X-ray spectral and temporal evolution of atoll source 4U 1820-30 with AstroSat: detection of high frequency quasi-periodic oscillation

Abstract

AstroSat/LAXPC and SXT observed the persistent neutron star low-mass X-ray binary 4U 1820-30 between 2016 and 2022. During these observations, the hardness-intensity diagram (HID) and color-color diagram (CCD) indicated that the source was in the banana state. We divided the CCD into 11 segments for spectral and timing analyses. For each segment in the CCD, we modeled the spectral data using two distinct approaches over the 0.7-20.0 keV band. A combination of a multi-color-disk component with an inner disk temperature of around 0.6 keV and Comptonized emission from the boundary layer (BL)/ hot corona provided the best description of the X-ray spectral data of this source. The truncation radius was found to be in the range of $\sim$ 19-40 km. The Comptonized component has an optical depth in the range of $\sim 7 - 13$ with electron temperature in the range of $\sim 2.5 - 3.8$ keV. The optical depth of the corona varies significantly along the position on the CCD, while $\sim$ 80\% of the X-ray flux comes from the Comptonized component. We discuss possible physical scenarios to explain the relationship between the spectral evolution and motion of the source along the CCD. The timing analysis revealed kHz QPOs peaks at $\sim 710$ Hz and $\sim 740$ Hz in the lower left banana branch. An energy-dependent study indicates that these QPOs are stronger in the high-energy band.

Figures (5)

• Figure 1: The Hardness Intensity Diagram of 4U 1820-30. We utilized MAXI count rates in the energy range of 2.0–20.0 keV for the intensity. Following marino2023accretion, we estimated the hardness as the ratio of the BAT count-rate (15.0–50.0 keV) to the MAXI count-rate in the soft band (2.0–4.0 keV) obtained on the same day. The position in the HID during the dates of the individual AstroSat observations is highlighted with color points superimposed over the grey data points. The positions are labeled with progressive numbers, as shown in Table \ref{['Observation details']}. "LB" is at the bottom of the BS, and "UB" is at the top.
• Figure 2: The left column presents the 256-sec binned background-subtracted light curve within the energy range of 3.0-50.0 keV using LAXPC 20 data. In the right column, the CCD of 4U 1820-30 is constructed using three distinct energy bands: 3.0-5.0, 5.0-7.0, and 7.0-20.0 keV. Here the ratio of the count rates in the energy band 7.0-20.0 keV and 3.0-5.0 keV is defined as hard color, and the ratio of the count rates in the energy band 5.0-7.0 keV and 3.0-5.0 is defined as soft color. The light curve (left) and CCD (right) in the top panel corresponds to Epoch A, segmented into three parts: BA1, BA2, and BA3. The middle panel shows the light curve and CCD for Epoch B, divided into six segments named BB1, BB2, BB3, BB4, BB5, and BB6. The bottom panel is related to Epoch C, featuring two segments in the CCD, labelled BC1 and BC2. The highlighted areas indicate the portions used to generate the energy spectra and Power Density Spectra (PDS). Refer to the main text for more details.
• Figure 3: The PDS for CDD segment BB1 (left panel) and BB2 (right panel) in the energy band 3.0-50.0 keV. The PDS shows significant kHz QPOs at $\sim$ 710 Hz and $\sim$ 740 Hz in the BB1 and BB2 segments. The residuals are shown in units of standard deviations from the model.
• Figure 4: SXT (blue, 0.7-7.0 keV) and LAXPC20 (red, 5.0-20.0 keV) unfolded spectra of Segment BA1 (left panel) and BB1 (right panel), fit with the Model 3 - constant*tbabs*(Comptb+diskbb). The residuals are shown in units of standard deviations from the model
• Figure 5: Evolution of the spectral parameters for 4U 1820-30 along the CCD, obtained using Model 3: constant*tbabs*(Comptb+diskbb) applied to the combined SXT and LAXPC20 data. From left to right, the vertical dashed lines indicate the transitions between Epoch A and Epoch B, and between Epoch B and Epoch C.