Detection of Low-Frequency QPO From X-ray Pulsar XTE J1858+034 During Outburst in 2019 with NuSTAR
aa r X i v : . [ a s t r o - ph . H E ] J a n Detection of Low-Frequency QPO From X-ray Pulsar XTEJ1858+034 During Outburst in 2019 with NuSTAR
Manoj Mandal
Midnapore City College, Kuturia, Bhadutala, West Bengal, India 721129 andSabyasachi Pal
Indian Centre for Space Physics, 43 Chalantika, Garia Station Road, Kolkata, India 700084Midnapore City College, Kuturia, Bhadutala, West Bengal, India 721129
ABSTRACT
We study the timing properties of XTE J1858+034 using the Nuclear Spec-troscopic Telescope Array (NuSTAR) and Burst Alert Telescope onboard Swiftduring the outburst in October–November 2019. We have investigated for Quasi-Periodic Oscillation (QPO) during the outburst and detected a low-frequencyQPO at ∼
196 mHz with ∼
6% RMS variability from the NuSTAR observation.The QPO is fitted and explained with the model – power law and a Lorentziancomponent. We have also studied the variation of QPO frequency with energy.The beat frequency model and Keplerian frequency model both are suitable toexplain the origin of the QPOs for the source. Regular pulsations and QPOs arefound to be stronger in high energy which suits the beat frequency model. Thevariation of the hardness ratio is studied over the outburst which does not showany significant variation.
Subject headings: accretion, accretion disks - stars: pulsar: individual: XTEJ1858+034.
1. Introduction
In February 1998, the hard X-ray transient XTE J1858+034 was discovered by Remillard & Levine(1998) using All-Sky Monitor (ASM) onboard Rossi X-ray Timing Explorer (RXTE) dur-ing an outburst. Series of observations were made after its discovery with the ProportionalCounter Array (PCA) onboard RXTE, and frequent pulsation with a period of 221 . ± . ∼
25% was found to be single-peaked and sinusoidal during the 1998 outburst(Takeshima et al. 1998; Paul & Rao 1998). From the timing analysis, the presence of Quasi-Periodic Oscillation (QPO) at 110 mHz was discovered during 1998 outburst with around7% root mean square (RMS) fluctuation (Paul & Rao 1998) and did not find any correlationbetween instantaneous X-ray flux with QPO frequency. Paul & Rao (1998) also looked forthe most probable model to explain the origin of the QPO and suggested that both the beatfrequency model and Keplerian frequency model are suitable to explain the origin of theQPOs for the source. They found the regular pulsations and QPOs are stronger in higherenergy range which supports the beat frequency model. There was a second outburst in2004 March and again in April which was first detected with Integral (Molkov et al. 2004)and also followed up with RXTE. During April-May 2004 a strong outburst was detectedusing Integral (Doroshenko et al. 2008), the hardness ratio did not show any significant vari-ation during the outburst and the spin period was measured to be 220.4 ±
2. Observation and data analysis
We followed up the evolution of the outburst using Swift/BAT. We analyzed the NuS-TAR data close to the peak of the 2019 outburst. We used the
HEASOFT v6.27.2 for the data 3 – F l ux ( C r a b ) Time (MJD-58765)
Fig. 1.— Flaring activity of XTE J1858+034 detected by Swift/BAT (15–50 keV) duringthe recent outburst (October–November 2019). The blue arrow shows the time of NuSTARobservation.reduction and analysis.NuSTAR was launched On the 13th of June 2012. The observatory comes with two co-aligned, identical X-ray telescope systems operating in a wide energy range from 3–79 keV.Separate solid-state CdZnTe pixel detector systems in each telescope usually referred to asFocal Plane Modules A and B (FPMA and FPMB; Harrison et al. (2013)), have a spectralenergy resolution of 400 eV (FWHM) at 10 keV. NuSTAR performed an observation of XTEJ1858+034 close to the peak of the outburst in the declining phase (MJD 58790) with atotal exposure time of 43.68 ks (Obs. Id – 90501348002). The observation log of NuSTAR isshown in the Table 1. The data is reduced using the
NuSTARDAS pipeline version v0.4.7 (2019-11-14) provided under
HEASOFT v6.27.2 with latest
CALDB . We have extracted clean eventfiles using
DS9 version 8.1. We have generated the light curves of the source and backgroundfrom circular regions centering the source with radii of 50 ′′ and 90 ′′ using NUPRODUCTS scriptsprovided by the
NuSTARDAS pipeline. The lcmath tool is used to combine light curves of theNuSTAR modules to improve the statistics for the timing analysis.Table 1: Log of NuSTAR observationInstrument Start time Date Exposure Observation Id(MJD) (yyyy-mm-dd) (ks)NuSTAR 58790.29754 2019-11-03 43.68 90501348002 4 –BAT onboard the Swift observatory (Gehrels et al. 2004) is sensitive in hard X-ray (15-50 keV) consists of array of CdZnTe detectors (Krimm et al. 2013). We have used the resultsof the BAT transient monitor during the outburst, which is provided by the BAT team.The light curves have been extracted from both the FPMA and FPMB science eventdata in different energy ranges with different bin sizes (0.01 s, 0.1 s, 1 s, and 10 s). Basicfiltering criteria and corrections are applied to get clean continuous science event data. Wehave extracted the power density spectrum of the source to detect the presence of QPO.Power density spectrum analysis has been done using the
POWSPEC routine of
FTOOLS v6.27.2which follows the FET algorithm. Power density spectrum with bin time 0.01 s is createdusing event mode data in the energy range 3–79 keV.
3. Results
The X-ray pulsar XTE J1858+034 went through an outburst detected by Swift/BAT with a maximum flux ∼ ∼ ∼ QPOs have been detected for different accretion-powered X-ray pulsars in the range1 mHz – 40 Hz (Psaltis 2006). We have generated a Power Density Spectrum (PDS)with 0.01 s bin time light curves for both the module of NuSTAR – FPMA and FPMBindependently. A QPO feature is prominent at 0.196 Hz with RMS variability of 5.2% forFPMA and for FPMB the QPO is prominent at 0.197 Hz with 6.3% RMS variability. Thepower density spectrum with prominent QPO is shown in Figure 2 along with the best-fitted model using NuSTAR/FPMA data. The QPO feature is well fitted with a constant,a power-law component (with a power-law index -1.08), and a Lorentzian. QPO featurecentered nearly 196 mHz for NuSTAR/FPMA with width (LW) 0.06 Hz and normalization(LN) 0.75. Significant energy dependence of the RMS variation in the QPOs was observedin the previous outburst (Mukherjee et al. 2006). We have also investigated the energy https://swift.gsfc.nasa.gov/results/transients/ P o w e r Frequency (Hz)
Fig. 2.— The power spectrum of XTE J1858+034 generated from the 0.01 s binned lightcurve over the energy band 3–79 keV. The presence of QPO is observed at 0.196 Hz. The linerepresents the best-fitted model in frequency range 0.01–10.0 Hz consisting of a constant,power-law and a Lorentzian centered at the QPO frequency.dependence of the QPO feature. To study the energy dependence of QPO, we generatedPDS in different energy ranges. The QPOs in the energy range 30–50 keV and beyond thisrange is not significant. We have not found any significant variation of the QPO frequencywith energy.
4. Discussion
We present the results of timing analysis using the NuSTAR data collected during therecent outburst of XTE J1858+034 in 2019. From the timing analysis, the spin period ofthe X-ray pulsar has been found 218 . ± .
01 s. We study the hardness ratio using the ratioof the count rates with time from Swift/BAT (15–50 keV) and MAXI/GSC (2–20 keV). Wedo not observe any significant change in the hardness ratio during the outburst. A low-frequency QPO has been detected at 0.196 Hz from the power spectrum during the 2019outburst. It is well fitted by a constant, power-law, and a Lorentzian component. During the2004 outburst, the QPOs were also detected in the frequency range 140–185 mHz. The QPOfrequency in the recent outburst does not vary significantly with energy. Several modelsare used to explain the origin of QPOs (Lewin et al. 1988). Generally, the beat frequency 6 –model (Alpar & Shaham 1985), the Keplerian frequency model (van der Klis et al. 1987),and the magnetic disc precession model (Shirakawa & Lai 2002) are used to explain theQPOs in X-ray pulsars. For XTE J1858+034, the QPO frequency ( ν q ) is 196 mHz and spinfrequency ( ν s ) is 4.58 mHz. As ν q > ν s for this source, so the Keplerian frequency modelmay be applicable to explain the origin of QPO. According to this model, the QPOs aregenerated because of inhomogeneities in the Keplerian disk which attenuate the pulsar beamfrequently. As the pulsations and QPOs are stronger in high energy which favors the beatfrequency model, which is similar as observed during the 1998 outburst (Paul & Rao 1998).The beat frequency model implies that QPOs are generated due to the beat phenomenabetween the spin of the neutron star and the rotation of the innermost part of the disk(Paul & Rao 1998). Earlier both the Keplerian frequency model and beat frequency modelwere proposed to explain the origin of QPOs for this source (Paul & Rao 1998) during the1998 outburst and the pulsations and QPOs were found to be stronger in high energy, whichfavors the beat frequency model. Later in the 2004 outburst, the energy dependence of RMSvariability of QPO and dependence of QPO frequency on X-ray flux was observed, whichwas more suitable to explain using the beat frequency model (Mukherjee et al. 2006).
5. Conclusion
We report a low-frequency QPO for the X-ray pulsar XTE J1858+034 at 0.196 Hz with ∼
6% RMS variability during the 2019 outburst. The QPO frequency does not show anysignificant variation with energy. The power spectrum is well described with the power-lawand Lorentzian components. The origin of QPO for the source can be explained with thebeat frequency model. The hardness ratio does not show any significant variation over theoutburst.
Acknowledgements
This research has done using data collected by NuSTAR, a project led by Caltech,managed by NASA/JPL and funded by NASA, and has utilized the
NUSTARDAS softwarepackage, jointly developed by the ASDC (Italy) and Caltech (USA).
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