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Register a new userBATSE SD Observations of Hercules X-1
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Authors
P. E. Freeman
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The cyclotron line in the spectrum of the accretion-powered pulsar Her X-1 offers an opportunity to assess the ability of the BATSE Spectroscopy Detectors (SDs) to detect lines like those seen in some GRBs. Preliminary analysis of an initial SD pulsar mode observation of Her X-1 indicated a cyclotron line at an energy of approximately 44 keV, rather than at the expected energy of approximately 36 keV. Our analysis of four SD pulsar mode observations of Her X-1 made during high-states of its 35 day cycle confirms this result. We consider a number of phenomenological models for the continuum spectrum and the cyclotron line. This ensures that we use the simplest models that adequately describe the data, and that our results are robust. We find modest evidence (significance Q ~ 10^-4-10^-2) for a line at approximately 44 keV in the data of the first observation. Joint fits to the four observations provide stronger evidence (Q ~ 10^-7-10^-4) for the line. Such a shift in the cyclotron line energy of an accretion-powered pulsar is unprecedented.
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The accretion-powered pulsar Her X-1 was observed with Suzaku twice in its main-on state, on 2005 October 5-6 and 2006 March 29-30, for a net exposure of 30.5 ks and 34.4 ks, respectively. In the 2005 and 2006 observations, the source was detected at an average 10-30 keV intensity of 290 mCrab and 230 mCrab, respectively. The intrinsic pulse period was measured on both occasions at 1.23776 s by HXD-PIN, after barycentric and binary corrections. The pulse phase-averaged spectra in the energy range above 10 keV are well fitted by ``Negative and Positive power-law times EXponential (NPEX) model, multiplied by a fundamental cyclotron resonance scattering feature at ~36 keV which appears very significantly in the HXD-PIN data. The resonance profiles were reproduced successfully by the Lorentzian type scattering cross section, rather than by a Gaussian type alternative. The pulse phase-averaged HXD-GSO data, covering 50-120 keV, are featureless. However, in a differential spectrum between the pulse-decay phase and off-pulse phase, the second harmonic cyclotron resonance was detected in the GSO data at ~73 keV, with a depth of 1.6_-0.7^+0.9. This makes Her X-1 a 6th pulsar with established second harmonic resonance. Implications of these results are briefly discussed.
We present RXTE monitoring of the eclipsing X-ray binary Hercules X-1 conducted over the short-high state of July 1998. This was one of the last major short-high states before the source entered an anomalous low-state of activity. A comparison with previous epochs finds no evidence for special behavior during these observations. We determine orbital and pulsar spin periods to facilitate measurements period derivatives during the subsequent anomalous low state and during the next epoch of high-state activity. Spectrally, the decay of the short-high state and concurrent pre-eclipse dips are consistent with obscuration of a central X-ray source by a cloud of non-uniform column density. The standard model of a warped accretion disk of finite vertical scale height fits the characteristics of this absorber well. Pre-eclipse dips have durations a factor of a few longer than the characteristic durations of dips during main-high states. Pulse profile structure increases in complexity towards the tail of the short-high state suggesting changes in accretion curtain geometry.
We find line emission from the hydrogen- and/or helium-like ions of Ne, O, N and C in the low and short-on states of Her X-1, using the XMM-Newton Reflection Grating Spectrometer. The emission line velocity broadening is 200 < sigma < 500 km/s. Plasma diagnostics with the Ne IX, O VII and N VI He-alpha lines and the radiative recombination continua of O VII and N VII, indicate the gas is heated by photoionization. We use spectral models to measure the element abundance ratios N/O, C/O, and Ne/O, which quantify CNO processing in HZ Her. Photoexcitation and high-density effects are not differentiated by the measured He-alpha lines. We set limits on the location, temperature and density of the line emission region. The narrow emission lines can be attributed to reprocessing in either an accretion disk atmosphere and corona or on the X-ray illuminated face of HZ Her. In the main-on state, the bright continuum only allows the detection of interstellar absorption, plus O VII He-alpha emission lines with sigma = 3200 +- 700 km/s and complex profiles. Other broad lines may be present. The broad lines may originate in a region near the pulsar magnetosphere. Fe L lines are not detected.
The UV emission lines of Hercules X-1, resolved with the HST GHRS and STIS, can be divided into broad (FWHM 750 km/s) and narrow (FWHM 150 km/s) components. The broad lines can be unambiguously identified with emission from an accretion disk which rotates prograde with the orbit. The narrow lines, previously identified with the X-ray illuminated atmosphere of the companion star, are blueshifted at both phi=0.2 and phi=0.8 and the line flux at phi=0.2 is 0.2 of the flux at phi=0.8. Line ratio diagnostics show that the density of the narrow line region is log n=13.4+/-0.2 and the temperature is T=1.0+/-0.2x10^5 K. The symmetry of the eclipse ingress suggests that the line emission on the surface of the disk is left-right symmetric relative to the orbit. Model fits to the O V, Si IV, and He II line profiles agree with this result, but fits to the N V lines suggest that the receding side of the disk is brighter. We note that there are narrow absorption components in the N V lines with blueshifts of 500 km/s.
The long-term evolution of the centroid energy of the CRSF in Her X-1 is still a mystery. We report a new measurement from a campaign between {sl Insight}-HXMT and {sl NuSTAR} performed in February 2018. Generally, the two satellites show well consistent results of timing and spectral properties. The joint spectral analysis confirms that the previously observed long decay phase has ended, and that the line energy instead keeps constant around 37.5 keV after flux correction.
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