Luminosity dependence of the cyclotron line energy in 1A 0535+262 observed by Insight-HXMT during 2020 giant outburst
L. D. Kong, S. Zhang, L. Ji, P. Reig, V. Doroshenko, A. Santangelo, R. Staubert, S. N. Zhang, R. Soria, Z. Chang, Y. P. Chen, P. J. Wang, L. Tao, J. L. Qu
We report on a detailed spectral analysis of the transient X-ray pulsar 1A~0535+262, which underwent the brightest giant outburst ever recorded for this source from November to December 2020 with a peak luminosity of $1.2$ $\times10^{38}\ \rm erg\ s^{-1}$. Thanks to the unprecedented energy coverage and high cadence observations provided by Insight-HXMT, we were able to find for the first time evidence for a transition of the accretion regime. At high luminosity, above the critical luminosity $6.7\times10^{37}$ erg s$^{-1}$, the cyclotron absorption line energy anti-correlates with luminosity. Below the critical luminosity, a positive correlation is observed. The 1A~0535+262 becomes, therefore, the second source after V~0332+53, which clearly shows an anti-correlation above and transition between correlation and anti-correlation around the critical luminosity. The evolution of both the observed CRSF line energy and broadband X-ray continuum spectrum throughout the outburst exhibits significant differences during the rising and fading phases: that is, for a similar luminosity the spectral parameters take different values which results in hysteresis patterns for several spectral parameters including the cyclotron line energy. We argue that, similarly to V~0332+53, these changes might be related to different geometry of the emission region in rising and declining parts of the outburst, probably due to changes in the accretion disk structure and its interaction with the magnetosphere of the neutron star.
We report on a detailed spectral analysis of emission from X-ray pulsar 4U 1901+03 using data obtained by the Insight-HXMT and NuSTAR observatories during the 2019 outburst of the source. Thanks to the extensive coverage of the outburst by Insight-HXMT, we were able to investigate the spectral evolution of the source as a function of flux, and compare these results to the previous reports, focusing on the properties of a putative absorption feature at around 10 keV. In particular, we demonstrate that the broadband X-ray continuum of 4U 1901+03 can be well described with a two-component continu…
We present the observational results from a detailed timing analysis of the black hole candidate Swift J1658.2-4242 during its 2018 outburst with the observations of Hard X-ray Modulation Telescope (Insight-HXMT), Neutron Star Interior Composition Explorer (NICER) and AstroSat in 0.1-250keV. The evolution of intensity, hardness and integrated fractional root mean square (rms) observed by Insight-HXMT and NICER are presented in this paper. Type-C quasi-periodic oscillations (QPOs) observed by NICER (0.8-3.5Hz) and Insight-HXMT (1-1.6Hz) are also reported in this work. The features of the QPOs a…
We have performed phase-resolved spectral analysis of the accreting pulsar 1A~0535+262 based on observations of Insight-HXMT during the 2020 Type-II outburst of the source. We focus on the two-dimensional dependence of the cyclotron resonance scattering features (CRSFs) along the outburst time and at different phases. The fundamental CRSF line (f-CRSF) shows different time- and phase-dependent behaviors. At higher luminosity, the phase profile of the f-CRSF energy changes from a single peak to double peaks, with the transition occurring at MJD 59185. On the contrary, the first harmonic CRSF (1…
The prolific thermonuclear X-ray bursts from the outburst of the newly discovered millisecond pulsar MAXI~J1816--195 observed by Insight-HXMT and NICER
The low mass X-ray binary GX 339-4 went into a new outburst in 2021. At the end of the hard to soft transition of this outburst, Insight-HXMT found that the source rapidly alternated between low flux and high flux states on a timescale of hours. Two high flux states lasted only for a period comparable to the orbital period of the observatory. Time-resolved spectral analysis shows that the sudden changes of flux are confined in the hard X-ray band (>4 keV). The variable non-thermal emission, including the power-law continuum from the corona and the reflected emission from the inner accretion di…
We report the evolution of the X-ray pulsations of EXO 2030+375 during its 2021 outburst using the observations from \textit{Insight}-HXMT. Based on the accretion torque model, we study the correlation between the spin frequency derivatives and the luminosity. Pulsations can be detected in the energy band of 1--160 keV. The pulse profile evolves significantly with luminosity during the outburst, leading to that the whole outburst can be divided into several parts with different characteristics. The evolution of the pulse profile reveals the transition between the super-critical (fan-beam domin…
Cyclotron line scattering features are detected in a few tens of X-ray pulsars (XRPs) and used as direct indicators of a strong magnetic field at the surface of accreting neutron stars (NSs). In a few cases, cyclotron lines are known to be variable with accretion luminosity of XRPs. It is accepted that the observed variations of cyclotron line scattering features are related to variations of geometry and dynamics of accretion flow above the magnetic poles of a NS. A positive correlation between the line centroid energy and luminosity is typical for sub-critical XRPs, where the accretion result…
We present a detailed study of the high mass X-ray binary Vela X-1, using observations performed by Insight-HXMT in 2019 and 2020, concentrating on timing analysis and spectral studies including pulse phase-resolved spectroscopy. The cyclotron line energy is found to be at ~21-27 keV and 43-50 keV for the fundamental and first harmonic, respectively. We present the evolution of spectral parameters and find that two line centroid energy ratio E2/E1 evolved from ~2 before MJD 58900 to ~1.7 after that. The harmonic cyclotron line energy has no relation to the luminosity but the fundamental line e…
Timing and spectral studies of the Be/X-ray binary EXO 2030+375 using Insight-HXMT observations
Institut für Astronomie und Astrophysik, Universität Tübingen（蒂宾根大学天文与天体物理研究所）；ISDC Data Center for Astrophysics, Université de Genève（日内瓦大学天体物理学数据中心）；School of Physics and Astronomy, Sun Yat-Sen University（中山大学物理学院）；Institute of Astrophysics, Central China Normal University（华中师范大学天体物理研究所）
We report results on the timing analysis of the 2020 giant outburst of 1A 0535+262, using broadband data from Insight-HXMT. The analysis of the pulse profile evolution from the sub-critical luminosity to super-critical luminosity regime is presented for the first time. We found that the observed pulse profile exhibits a complex dependence on both energy and luminosity.A dip structure at the energy of the cyclotron resonant scattering features (CRSFs) is found for the first time in the pulse fraction-energy relation of 1A 0535+262, when the outburst evolves in a luminosity range from 4.8 $\time…
Revisiting the 2021 Outburst of the BHC MAXI J1803-298 Using NICER, NuSTAR, and Insight-HXMT Data
South-Western Institute for Astronomy Research, Yunnan University（西南天体物理研究所，云南大学）；Indian Center for Space Physics（印度空间物理中心）；Institute of Astronomy, National Tsing Hua University（国立清华大学天文研究所）
We report on the observation of the accreting pulsar GRO J1008-57 performed by Insight-HXMT at the peak of the source's 2017 outburst. Pulsations are detected with a spin period of 93.283(1) s. The pulse profile shows double peaks at soft X-rays, and only one peak above 20 keV. The spectrum is well described by the phenomenological models of X-ray pulsars. A cyclotron resonant scattering feature is detected with very high statistical significance at a centroid energy of $E_{\rm cyc}=90.32_{-0.28}^{+0.32}$ keV, for the reference continuum and line models, HIGHECUT and GABS respectively. Detecti…
We study the 2018 outburst of Aql X-1 via the monitor of all sky X-ray image (MAXI) data. We show that the outburst starting in February 2018 is a member of short-low class in the frame of outburst duration and the peak count rate although the outburst morphology is slightly different from the other fast-rise-exponential-decay (FRED) type outbursts with a milder rising stage. We study the partial accretion in the weak propeller stage of Aql X-1 via the MAXI data of the 2018 outburst. We report on the spectral analysis of 3 observations of Aquila X-1 obtained by Insight - hard X-ray modulation …
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.
Return of 4U~1730--22 after 49 years silence: the spectral properties of the 2021/2022 outbursts observed by NICER and Insight-HXMT and the soft-to-hard state transition caused by the propeller effect
The recently discovered neutron star transient Swift J0243.6+6124 has been monitored by {\it the Hard X-ray Modulation Telescope} ({\it Insight-\rm HXMT). Based on the obtained data, we investigate the broadband spectrum of the source throughout the outburst. We estimate the broadband flux of the source and search for possible cyclotron line in the broadband spectrum. No evidence of line-like features is, however, found up to $\rm 150~keV$. In the absence of any cyclotron line in its energy spectrum, we estimate the magnetic field of the source based on the observed spin evolution of the neutr…
On April 27, 2020, the soft gamma ray repeater SGR J1935+2154 entered its intense outburst episode again. Insight-HXMT carried out about one month observation of the source. A total number of 75 bursts were detected during this activity episode by Insight-HXMT, and persistent emission data were also accumulated. We report on the spin period search result and the phase distribution of burst start times and burst photon arrival times of the Insight-HXMT high energy detectors and Fermi Gamma-ray Burst Monitor (GBM). We find that the distribution of burst start times is uniform within its spin pha…
The bright black hole X-ray binary 4U 1543-47 during 2021 outburst. A clear state transition from super-Eddington to sub-Eddington accretion revealed by Insight-HXMT
In 2015 October, the Be/X-ray binary 4U 0115+63 underwent a type II outburst, reaching an X-ray luminosity of $\sim$ {\DP{38}\es}. During the outburst, Nuclear Spectroscopic Telescope Array ({\it NuSTAR}) performed two Target of Opportunity observations. Using the broad-band spectra from {\it NuSTAR} (3$-$79 keV), we have detected multiple cyclotron lines of the source, i.e., $\sim$ 12, 16, 22 and 33/35 keV. Obviously, the 16 keV line is not a harmonic component of the 12 keV line. As described by the phase-dependent equivalent widths of these cyclotron lines, the 16 keV and 12 keV lines are t…
We studied the millihertz quasi-periodic oscillation (mHz QPO) in the 2020 outburst of the Be/X-ray binary 1A 0535+262 using Insight-HXMT data over a broad energy band. The mHz QPO is detected in the 27-120 keV energy band. The QPO centroid frequency is correlated with the source flux, and evolves in the 35-95 mHz range during the outburst. The QPO is most significant in the 50-65 keV band, with a significance of ~ 8 sigma, but is hardly detectable (<2 sigma) in the lowest (1-27 keV) and highest (>120 keV) energy bands. Notably, the detection of mHz QPO above 80 keV is the highest energy at wh…
We report on changes of the cyclotron resonance energies of the recurrent transient pulsar, X0331+53 (V0332+53). The whole RXTE data acquired in the 2004-2005 outburst were utilized. The 3-80 keV source luminosity varied between 1.7x10^36 and 3.5x10^38 ers/s, assuming a distance of 7 kpc. We confirmed that the fundamental cyclotron resonance energy changed from ~22 to ~27 keV in a clear anti-correlation to the source luminosity, and without any hysteresis effects between the rising and declining phases of the outburst. In contrast, the second harmonic energy changed from ~49 to ~54 keV, implyi…
New insight into the hard X-ray emission influenced by the type-\uppercase\expandafter{\romannumeral1} bursts observed by Insight-HXMT during outburst of 4U 1636--536
We report the results of AstroSat and NuSTAR observations of the Be/X-ray binary pulsar SXP 15.3 in the Small Magellanic Cloud during its outburst in late 2017, when the source reached a luminosity level of ~ 10^{38} erg s^{-1}, close to the Eddington limit. The unprecedented broadband coverage of the source allowed us to perform timing and spectral analysis between 3 and 80 keV. The pulse profile exhibits a significant energy dependence, and morphs from a double peaked profile to a single broad pulse at energies >15 keV. This can be explained by a spectral hardening during an intensity dip se…
We report on the firm detection of a cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of the Be X-ray binary pulsar, GRO J1008-57, achieved by the Suzaku Hard X-ray Detector during a giant outburst which was detected by the MAXI Gas Slit Camera in 2012 November. The Suzaku observation was carried out on 2012 November 20, outburst maximum when the X-ray flux reached $\sim 0.45$ Crab in 4-10 keV, which corresponds to a luminosity of $1.1 \times 10^{38}$ erg s$^{-1}$ in 0.5--100 keV at 5.8 kpc. The obtained broadband X-ray spectrum from 0.5 keV to 118 keV revealed a significant…
We present results from timing and spectral analysis of the HMXB X-ray pulsar IGR J19294+1816 observed using \asr during its recent Type-I outburst in October, 2019. AstroSat observations sampled the outburst at the decline phase right after the outburst peak. We carried out timing analysis on the light curves obtained using the Large Area X-ray Proportional Counter (LAXPC) instrument on board AstroSat and measured a spin period of 12.485065$\pm$0.000015 s. The peak in the power density spectrum (PDS) corresponding to the spin period of 12.48 s also shows a broadened base. We also detected a Q…
GRO J1750-27, discovered during an outburst in 1995 with CGRO-BATSE, is one of the farthest known galactic Be-X-ray binary systems. This relatively poorly studied system recently went into an outburst in September 2021. The source was observed during the latest outburst using the NuSTAR telescope during the rising phase of the outburst. We estimate the spin period of the source to be 4.45 s using which we produced energy-resolved pulse profiles between 3 and 65 keV. We find that the profile is double-peaked at low energies (<18 keV) while evolving into a single peak at higher energies (>18 keV…
Swift J1626.6-5156 is a Be/X-ray binary that was in outburst from December 2005 until November 2008. We have examined RXTE/PCA and HEXTE spectra of three long observations of this source taken early in its outburst, when the PCA 2-20 keV count rate was >70 counts/s/PCU, as well as several combined observations from different stages of the outburst. The spectra are best fit with an absorbed cutoff power law with a ~6.4 keV iron emission line and a Gaussian optical depth absorption line at ~10 keV. We present strong evidence that this absorption-like feature is a cyclotron resonance scattering f…
After in quiescence for 49 years, 4U~1730--22 became active and had two outbursts in 2021 \& 2022; ten thermonuclear X-ray bursts were detected with Insight-HXMT. Among them, the faintest burst showed a double-peaked profile, placing the source as the 5th accreting neutron star (NS) exhibiting double/triple-peaked type-I X-ray bursts; the other bursts showed photospheric radius expansion (PRE). The properties of double-peaked non-PRE burst indicate that it could be related to a stalled burning front. For the five bright PRE bursts, apart from the emission from the neutron star (NS) surface, we…
We present the analysis of RXTE monitoring data obtained during the January 2009 outburst of the hard X-ray transient 1A 1118-61. Using these observations the broadband (3.5-120 keV) spectrum of the source was measured for the first time ever. We have found that the broadband continuum spectrum of the source is similar to other accreting pulsars and is well described by several conventionally used phenomenological models. We have discovered that regardless of the applied continuum model, a prominent broad absorption feature at ~55 keV is observed. We interpret this feature as a cyclotron reson…
We report the discovery of a cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of GX 304-1, obtained by RXTE and Suzaku during major outbursts detected by MAXI in 2010. The peak intensity in August reached 600 mCrab in the 2-20 keV band, which is the highest ever observed from this source. The RXTE observations on more than twenty occasions and one Suzaku observation revealed a spectral absorption feature at around 54 keV, which is the first CRSF detection from this source. The estimated strength of surface magnetic field, $4.7 \times 10^{12}$ G, is one of the highest among b…
We study the timing and spectral properties of the low-magnetic field, transient magnetar SWIFT J1822.3-1606 as it approached quiescence. We coherently phase-connect the observations over a time-span of ~500 days since the discovery of SWIFT J1822.3-1606 following the Swift-BAT trigger on 2011 July 14, and carried out a detailed pulse phase spectroscopy along the outburst decay. We follow the spectral evolution of different pulse phase intervals and find a phase and energy-variable spectral feature, which we interpret as proton cyclotron resonant scattering of soft photon from currents circula…
We report on the analysis of NuSTAR observations of the Be-transient X-ray pulsar V 0332+53 during the giant outburst in 2015 and another minor outburst in 2016. We confirm the cyclotron-line energy-luminosity correlation previously reported in the source and the line energy decrease during the giant outburst. Based on 2016 observations, we find that a year later the line energy has increased again essentially reaching the pre-outburst values. We discuss this behaviour and conclude that it is likely caused by a change of the emission region geometry rather than previously suggested accretion-i…
We present the results of the pulse-amplitude-resolved spectroscopy of the accreting pulsar V 0332+53 using the NuSTAR observations of the source in 2015 and 2016. We investigate the dependence of the energy of the cyclotron resonant scattering feature (CRSF) as a function of X-ray luminosity on timescales comparable with the spin period of the pulsar within individual observations, and the behavior on longer timescales within and between the two observed outbursts. We confirm that in both cases the CRSF energy is negatively correlated with flux at luminosities higher than the critical luminos…
