Gas Slit Camera Research Articles

High-velocity Blue-shifted Fe xxv Heα Line during a Superflare of the RS Canum Venaticorum–type Star IM Peg

The Monitor of All-sky X-ray Image (MAXI) detected a superflare, releasing 5 × 1037 erg in 2−10 keV, of the RS CVn-type star IM Peg at 10:41 UT on 2023 July 23 with its Gas Slit Camera (2−30 keV). We conducted X-ray follow-up observations of the superflare with the Neutron Star Interior Composition Explorer (NICER; 0.2−12 keV) starting at 16:52 UT on July 23 until 06:00 UT on August 2. NICER X-ray spectra clearly showed emission lines of the Fe xxv Heα and Fe xxvi Lyα for ∼1.5 days since the MAXI detection. The Fe XXV Heα line was blueshifted with its maximum Doppler velocity reaching −2200 ± 600 km s−1, suggesting an upward-moving plasma during the flare, such as a coronal mass ejection (CME) and/or chromospheric evaporation. This is the first case that the Fe xxv Heα line is blueshifted during a stellar flare, and its velocity overwhelmingly exceeds the escape velocity of the star (−230 km s−1). One hour before the most pronounced blueshift detection, a signature of the reheating of the flare plasma was observed. We discuss the origin of the blueshift, a CME, or high-velocity chromospheric evaporation.

LCGCT: A light curve generator in customisable-time-bin based on time-series database

In the era of time-domain astronomy, scientists often need to generate light curves with varying time-bin. However, an increase in time resolution typically leads to a substantial increase in data transmission. To enhance the data processing efficiency in time-domain astronomy, we propose a novel time-series data model for storing time-series observation data, and we construct the LCGCT, a tool designed to produce light curves with customisable time bins. To validate our approach, we utilise the 7-year MAXI/GSC (Gas Slit Camera of the Monitor of All-sky X-ray Image) X-ray source catalogue, incorporating its 24-h binned light curves for a comparative analysis with our approach. The results obtained confirm the accuracy and effectiveness of our proposed approach. Subsequently, we compare the storage capacity and query performance of LCGCT with a PostgreSQL-based implementation, and results show that LCGCT conserves 75% of the storage space and achieves three times the query speed. Owing to its noteworthy storage and query performance, our proposed time-series data model-based LCGCT can be used in time-domain astronomical projects with high time resolution.

A comprehensive study of orbital evolution of LMC X-4: existence of a second derivative of the orbital period

ABSTRACT We report here results from pulse arrival time delay analysis of the eclipsing high-mass X-ray binary (HMXB) pulsar LMC X-4 using observations made with the Rossi X-ray Timing Explorer, XMM–Newton, NuSTAR (Nuclear Spectroscopic Telescope ARray), and AstroSat. Combining the orbital parameters determined from these observations with the historical measurements dating back to 1998, we have extended the Tπ/2 epoch history of LMC X-4 by about 4600 binary orbits spanning about 18 yr. We also report mid-eclipse time measurements (Tecl) using data obtained from wide-field X-ray monitors of MAXI-GSC (Monitor of All-sky X-ray Image – Gas Slit Camera) and Swift-BAT (Burst Alert Telescope). Combining the new Tπ/2 and Tecl estimates with all the previously reported values, we have significantly improved the orbital evolution measurement, which indicates that the orbital period is evolving at a time-scale ($P_{\rm orb}/\dot{P}_{\rm orb}$) of about 0.8 Myr. For the first time in an accreting X-ray pulsar system, we confirm the existence of a second derivative of the orbital period, having an evolution time-scale ($\dot{P}_{\mathrm{ orb}}/\ddot{P}_{\mathrm{ orb}}$) of about 55 yr. Detection of a second derivative of the orbital period in LMC X-4 makes its orbital evolution time-scale more uncertain, which may also be true for other HMXBs. Independent solutions for the orbital evolution measurement using the mid-eclipse data and the pulse timing data are consistent with each other, and help us put an upper limit of 0.009 on the eccentricity of the binary system.

The spin measurement of MAXI J1348-630 using the Insight-HXMT data

ABSTRACT We report the results of fitting Insight-HXMT data to the black hole X-ray binary MAXI J1348-430, which was discovered on January 26th, 2019, We also inferred the ionization state and iron abundancewith the Gas Slit Camera on-board MAXI. Several observations at the beginning of the first burst were selected, with a total of 10 spectra. From the residuals of fits using disc plus power-law models, X-ray reflection signatures were clearly visible in some of these observations. We use the state-of-the-art $\tt {relxill}$ series reflection model to fit six spectra with distinct reflection signatures and a joint fit to these spectra. In particular, we focus on the results for the black hole spin values. Assuming Rin = RISCO, the spin parameter is constrained to be $0.82^{+0.04}_{-0.03}$ with 90 per cent confidence level (statistical only).

Detailed analysis on the reflection component for the black hole candidate MAXI J1348-630

ABSTRACT The black hole candidate MAXI J1348-630 was discovered in 2019 January 26, with the Gas Slit Camera (GSC) on-board MAXI. We report a detailed spectral analysis of this source by using the archived data of NuSTAR. A total of nine observations covered the complete outburst evolution of MAXI J1348-630 from the hard state to the soft state and finally back to the hard state. Additionally, the intermediate state is found in the transition from the hard state to the soft state. We use the state-of-the-art reflection model relxill family to fit all the nine spectra, and the spectra from two focal plane module detectors of NuSTAR are jointly fitted for each observation. In particular, we concentrate on the results of the black hole spin parameter and the inclination of the accretion disc. Based on the analysis of the inner radius of the accretion disc, we obtain the spin parameter $a_* =0.78_{-0.04}^{+0.04}$, and the inclination angle of the inner disc $i = 29.2_{-0.5}^{+0.3}$°. Furthermore, we also find that when the black hole is in the hard state, the accretion disc would show a significant truncation. The high iron abundance and ionization of the accretion disc obtained in the fitting results can be possibly explained by the high density of the accretion disc.

On the Impact of an Intermediate Duration X-Ray Burst on the Accretion Environment in IGR J17062–6143

Abstract We report on a spectroscopic analysis of the X-ray emission from IGR J17062−6143 in the aftermath of its 2020 June intermediate duration Type I X-ray burst. Using the Neutron Star Interior Composition Explorer, we started observing the source 3 hr after the burst was detected with the Monitor of All-sky X-ray Image Gas Slit Camera, and monitored the source for the subsequent 12 days. We observed the tail end of the X-ray burst-cooling phase, and find that the X-ray flux is severely depressed relative to its historic value for a three-day period directly following the burst. We interpret this intensity dip as the inner accretion disk gradually restoring itself after being perturbed by the burst irradiation. Superimposed on this trend we observed a 1.5 day interval during which the X-ray flux is sharply lower than the wider trend. This drop in flux could be isolated to the nonthermal components in the energy spectrum, suggesting that it may be caused by an evolving corona. Additionally, we detected a 3.4 keV absorption line at 6.3σ significance in a single 472 s observation while the burst emission was still bright. We tentatively identify the line as a gravitationally redshifted absorption line from burning ashes on the stellar surface, possibly associated with 40Ca or 44Ti.

Discovery of the Black Hole X-Ray Binary Transient MAXI J1348–630

Abstract We report the first half-year monitoring of the new Galactic black hole candidate MAXI J1348–630, discovered on 2019 January 26 with the Gas Slit Camera on board the Monitor of All-sky X-ray Image (MAXI). During the monitoring period, the source exhibited two outburst peaks, where the first peak flux (at T = 14 days from the discovery of T = 0) was ∼4 Crab (2–20 keV) and the second one (at T = 132 days) was ∼0.4 Crab (2–20 keV). The source exhibited distinct spectral transitions between the high/soft and low/hard states and an apparent “q”-shape curve on the hardness-intensity diagram, both of which are well-known characteristics of black hole binaries (BHBs). Compared to other bright black hole transients, MAXI J1348–630 is characterized by its low disk temperature (∼0.75 keV at the maximum) and high peak flux in the high/soft state. The low peak temperature leads to a large innermost radius that is identified as the innermost stable circular orbit, determined by the black hole mass and spin. Assuming the empirical relation between the soft-to-hard transition luminosity (L trans) and the Eddington luminosity (L Edd), L trans/L Edd ≈ 0.02, and a face-on disk around a non-spinning black hole, the source distance and the black hole mass are estimated to be D ≈ 4 kpc and , respectively. The black hole is more massive if the disk is inclined and the black hole is spinning. These results suggest that MAXI J1348–630 may host a relatively massive black hole among the known BHBs in our Galaxy.

Monitoring the Superorbital Period Variation and Spin Period Evolution of SMC X-1

Abstract The X-ray pulsar SMC X-1 shows a superorbital modulation with an unstable cycle length in the X-ray band. We present its timing behaviors, including the spin, orbital, and superorbital modulations, beyond the end of the Rossi X-ray Timing Explorer mission. The time-frequency maps derived by the wavelet Z-transform and the Hilbert–Huang transform suggest that a new superorbital period excursion event occurred in ∼MJD 57,100 (2015 March). This indicates that the excursion is recurrent and probably (quasi)periodic. The hardness ratio obtained with the Monitor of All-sky X-ray Image (MAXI) suggests increased absorption during the transition from the high to the low state in the superorbital cycle. Compared to the regular epochs, the superorbital profile during the excursion epochs has a shallower and narrower valley, likely caused by a flatter warp. By tracking the spin period evolution with the MAXI Gas Slit Camera in 2–20 keV, we derive an averaged spin-up rate of ν ˙ = 2.515 ( 3 ) × 10 − 11 s−2 during the period between MJD 55,141 (2009 November) and 58,526 (2019 February). We obtain no positive correlation between the spin frequency residual and the superorbital frequency, but a torque change accompanying the superorbital period excursion is possible. We suggest that the accretion torque on the neutron star could be changed by various mechanisms, including the change of mass accretion rate and the warp angle. We update the value of the orbital decay as P ˙ orb / P orb = − 3.380 ( 6 ) × 10 − 6 yr−1. Finally, we reconfirm the detection of the superorbital modulation in the optical band and its coherence in phase with the X-ray modulation.

Cosmological implications of the composite spectra of galactic X-ray binaries constructed using MAXI data

We have investigated the long term average spectral properties of galactic X-ray binaries in the energy range of 3-20 keV, using long term monitoring data from MAXI -Gas Slit Camera (GSC). These long term average spectra are used to construct separately the composite spectra of galactic High Mass X-ray binaries (HMXBs) and Low Mass X-ray binaries (LMXBs). These composite spectra can be described empirically with piece-wise power-law with three components. X-rays from HMXBs are considered as important contributors to heating and ionization of neutral hydrogen in the intergalactic medium during the Epoch of Reionization. Using the above empirical form of the composite HMXB spectra extrapolated to lower energies as an input, we have studied the impact of these sources on the 21-cm signal using the outputs of N-body simulation and 1D radiative transfer. The heating due to the composite spectrum is less patchy compared to power-law spectrum with a spectral index $\alpha = 1.5$, used in previous studies. The amplitude of the heating peak of large scale power spectrum, when plotted as a function of the redshift, is less for the composite spectrum.

Discovery and state transitions of the new Galactic black hole candidate MAXI J1535−571

Abstract We report on the detection and subsequent X-ray monitoring of the new Galactic black hole candidate MAXI J1535−571 with the Gas Slit Camera onboard Monitor of All-sky X-ray Image (MAXI/GSC). After the discovery on 2017 September 2, made independently with MAXI and the Swift/BAT, the source brightened gradually, and in a few weeks reached the peak intensity of ∼5 Crab, or ∼1.6 × 10−7 erg cm−2 s−1 in terms of the 2–20 keV flux. On the initial outburst rise, the 2–20 keV MAXI/GSC spectrum was described by a power-law model with a photon index of ≲ 2, while after a hard-to-soft transition, which occurred on September 18, the spectrum required a disk blackbody component in addition. At around the flux peak, the 2–8 keV and 15–50 keV light curves showed quasi-periodic and anti-correlated fluctuations with amplitudes of 10%–20%, on a time scale of ∼1 d. Based on these X-ray properties obtained with the MAXI/GSC, with additional information from the Swift/BAT, we discuss the evolution of the spectral state of this source, and give constraints on its system parameters.

An application of the Ghosh & Lamb model to the accretion-powered X-ray pulsar X Persei

Abstract The accretion-induced pulse period changes of the Be/X-ray binary pulsar X Persei were investigated over the period of 1996 January to 2017 September. This study utilized the monitoring data acquired with the RXTE All-Sky Monitor in 1.5–12 keV and the MAXI Gas-Slit Camera in 2–20 keV. The source intensity changed by a factor of 5–6 over this period. The pulsar was spinning down for 1996–2003, and has been spinning up since 2003, as already reported. The spin-up/down rate and the 3–12 keV flux, determined every 250 d, showed a clear negative correlation, which can be successfully explained by the accretion torque model proposed by Ghosh and Lamb (1979, ApJ, 234, 296). When the mass, radius, and distance of the neutron star were allowed to vary over a range of 1.0–2.4 solar masses, 9.5–15 km, and 0.77–0.85 kpc, respectively, the magnetic field strength of B = (4–25) × 1013 G gave the best fits to the observations. In contrast, the observed results cannot be explained by the values of B ∼ 1012 G previously suggested for X Persei, as long as the mass, radius, and distance are required to take reasonable values. Assuming a distance of 0.81 ± 0.04 kpc as indicated by optical astrometry, the mass of the neutron star is estimated as M = 2.03 ± 0.17 solar masses.

Orbital resolved spectroscopy of GX 301–2: wind diagnostics

AbstractGX 301–2, a bright high-mass X-ray binary with an orbital period of 41.5 days, exhibits stable periodic orbital intensity modulations with a strong pre-periastron X-ray flare. Several models have been proposed to explain the accretion at different orbital phases. In Islam & Paul (2014), we presented results from an orbital resolved spectroscopic study of GX 301–2 using data from MAXI Gas Slit Camera. We have found a strong orbital dependence of the absorption column density and equivalent width of the iron emission line. A very large equivalent width of the iron line along with a small value of the column density in the orbital phase range 0.1–0.3 after the periastron passage indicates the presence of high density accretion stream. We aim to further investigate the characteristics of the accretion stream with an AstroSat observation of the system.

MAXI upper limits of the electromagnetic counterpart of GW170817

Abstract We report on the Monitor of All-sky X-ray Image (MAXI) observation of the gravitational-wave (GW) event GW170817 and the electromagnetic counterpart of GW170817. GW170817 is a binary neutron star coalescence candidate detected by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and Advanced Virgo detectors, and it is the first event for which the optical counterpart has been discovered. In the MAXI observation, the Gas Slit Camera (GSC) covered approximately 62% of the sky region of the GW event within the 90% probability during the first 92 min orbital period after the trigger. No significant X-ray transient was detected in the error region, and the upper limit of the average flux with a significance of 3σ in the 2–10 keV band was 53/26 mCrab (one-orbit observation/one-day observation). In the optical counterpart of GW170817, the observational window of the GSC at the position started 20 s after the GW trigger, but the high-voltage power supply of the GSC was unfortunately off at the time because the International Space Station (ISS) was entering a high-particle-background region.The first observation of the position by the GSC was eventually performed 16797 s (4.6 hr) after the GW trigger, yielding the 3σ upper limit of 8.60 × 10−9 erg cm−2 s−1 in the 2–10 keV band, though it was the earliest X-ray observation of the counterpart.

Effect of non-stationary accretion on spectral state transitions: An example of a persistent neutron star LMXB 4U1636–536

Observations of black hole and neutron star X-ray binaries show that the luminosity of the hard-to-soft state transition is usually higher than that of the soft-to-hard state transition, indicating additional parameters other than mass accretion rate are required to interpret spectral state transitions. It has been found in some individual black hole or neutron star soft X-ray transients that the luminosity corresponding to the hard-to-soft state transition is positively correlated with the peak luminosity of the following soft state. In this work, we report the discovery of the same correlation in the single persistent neutron star low mass X-ray binary (LMXB) 4U 1636–536 based on data from the All Sky Monitor (ASM) on board RXTE, the Gas Slit Camera (GSC) on board MAXI and the Burst Alert Telescope (BAT) on board Swift. We also found such a positive correlation holds in this persistent neutron star LMXB in a luminosity range spanning about a factor of four. Our results indicate that non-stationary accretion also plays an important role in driving X-ray spectral state transitions in persistent accreting systems with small accretion flares, which is much less dramatic compared with the bright outbursts seen in many Galactic LMXB transients.

Detection of the thermal component in GRB 160107A

Abstract We present the detection of a blackbody component in gamma-ray burst GRB 160107A emission by using the combined spectral data of the CALET Gamma-ray Burst Monitor (CGBM) and the MAXI Gas Slit Camera (GSC). MAXI/GSC detected the emission ∼45 s prior to the main burst episode observed by the CGBM. The MAXI/GSC and the CGBM spectrum of this prior emission period is fitted well by a blackbody with temperature $1.0^{+0.3}_{-0.2}$ keV plus a power law with a photon index of −1.6 ± 0.3. We discuss the radius of the photospheric emission and the main burst emission based on the observational properties. We stress the importance of coordinated observations via various instruments collecting high-quality data over a broad energy coverage in order to understand the GRB prompt emission mechanism.

Correlation between the luminosity and spin-period changes during outbursts of 12 Be binary pulsars observed by the MAXI/GSC and the Fermi/GBM

Abstract To study observationally the spin-period changes of accreting pulsars caused by the accretion torque, the present work analyzes X-ray light curves of 12 Be binary pulsars obtained by the MAXI Gas-Slit Camera all-sky survey and their pulse periods measured by the Fermi Gamma-ray Burst Monitor pulsar project, both covering more than six years, from 2009 August to 2016 March. The 12 objects were selected because they are accompanied by clear optical identification and accurate measurements of surface magnetic fields. The luminosity L and the spin-frequency derivatives $\dot{\nu }$, measured during large outbursts with L ≳ 1 × 1037 erg s−1, were found to follow approximately the theoretical relations in the accretion torque models, represented by $\dot{\nu } \propto L^{\alpha }$ (α ≃ 1), and the coefficient of proportionality between $\dot{\nu }$ and Lα agrees, within a factor of ∼3, with that proposed by Ghosh and Lamb (1979b, ApJ, 234, 296). In the course of the present study, the orbital elements of several sources were refined.

X-Ray Transients Observed with MAXI

AbstractMAXI (Monitor of All-sky X-ray Image) is an astronomical mission onboard the International Space Station. It started observations in August 2009. The Gas Slit Camera of MAXI is sensitive to X-rays in the energy range 2–30 keV. Most of the sky is scanned every 90 min with the orbital revolution of the ISS. With this unbiased monitoring, MAXI has detected numerous outbursts from known and unknown X-ray sources. MAXI discovered 18 X-ray novæ in seven years, including seven neutron star binaries, six black hole binaries (+candidates) and four unidentified sources. Other results include detections of superluminous stellar flares, a super-Eddington luminous flare from a white dwarf+Be Star binary near the SMC, and monitoring of recurrent outbursts from Be neutron-star binaries. Variations in X-ray-bright AGNs such as Cen A and Mrk 421 have been also monitored. This talk presented the highlights of the MAXI observations of variable sources, including the search for X-ray counterparts of gravitational-wave events.

X-ray upper limits of GW151226 with MAXI

Abstract The error region of the the gravitational-wave (GW) event GW151226 was observed with Monitor of All-sky X-ray Image (MAXI). MAXI was operating at the time of GW151226, and continuously observed up to 4 min after the event. MAXI covered about 84% of the 90% error region of the GW event during the first 92 min orbit after the event. No significant X-ray transient was detected in the GW error region. A typical 3 σ Gas Slit Camera upper limit for a scan is 1.2 × 10−9 erg cm−2 s−1 in the 2–20 keV band. The autodetection (MAXI nova-search) systems detected a short excess event with a low significance (2.85 σ) from 5257 s to 5260 s after the GW trigger. Finally, we discuss the sensitivity of MAXI to long X-ray emissions of short gamma-ray bursts, which are expected to accompany GW events.

Hard-tail emission in the soft state of low-mass X-ray binaries and their relation to the neutron star magnetic field

Abstract Average hard-tail X-ray emission in the soft state of nine bright Atoll low-mass X-ray binaries containing a neutron star (NS-LMXBs) are investigated by using the light curves of MAXI/GSC (Gas Slit Camera) and Swift/BAT (Burst Alert Telescope). Two sources (4U 1820−30 and 4U 1735−44) exhibit a large hardness ratio (15–50 keV/2–10 keV: HR >0.1), while the other sources distribute at HR ≲ 0.1. In either case, HR does not depend on the 2–10 keV luminosity. Therefore the difference of HR is due to the 15–50 keV luminosity, which is Comptonized emission. The Compton cloud is assumed to be around the neutron star. The size of the Compton cloud would affect the value of HR. Although the magnetic field of an NS-LMXB is weak, we could expect a larger Alfvén radius than the innermost stable circular orbit or the neutron star radius in some sources. In such cases, the accretion inflow is stopped at the Alfvén radius and would create a relatively large Compton cloud. This would result in the observed larger Comptonized emission. By attributing the difference of the size of Compton cloud to the Alfvén radius, we can estimate the magnetic fields of neutron stars. The obtained lower/upper limits are consistent with the previous results.

Search for soft X-ray flashes at the fireball phase of classical/recurrent novae using MAXI/GSC data

Abstract We searched for precursive soft X-ray flashes (SXFs) associated with optically discovered classical or recurrent novae in the data of five years’ all-sky observations with the Gas Slit Camera (GSC) of the Monitor of All-sky X-ray Image (MAXI). We first developed a tool to measure the fluxes of point sources by fitting the event distribution with a model that incorporates the point-spread function (PSF-fit) to minimize the potential contamination from nearby sources. Then we applied the PSF-fit tool to 40 classical/recurrent novae that were discovered in optical observations from 2009 August to 2014 August. We found no precursive SXFs with significance above the 3 σ level in the energy range of 2–4 keV between td − 10 d and td, where td is the date when each nova was discovered. We obtained the upper limits for the bolometric luminosity of SXFs, and compared them with the theoretical prediction and that observed for MAXI J0158−744. This result could constrain the population of massive white dwarfs with a mass of roughly 1.40 solar mass, or larger, in binary systems.