreading.Comment: Accepted for publication in the Astrophysical Journa

Accreting on the edge: a luminosity-dependent cyclotron line in the Be/X-ray Binary 2S 1553-542 accompanied by accretion regimes transition

Accreting X-ray pulsars (XRPs) undergo luminous X-ray outbursts during which the luminosity-dependent spectral and timing features of the neutron star's emission can be analyzed in detail, thus shedding light on the accretion regime at work. We took advantage of a monitoring campaign performed with NuSTAR, Swift/XRT, AstroSat and NICER, to follow the Be/X-ray Binary 2S 1553-542 along one of its rare outbursts and trace its spectral and timing evolution. We report the discovery of a luminosity-dependent cyclotron line energy for the first time in this source. The pulse profiles and pulsed fraction also show variability along the outburst, consistently with the interpretation that the source transitions from the sub-critical to the super-critical accretion regime, separated by a critical luminosity of L$_{crit}\approx4\times10^{37}$ erg/s.Comment: Accepted on ApJ. 11 pages, 7 figures, 3 table