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Register a new userDiscovery of a pulsating Fe Ka line in GX 301-2
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We report on the detection of a pulsating Fe Ka line in the High Mass X-ray Binary (HMXB) GX 301-2, from a 40-ks Chandra observation near periastron. The pulsations in the Fe Ka emission appeared only in the first 7 ks of the observation, with a period and phase profile similar to those of the continuum. The presence of pulsed fluorescent lines is an unusual property in HMXBs. After 7 ks, the continuum flux increased by a factor of three, the Fe Ka flux increased only by about 10%, and the pulsating signal in the line disappeared. Finally, in the second half of the observation, both the continuum and the line flux dropped by a similar factor of 2. We suggest that the pulsating component of the Fe Ka line is coming from a transient non-isotropic distribution of dense gas around the neutron star, for example an accretion stream induced by periastron passage, or from the illuminated surface of the donor star.
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We present two observations of the high-mass X-ray binary GX 301-2 with NuSTAR, taken at different orbital phases and different luminosities. We find that the continuum is well described by typical phenomenological models, like a very strongly absorbed NPEX model. However, for a statistically acceptable description of the hard X-ray spectrum we require two cyclotron resonant scattering features (CRSF), one at ~35 keV and the other at ~50 keV. Even though both features strongly overlap, the good resolution and sensitivity of NuSTAR allows us to disentangle them at >=99.9% significance. This is the first time that two CRSFs are seen in GX 301-2. We find that the CRSFs are very likely independently formed, as their energies are not harmonically related and, if it were a single line, the deviation from a Gaussian shape would be very large. We compare our results to archival Suzaku data and find that our model also provides a good fit to those data. We study the behavior of the continuum as well as the CRSF parameters as function of pulse phase in seven phase bins. We find that the energy of the 35 keV CRSF varies smoothly as function of phase, between 30-38 keV. To explain this variation, we apply a simple model of the accretion column, taking the altitude of the line-forming region, the velocity of the in-falling material, and the resulting relativistic effects into account. We find that in this model the observed energy variation can be explained simply due to a variation of the projected velocity and beaming factor of the line forming region towards us.
We present the results of an in-depth study of the long-period X-ray pulsar GX 301-2. Using archival data of INTEGRAL, RXTE ASM, and CGRO BATSE, we study the spectral and timing properties of the source. Comparison of our timing results with previously published work reveals a secular decay of the orbital period at a rate of simeq -3.25 times 10^{-5} d yr^{-1}, which is an order of magnitude faster than for other known systems. We argue that this is probably result either of the apsidal motion or of gravitational coupling of the matter lost by the optical companion with the neutron star, although current observations do not allow us to distinguish between those possibilities. We also propose a model to explain the observed long pulse period. We find that a very strong magnetic field B sim 10^{14} G can explain the observed pulse period in the framework of existing models for torques affecting the neutron star. We show that the apparent contradiction with the magnetic field strength B_{CRSF} sim 4 times 10^{12} G derived from the observed cyclotron line position may be resolved if the line formation region resides in a tall accretion column of height sim 2.5 - 3 R_{NS}. The color temperature measured from the spectrum suggests that such a column may indeed be present, and our estimates show that its height is sufficient to explain the observed cyclotron line position.
X-ray fluorescent lines are unique features of the reflection spectrum of the cold torus when irradiated by the central AGN. Their intrinsic line widths can be used to probe the line-emitting region. The line widths of the Fe Ka line measured from the first order Chandra High Energy Grating (HEG) spectra are $3-5$ times larger than those measured with the Si Ka line for Circinus, Mrk 3, and NGC 1068. Because the observed Si Ka and Fe Ka lines are not necessarily coming from the same physical region, it is uncertain whether the line widths of the Fe Ka line are over-estimated or not. We measured the intrinsic line widths of the Fe Ka line of several nearby bright AGN using the second and third order Chandra HEG spectra, whose spectral resolutions are better than the first order data. We found the measured widths are all smaller than those from the first order data. The results clearly show that the widths of the Fe Ka line measured from the first order HEG data are over-estimated. It indicates that the Fe Ka lines of the studied sources are originating from regions around the cold dusty torus.
GX 301-2 provides a rare opportunity to study both disk and wind accretion in a same target. We report Insight-HXMT observations of the spin-up event of GX 301-2 happened in 2019 and compare with those of wind-fed state. The pulse profiles of the initial rapid spin-up period are dominated by one main peak, while those of the later slow spin-up period are composed of two similar peaks, as those of wind-fed state. These behaviors are confirmed by Fermi/GBM data, which also show that during the rapid spin-up period, the main peak increases with luminosity up to $8times10^{37}$ erg s$^{-1}$, but the faint peak keeps almost constant. The absorption column densities during the spin-up period are $sim1.5times10^{23}$ cm$^{-2}$, much less than those of wind-fed state at similar luminosity ($sim9times10^{23}$ cm$^{-2}$), supporting the scenario that most of material is condensed into a disk during the spin-up period. We discuss possible differences between disk and wind accretion that may explain the observed different trend of pulse profiles.
We investigate the absorption and emission features in observations of GX 301-2 detected with Insight-HXMT/LE in 2017-2019. At different orbital phases, we found prominent Fe Kalpha, Kbeta and Ni Kalpha lines, as well as Compton shoulders and Fe K-shell absorption edges. These features are due to the X-ray reprocessing caused by the interaction between the radiation from the source and surrounding accretion material. According to the ratio of iron lines Kalpha and Kbeta, we infer the accretion material is in a low ionisation state. We find an orbital-dependent local absorption column density, which has a large value and strong variability around the periastron. We explain its variability as a result of inhomogeneities of the accretion environment and/or instabilities of accretion processes. In addition, the variable local column density is correlated with the equivalent width of the iron Kalpha lines throughout the orbit, which suggests that the accretion material near the neutron star is spherically distributed.
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