NICER, NuSTAR, and Insight-HXMT Views to the Newly Discovered Black Hole X-Ray Binary Swift J1727.8-1613

Swift J1727.8–1613 is a black hole X-ray binary newly discovered in 2023. We perform spectral analysis with simultaneous Insight-HXMT, NICER, and NuSTAR observations when the source was approaching the hard intermediate state. Such a joint view reveals an additional hard component apart from the normally observed hard component with reflection in the spectrum, to be distinguished from the usual black hole X-ray binary systems. By including this extra component in the spectrum, we have measured a high spin of 0.98−0.07+0.02 and an inclination of around 40−0.8+1.2 °, which is consistent with NICER results reported before. However, we find that the additional spectral component cannot be exclusively determined due to the model degeneracy. Accordingly, a possible jet/corona configuration is adjusted to account for the spectral fitting with different model trials. The extra component may originate either from a relativistic jet or a jet base/corona underneath a slow jet.


Introduction
A black hole X-ray binary (BHXRB) consists of a companion star and a black hole (BH).BHXRBs can be classified into two categories: low-mass X-ray binaries, where the companion star has a low mass (1M e ), and high-mass X-ray binaries, where the companion star has a higher mass.Additionally, BHXRBs can be further classified as either persistent or transient sources, depending on their behavior (Tetarenko et al. 2016;Sreehari et al. 2018).For transient sources that remain in a quiescent state with a very low accretion rate for a long time, the accretion disk is a standard Shakura-Sunyaev disk, and the inner radius of the disk is truncated at a distance far from the BH.At this time, the accretion rate of the disk is very low, and the temperature is also very low (Shakura & Sunyaev 1973).
Due to the low accretion rate, the temperature of the disk remains relatively low, and the material within the disk predominantly exists in a neutral state.As the accreted material accumulates in the disk, its temperature gradually increases to the ionization temperature of hydrogen (∼10 5 K), causing ionization of hydrogen.This ionization process triggers thermal and viscous instabilities, which lead to an outward transfer of angular momentum, inward motion of the material, and an increase in the rate of accretion, producing an X-ray outburst (Cannizzo et al. 1995;Lasota 2001;Belloni et al. 2011;Corral-Santana et al. 2016).
BH X-ray outbursts are commonly represented by the hardness-intensity diagram (HID), which allows for their classification into different spectral states.These states include the low/hard states (LHSs), high/soft states (HSSs), and intermediate states (IMSs) based on the position of the trajectory of the outbursts on the HID (Belloni et al. 2005;Motta et al. 2009).The characterization varies from one spectral state to another.In the LHS, the emission is mainly from the corona/jet, the nonthermal component dominates, and the photon index Γ is around 1.5.Hard tails are sometimes present in the spectrum usually thought to be related to the jet (Reig & Kylafis 2016;Kong et al. 2021).In the soft state, nonthermal radiation from the disk dominates, and the energy spectrum is softer, with a higher spectral index around 2.1-3.7 (Esin et al. 1997;McClintock & Remillard 2006).The IMS serves as an intermediary between the LHS and HSS and can be further divided into hard and soft intermediate states (Belloni et al. 2005;Homan & Belloni 2005).
For BHs, their mass and spin are two crucial parameters that play a significant role in understanding their origin and evolution.The spin of a BH provides valuable insights into how it interacts with its surrounding accretion environment and influences various processes such as jet production.There are typically two methods used for measuring the spin of BHs.The first method is relativistic reflection, which involves estimating the spin by analyzing the profile of iron lines and the Compton hump near ∼20 keV (Brenneman & Reynolds 2006;Miller et al. 2009).The second method is through continuum fitting, which requires the inner accretion disk to be in the innermost stable circular orbit (ISCO; Zhang et al. 1997;Li et al. 2005).
Swift J1727.8-1613 was first detected by the SWIFT/BAT on 2023 August 24.with further observations in X-ray, radio, and optical wavelengths; it was subsequently identified and confirmed as a low-mass BH X-ray binary (Castro-Tirado et al. 2023;Miller-Jones et al. 2023;Nakajima et al. 2023;O'Connor et al. 2023).In the hard X-ray band, there are prominent quasiperiodic oscillation (QPO) frequencies observed within the range of 0.46-0.88Hz (Palmer & Parsotan 2023).Draghis et al. (2023) obtained an interstellar absorption of 0.41± 0.01 × 10 22 cm −2 by fitting the spectrum of Neutron Star Interior Composition Explorer (NICER).Additionally, a relativistically broadened Fe K emission line was detected at the 5-7 keV range; for the first time the spin of the BH and the inclination of the system were reported as a 0.995 0.004 0.001 = -+ and θ = 47.9 ± 0.03°, respectively.Furthermore, in the residuals of the fit, an absorption feature near 7 keV was observed, suggesting the presence of an accretion disk wind.
In this paper, we analyze the outburst data of Swift J1727.8-1613collected simultaneously by snapshots of Insight-HXMT, NICER, and NuSTAR and thus present the first views of the spectral properties of Swift J1727.8-1613 in a rather broad energy band.In Section 2, we describe the observations and data reduction.The detailed results are presented in Section 3. The results are discussed and the conclusions are presented in Section 4.
Insight-HXMT started to observe Swift J1727.8-1613 on 2023 August 25.We take an observation that was carried out almost simultaneously with the NuSTAR observation (see Table 1) when the source evolved to approach the intermediate state (see Figure 1).We extract the data from ME and HE using the Insight-HXMT Data Analysis software HXMTDAS v2.05.LE data are not included due to temporary calibration issues.Since the LE is an SCD detector, there is a pileup effect, which is in the process of being calibrated.The data are filtered with the criteria recommended by the Insight-HXMT Data Reduction Guide v2.0511 Xspec v12.13.112 used to perform analysis of spectrum.Due to poor calibration, the energy bands considered for spectral analysis are ME 10-28 keV and HE 28-140 keV.One percent systematic error is added to data, and errors are estimated via Markov Chain Monte Carlo (MCMC) chains with a length of 20,000.

NuSTAR
The Nuclear Spectroscopic Telescope Array (NuSTAR) is the first mission to use focusing telescopes to image the sky in the high-energy X-ray (3-79 keV) region of the electromagnetic spectrum, which was launched at 9 am PDT, 2012 June 13 (Harrison et al. 2013).The NuSTAR observation carried out simultaneously with Insight-HXMT (see Table 1) is adopted to investigate the spectral properties of Swift J1727.8-1613 in a broad energy band.We extract NuSTAR filtered data using the standard pipeline program nupipeline.Since Swift J1727.8-1613 is very bright, we make the parameter statusexpr="(STATUS==b0000xxx00xxxx000) &&(SHIELD==0)" followed by nuproduct to extract the light curves and spectrum.The spectrum and light curves were extracted from a 120″ circle region centered on the source, and the background was generated from a 60″ circle region away from the source.We adopt both FPMA and FPMB data for spectral analyses.

NICER
The X-ray Timing Instrument of NICER is an International Space Station payload, which was launched by the Space X Falcon 9 rocket on 2017 June 3 (Gendreau et al. 2016).NICER has a large effective area and high temporal resolution in the soft X-ray band (0.2-12 keV), which may well fit the blackbody component at low temperatures.
The observation from NICER that was observed simultaneously with NuSTAR and Insight-HXMT (see Table 1) extends the investigation of the spectral properties of Swift J1727.8-1613 to lower energy bands.NICER data are reduced using the standard pipeline tool NICERl213 .We extract light curves using NICERl3-lc14 in 1-10 keV.We use NICERl3spect15 to extract the spectrum, with the "niback-gen3C50"16 model to estimate the background for the spectral analysis; NICERl3-spect also applies the systematic error using niphasyserr automatically.For the fitting of the spectrum, we choose an energy range of 1-10 keV because NICER below 1 keV has significant residual due to calibration problems.

Hardness Ratio
As shown in Table 1, Swift J1727.8-1613 was observed on MJD 60185 by NuSTAR with ObsID 90902330002, by NICER with ObsID 6703010101, and by Insight-HXMT ME and HE with ObsID P061433800302.To investigate the timing of this observation during the Swift J1727.8-1613outburst, we construct the MAXI HID of Swift J1727.8-1613 with time bin 1 day.As shown in Figure 1, the hardness is defined as the ratio of 4-10 keV to 2-4 keV count rate.The red dot is the time of joint NuSTAR, NICER, and Insight-HXMT observations.The flux of Swift J1727.8-1613exhibits a rapid increase, and the observation corresponds to the end of the hard state of the outburst.

The Spectral Analysis
We conduct an analysis of the spectrum of Swift J1727.8-1613 using the FPMA and FPMB data from NuSTAR in the energy range of 3-78 keV.The first trial for the spectral model is constant * TBABS * (diskbb+relxill), where TBABS accounts for the interstellar absorption (Wilms et al. 2000) by considering photoelectric cross sections provided by Verner et al. (1996).We add the relativistic reflection model RELXILL to fit the reflection component in the spectrum (Dauser et al. 2016).During fittings, we fix the two emissivity indices at 3, and the inner radius of the disk at the ISCO.At this time, there is a hard tail in the spectrum component, with χ 2 /(d.o.f.) = 1.14 (see Figure 2(a)).We also try to replace relxill with relxillcp.The difference between relxillcp and relxill is that they have different incidence spectra, with the former being an nthcomp Comptonization continuum and the latter being a standard highenergy cutoff power law.However, the fit for the model of the latter is not satisfactory, with χ 2 /(d.o.f.) = 1.21 (The residuals are shown in Figure 2(b)).In order to better investigate the "hard tail" and improve the fit by including the simultaneous observations from Insight-HXMT ME and HE, the maximum energy of NuSTAR is limited to 78 keV.Again, as shown in Figure 2(c), the reduced χ 2 /(degree of freedom) is as large as 1.37, and a significant residual shows up at the high-energy band.To account for the structure observed in the high-energy bands of the residuals, we add a cutoffpl component to the model.Additionally, we used plabs instead of constants to correct for the differences in spectral index between NICER, NuSTAR, and Insight-HXMT (Zdziarski et al. 2021).We set K and ΔΓ fixed at 1 and 0, respectively, for the NICER and have a fit of χ 2 /(dof) = 1.02 (see Figure 2(d)), with a spectral model of plabs * tbabs(diskbb+relxill+cutoffpl).To investigate the properties of the disk, we add simultaneous observation from NICER for joint fitting and also add diskbb to the model to account for the multi-temperature blackbody of the accretion disk (Mitsuda et al. 1984).We adopted model 1, which is plabs * tbabs(diskbb+ relxill+cutoffpl), χ 2 /(d.o.f.) = 2601.42/3015= 0.86.In order to investigate whether model plabs may have an impact on our fitting results, we perform a series of trials.First, we replace the plabs with constants and allow only a normalization constant to float between the spectra.Second, we remove the constants but allow each individual component normalization to vary.Third, we allow each component normalization and shape parameters to vary.We find that the normalization and shape parameters are allowed to vary for all components, and the results are essentially the same as those we fitted with plabs.So the extra component can be regarded as robust.
Since in model 1 both the incident spectrum of the reflection and the additional component are cutoff power laws, we find that an exchange of the spectral index and cutoff energy between these two components does not change the fitting goodness but results in completely different descriptions upon the additional spectral component.As shown in Figure 3, we find that when the Γ and the E cut of the reflected component are larger, the extra component shows up at the intermediate energy band as a rather broad hump.The count rates of FPMA and FPMB in NuSTAR exhibit differences, but they do not affect the spectral shape.On the contrary, when the Γ and the E cut of the reflected component are set to small values, the extra component takes a "hard tail" located at the high-energy band, and the probability distribution of the parameters is shown in Figure 4.As shown in Figures 4 and 5, we find that the fitting results are all reliable.To reduce such a degeneracy in the two spectral components, we replace cutoffpl in model 1 with comptt with geometry as a disk, so model 2 is plabs * tbabs(diskbb+relxill +comptt) and results in χ 2 /(dof) = 2620.02/3014= 0.87.As shown in the left panel of Figure 6, in this case, the extra component shows up as the "hard tail" at the high-energy band.We also find that the relxill in model 1 can be replaced with relxillcp to form model 3: plabs * tbabs(diskbb+relxillcp+cutoffpl), which can result in an equally good fit with χ 2 /(dof) = 0.87.As shown in the right panel of Figure 6, the extra component again appears as a broad hump in the mediate energy band.
The parameters from the joint spectral fitting with different models are shown in Tables 2 and 3, where Swift J1727.8-1613 is revealed as a high-spin, medium-inclination system: the spin and the inclination angle are measured as 0.98 0.07 0.02 -+ and 40 0.8 1.2 -+ °, respectively.A component in addition to the reflection component can either behave as a high-energy "hard tail," with the flux of the reflection component significantly higher than the flux of the "hard tail," or a broad hump at the intermediate energy band, with its flux comparable to the flux of the reflection component.

Discussion and Conclusion
We have carried out spectral analysis of the 2023 outburst of Swift J1727.8-1613, which was observed simultaneously by Insight-HXMT, NICER, and NuSTAR on MJD 60185.Such joint observations reveal the properties of the newly discovered BH system Swift J1727.8-1613: the system has a moderate inclination and harbors a highly spinning This is consistent with the results first reported with NICER data (Draghis et al. 2023).Most interestingly, this system distinguishes itself from a normal BH X-ray binary system by showing an additional component in the spectrum apart from the reflection one.
A spectral analysis solely with NuSTAR observation shows a hint of the need for a hard X-ray tail.Such a component shows up significantly by adding the simultaneous Insight-HXMT observations, which significantly extends the upper bound of the spectrum from 75 keV of NuSTAR to roughly 140 keV.Accordingly, an additional cutoffpl component is observed apart from the reflection one.Obviously, such a spectrum is rather peculiar and distinguishable from most of the normal BH X-ray binary systems in their outbursts.
BH X-ray binary systems usually have a spectrum composed of disk componization and reflection components before entering the soft state of their outbursts but with rare additional nonthermal components, e.g., the hard tail.So far the hard tails have been observed in a few systems.One is from Cyg X-1, mostly in its relatively soft state but not the hard state (Cadolle Bel et al. 2006;Laurent et al. 2011;Tomsick et al. 2014).Systems reminiscent of Swift J1727.8-1613 may be the outbursts from MAXI J1820+070 and MAXI J1535-571.For MAXI J1820+070, the spectrum observed by Insight-HXMT during the hard state can be well fitted by two reflection components: one from the inner disk contributing the broadened iron line and the second one illuminating the outer part (You et al. 2021), with both illuminators from the jet.However, Kawamura et al. (2023) performed a detailed spectral-timing analysis to investigate the nature of the accretion flow in BH binaries.By fitting jointly the energy spectrum, the time lag, and the QPO rms spectrum, they found that the hard X-rays at energies above roughly 50 keV cannot be well covered by the spectral models, which again may point to the need of a jet emission to account for the possible hard tail.MAXI J1535-571 may be so far the sample most reminiscent of Swift J1727.8-1613: in its hard intermediate state a significant hard tail was observed in addition to the disk reflection component (Kong et al. 2020).Obviously, all these hard tails tend to more or less be understood by introducing jet-like structures that produce hard X-rays.We also notice that Reig & Kylafis (2016) suggested that hard tails may be related to relativistic jets formed by massive collimated magnetic fields near BHs.
Our spectral analysis of Swift J1727.8-1613suggests two possibilities for describing the extra spectral component.One possibility is that we observed a hard tail at high energies.The presence of an additional hard component, along with the reflection component, in the spectrum of Swift J1727.8-1613provides an additional sample in such a hard-tail family of BHXRBs.An alternative possibility is that the extra spectral component may take a broad hump shape in the mediate energy band.We constitute in what follows a joint jet-corona scenario and adjust the configuration to account for both cases of the inferred possible extra spectral component.
By introducing Swift J1727.8-1613 in a jet-corona scenario, one key point is how to handle the additional component that may contribute to less disk reflections.If the extra component is a hard tail, one possibility is that the jet responsible for the hard tail has a large velocity and thus results in less illumination of the disk.In our fitting (M2), we replace comptt with relxilllpcp to form model 4 (M4) but encounter difficulty in constraining the height and reflection fraction.Consequently, if we fix them at 500 R g and 0.2, respectively, the velocity of the jet is estimated around 0.79 c 0.1 0.1 -+ (see Table 4).We notice that having such a large velocity is independent of how to

Note.
a We fixed the Γ of NICER to 0 and used the ΔΓ to account for the relative index corrections between NuSTAR and Insight-HXMT ME and HE compared to NICER.choose the height.As a result, the contribution of the jet producing the hard tail to the reflection component is small (see the left side of Figure 7).The reflection component mainly comes from the high jet base/corona, which hangs above the disk.The corona has the potential to block the jet if the jet base/corona has a relatively large extension in the direction perpendicular to the jet and a large optical depth.The optical depth is estimated as about 3.61 according to the parameter Γ and E cut listed in Table 2. Therefore, this blocking may lead to even less jet contribution to the disk reflection.Spectral fittings show the extra component of a broad hump can also show up in the mediate energy band.In our investigation of M5, where we replace cutoffpl in M3 with relxilllp and fix its reflection parameters, we find that while the velocity and height of this additional reflection component cannot be constrained, the reflection fraction has a rather small value.As shown in Table 4, assuming that the additional reflection component originates from the corona, we fix the velocity at 0 and obtain a reflection fraction of approximately 0.006 0.005 0.010 -+ . As shown on the right side of Figure 7, the jet base/corona can be located at a site very close to the BH, and the jet on top of the jet base/corona dominates the disk illumination.In such a configuration, a flat jet base/corona located close to the BH can be viewed by the disk from mostly , respectively (see Table 4).The corona may resist gravity through a magnetic field to allow it to exist at several times R g above the BH.You et al. (2023) discovered that the presence of a sufficiently strong magnetic field in the radial direction of the disk can halt the accretion flow, producing a magnetically arrested disk (MAD) in MAXI J1820+070.The strength of the magnetic field in both the vertical and radial directions is typically considered to be of the same order of magnitude, and we hypothesize the existence of an extremely strong magnetic field in the vertical direction to counteract gravity.
These results support further the scenario of having a low speed jet and a jet base/corona underneath located close to the BH, as shown on the right side of Figure 7.
Another issue is how the hard component without obvious reflection is produced.It is commonly believed that hard X-ray photons from BH X-ray binaries are produced by inverse  Compton scattering of low-energy photons (also called seed photons) by hot or high-energy electrons, as the comptt model describes.The seed photons are commonly believed to be produced by the thermal emission from the disk.However, the lack of reflection means that the hard X-ray photons do not illuminate the disk effectively, and consequently the disk photons also cannot reach the jet/corona producing the hard photons.It seems the only option out is that the seed photons are produced inside the jet/corona, perhaps through synchrotron radiation of high-energy electrons in strong magnetic fields.This is similar to the so-called synchrotron-self Comptonization process commonly used in describing the high-energy spectra of the jets of blazars and gamma-ray bursts.Further theoretical studies are needed to understand this problem.
In summary, simultaneous observations from Insight-HXMT, NICER, and NuSTAR provide the first view of the possible intrinsic properties of the newly discovered BH X-ray binary system Swift J1727.8-1613.Apart from revealing the spin of the BH and the inclination of the system, the additional nonthermal spectral component discovered for Swift J1727.8-1613 may need a jet-corona configuration to account for the results from different trails of spectral fittings.Notes.All parameters that do not have associated errors are fixed based on the previous fitting results for models M2, M3, and M1. a All parameters that do not have associated errors are fixed, and "a," "i," "Afe," and "logxi" are linked.b We fixed the Γ of NICER to 0 and used the ΔΓ to account for the relative index corrections between NuSTAR and Insight-HXMT ME and HE compared to NICER.

Figure 2 .
Figure 2. Spectral fitting residuals for different models.The black, red, green, and blue points represent spectral residuals from NuSTAR FPMA, FPMB and Insight-HXMT ME, HE respectively.

Figure 3 .
Figure 3.The simultaneous broadband spectrum of Swift J1727.8-1613 is observed from NICER (black), NuSTAR/FPMA (red), NuSTAR/FPMB (green), Insight-HXMT/ME (blue), and Insight-HXMT/HE (cyan).Left panel: the spectrum of model 1, with a relatively high Γ and E cut for the reflection component.The bottom panel is the residual of model 1 after removing cutoffpl.Right panel: the spectrum of model 1, with a relatively low Γ and E cut for the reflection component.The bottom panel is the residual of model 1 after removing cutoffpl.

Figure 4 .
Figure 4.An illustration of one-and two-dimensional projections of the posterior probability distributions derived from the MCMC analysis for the parameters in M1 * .

Figure 5 .
Figure 5.An illustration of one-and two-dimensional projections of the posterior probability distributions derived from the MCMC analysis for the parameters in M1.

Figure 7 .
Figure7.The schematic of the corona/jet of Swift J1727.8-1613.Left: the jet has high speed and may as well be blocked by the extended corona underneath and thus provides less disk reflection.This configuration corresponds to the fitting results for models M1 * and M2, as shown in the right panel of Figure3and the left panel of Figure6.Right: reflection is mainly contributed by the low speed jet and the jet base/corona underneath is mostly edge-on with respect to the view of the disk and thus provides less reflection than the jet.This configuration corresponds to the fitting results for models M1 and M3, as shown in the left panel of Figure3and the right panel of Figure6.

Table 2
The Results of Spectral Fitting the NICER+NuSTAR+Insight-HXMT Data for Model M1

Table 3
The Results of Spectral Fitting the NICER+NuSTAR+Insight-HXMT Data for Models M2 and M3 We fixed the Γ of NICER to 0 and used the ΔΓ to account for the relative index corrections between NuSTAR and Insight-HXMT ME and HE compared to NICER. a

Table 4
The Results of Spectral Fitting the NICER+NuSTAR+Insight-HXMT Data for Models M4, M5, and M6