7 research outputs found
Black hole spin measurements in LMC X-1 and Cyg X-1 are highly model-dependent
The black-hole spin parameter, , was measured to be close to its maximum
value of 1 in many accreting X-ray binaries. In particular,
was found in a number of studies of LMC X-1. These measurements were claimed to
take into account both statistical and systematic uncertainties. We perform new
measurements using a recent simultaneous observation by NICER and NuSTAR,
providing a data set of high quality. We use the disk continuum method together
with improved models for coronal Comptonization. With the standard relativistic
disk model and optically thin Comptonization, we obtain values of similar
to those obtained before. We then consider modifications to the standard model.
Using a color correction of 2, we find --0.84. We then
consider disks with dissipation in surface layers. To account for that, we
assume the standard disk is covered by a warm and optically thick Comptonizing
layer. Our model with the lowest yields then . In order to test the presence of such effects in other
sources, we also study an X-ray observation of Cyg X-1 by Suzaku in the soft
state. We confirm the previous findings of using the standard model,
but then we find a weakly constrained when
including an optically thick Comptonizing layer. We conclude that
determinations of the spin using the continuum method can be highly sensitive
to the assumptions about the disk structure.Comment: ApJ, in press,13 page
The Correlation between X-ray spectral slope and FeKalpha line energy in radio-quiet active galactic nuclei
A significant correlation between FeKalpha line energy and X-ray spectral
slope has been discovered among radio-quiet active galactic nuclei. The
ionization stage of the bulk of the FeKalpha emitting material is not the same
in all active galactic nuclei and is related to the shape of the X-ray
continua. Active galactic nuclei with a steep X-ray spectrum tend to have a
fluorescence FeKalpha line from highly ionized material. In the narrow-line
Seyfert 1 galaxies with steeper X-ray spectrum (Gamma_X > 2.1), the FeKalpha
line originates from highly ionized material. In the Seyfert 1 galaxies and
quasars with flatter X-ray spectrum (Gamma_X < 2.1), bulk of the FeKalpha
emission arises from near neutral or weakly ionized material. The correlation
is an important observational characteristic related to the accretion process
in radio quiet active galactic nuclei and is driven by a fundamental physical
parameter which is likely to be the accretion rate relative to the Eddington
rate.Comment: 4 pages, To apear in ApJ Letter
Soft X-ray and FUV observations of Nova Her 2021 (V1674 Her) with AstroSat
Nova Her 2021 or V1674 Her was one of the fastest novae to be observed so far. We report here the results from our timing and spectral studies of the source observed at multiple epochs with AstroSat. We report the detection of a periodicity in the source in soft X-rays at a period of 501.4–501.5 s which was detected with high significance after the peak of the super-soft phase, but was not detected in the far ultraviolet (FUV) band of AstroSat. The shape of the phase-folded X-ray light curves has varied significantly as the nova evolved. The phase-resolved spectral studies reveal the likely presence of various absorption features in the soft X-ray band of 0.5–2 keV, and suggest that the optical depth of these absorption features may be marginally dependent on the pulse phase. Strong emission lines from Si, N, and O are detected in the FUV, and their strength declined continuously as the nova evolved and went through a bright X-ray state.Fil: Bhargava, Yash. No especifÃca;Fil: Dewangan, Gulab Chand. International Centre Of Theoretical Science. Tata Institute Of Fundamental Research; EspañaFil: Anupama, G. C.. No especifÃca;Fil: Kamath, U. S.. No especifÃca;Fil: Sonith, L. S.. No especifÃca;Fil: Pal Singh, Kulinder. ndian Institute of Science Education and Research Mohali; IndiaFil: Drake, J. J.. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Beardmore, A. University of Leicester; Reino UnidoFil: Luna, Gerardo Juan Manuel. Secretaria de Investigacion ; Universidad Nacional de Hurlingham; . Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Orio, M.. Istituto Nazionale di Astrofisica; ItaliaFil: Page, K. L.. University of Leicester; Reino Unid
Black Hole Spin Measurements in LMC X-1 and Cyg X-1 Are Highly Model Dependent
The black hole spin parameter, a _* , was measured to be close to its maximum value of 1 in many accreting X-ray binaries. In particular, a _* ≳ 0.9 was found in a number of studies of LMC X-1. These measurements were claimed to take into account both statistical and systematic uncertainties. We perform new measurements using a recent simultaneous observation by NICER and NuSTAR, providing a data set of high quality. We use the disk continuum method together with improved models for coronal Comptonization. With the standard relativistic disk model and optically thin Comptonization, we obtain values of a _* similar to those obtained before. We then consider modifications to the standard model. Using a color correction of 2, we find a _* ≈ 0.64–0.84. We then consider disks with dissipation in surface layers. To account for that, we assume the standard disk is covered by a warm and optically thick Comptonizing layer. Our model with the lowest χ ^2 then yields . In order to test the presence of such effects in other sources, we also study an X-ray observation of Cyg X-1 by Suzaku in the soft state. We confirm the previous findings of a _* > 0.99 using the standard model, but then we find a weakly constrained when including an optically thick Comptonizing layer. We conclude that determinations of the spin using the continuum method can be highly sensitive to the assumptions about the disk structure