Hard X-rays and QPO in Swift J1727.8−1613: the rise and plateau of the 2023 outburst

ABSTRACT

We report on the detection of type-C quasi-periodic oscillations during the initial stages of the outburst of Swift J1727.8–1613 in 2023. Using data of the INTEGRAL observatory along with the data of the Mikhail PavlinskyART-XC telescope on board Spektr-RG and X-ray Telescope (XRT) of the Neil Gehrels Swift observatory the fast growth of the quasi-periodic oscillations (QPO) frequency was traced. We present a hard X-ray light curve that covers the initial stages of the 2023 outburst – the fast rise and plateau – and demonstrate that the QPO frequency was stable during the plateau. The switching from type-C to type-B QPO was detected with the beginning of the source flaring activity. We have constructed a broad-band spectrum of Swift J1727.8−1613 and found an additional hard cutoff power-law spectral component extending at least up to 250 keV. Finally, we have obtained an upper limit on the hard X-ray flux at the beginning of the optical outburst and estimated the delay of the hard X-ray outburst with respect to the optical one.

1 INTRODUCTION

Low-frequency quasi-periodic oscillations (LF QPO; see Wijnands & van der Klis 1999; Ingram & Motta 2019 for a review) of the X-ray flux are commonly observed during different stages of outbursts of low-mass X-ray binaries (LMXB). Usually, the first to emerge are type-C QPOs (Casella, Belloni & Stella 2005), which appear during the outburst rise phase, while the source is in a low/hard state (Tanaka & Shibazaki 1996; Remillard & McClintock 2006). Despite significant theoretical efforts (see e.g. Syunyaev 1973 for one of earlier predictions of QPO in black hole systems), there is no model that could reproduce the entire plethora of observed phenomena. Different models attribute the observed variability to Lense–Thirring precession of the central parts of the accretion flow (Stella & Vietri 1998; Ingram, Done & Fragile 2009), oscillations in the corona (Cabanac et al. 2010), coupling between the accretion disc and the Comptonizing corona (Mastichiadis, Petropoulou & Kylafis 2022), etc. (see Ingram & Motta 2019 for discussion of other models).

Although it is interesting to trace the QPO evolution all the way to the start of the outburst, that is rarely possible given the unpredictable nature of LMXB X-ray outbursts (although sometimes optical monitoring could warn us about an upcoming outburst, Russell et al. 2019), limited sensitivity of all-sky X-ray monitors, and the shortness of the rise stage, which usually lasts only few days.

A new X-ray transient, dubbed Swift J1727.8−1613, was detected in hard X-rays on 2023 August 24 (MJD60180) by both Swift Burst Alert Telescope (BAT, Page et al. 2023) and INTEGRAL (IBAS weak trigger 10373/0; Mereghetti et al. 2003).Later on, a number of optical (Baglio et al. 2023; Wang & Bellm 2023) and X-ray telescopes (Dovciak et al. 2023; Liu et al. 2023; Negoro et al. 2023; O’Connor et al. 2023; Sunyaev et al. 2023) also observed it. Optical spectroscopy (Castro-Tirado et al. 2023) revealed the presence of bright emission lines of hydrogen and helium, leading to the classification of the source as an LMXB candidate. X-ray spectral and temporal (Draghis et al. 2023; Palmer & Parsotan 2023) characteristics of the source were also typical for LMXB outbursts.

In this paper, we report on the detection of type-C QPO in the earliest stages of the outburst of Swift J1727.8−1613 with different X-ray telescopes. We were able to trace the evolution of the QPO frequency in detail during the first two weeks of the outburst, while the source transitioned from rapid rise to plateau. From simultaneous ART-XC  and INTEGRAL observations, we obtained and characterized the hard X-ray spectrum of the source in the broad-energy range of 5–230 keV during the plateau phase of the outburst, while in IBIS (Imager on Board the INTEGRAL Satellite) data accumulated throughout the outburst Swift J1727.8−1613  is detected up to 400 keV.

Because the wide-field telescopes aboard the INTEGRAL observatory are rarely used for aperiodic timing studies, we provide the public with an open data set, which could be used together with other multiwavelength observations in order to test models of LF QPO generation.

2 DATA ANALYSIS

To better characterize the early evolution of the source flux and to study its temporal properties, we used data from a number of X-ray telescopes. To construct a continuous long-term X-ray light curve of the outburst, we used data from MAXI (Matsuoka et al. 2009), covering the softer part of spectrum (2–20 keV). Because the MAXI light curve is contaminated by the nearby bright Galactic source GX 9 + 9, we decided not to use the MAXI data before MJD 60180.4, i.e. a few hours before the first hard X-ray detection.

We produced a long-term hard X-ray light curve using numerous serendipitous and target-of-opportunity INTEGRAL observations (see details in Section 2.1). To fill the gaps between adjacent INTEGRAL revolutions, we also used data from the Swift/BAT (Barthelmy et al. 2005) transient monitor (Krimm et al. 2013). Since the energy bands of the two instruments are slightly different, IBIS (30–80 keV) versus BAT (15–50 keV), we rescaled the BAT fluxes to match the IBIS measurements.

A number of observations of Swift J1727.8−1613 during the first days of the outburst were performed by the Swift/XRT telescope (Burrows et al. 2005). These data are crucial for us, since they enable a timing analysis. We selected observations performed in windowed timing mode (ObsIDs: 1186959005–006, performed on MJD60184–60185 for a total exposure of 10 ks) to produce 0.5–10 keV light curves for each snapshot.

For temporal and spectral studies we also employed SRG/ART-XC data from four dedicated observations, performed over first weeks of the outburst.

2.1 INTEGRAL observations

Thanks to the large field of view of the IBIS coded-mask telescope (Ubertini et al. 2003) on board the INTEGRAL observatory (Winkler et al. 2003), the sky region of the X-ray transient Swift J1727.8−1613 was covered during observations of the Galactic Centre region shortly before the outburst, during the rapid increase of the flux and during the outburst itself. In this work, we use the data acquired by the INTEGRAL Soft Gamma-Ray Imager (ISGRI) low-energy detector layer (Lebrun et al. 2003) of the IBIS telescope. For the timing analysis we also use data from the Joint European X-Ray Monitor (JEM-X) on board INTEGRAL, which is sensitive in the 3–35 keV energy range (Lund et al. 2003).

We reduced the IBIS/ISGRI data with the INTEGRAL data analysis software developed at IKI (see e.g. Krivonos et al. 2010, 2012; Churazov et al. 2014, and references therein), applying the latest energy calibration for the registered IBIS/ISGRI detector events with the INTEGRAL Offline Scientific Analysis (osa) version 11.2 provided by the INTEGRAL Science Data Centre (Courvoisier et al. 2003).

We produced a sky image for every individual INTEGRAL observation (referred to as a Science Window, or ScW). The flux scale in each ScW sky image was adjusted to the flux of the Crab Nebula measured in all available observations taken in 2023. Due to the loss of sensitivity at low energies caused by the gradual long-term degradation of the ISGRI detector, we set a low limit of 25 keV for our analysis. For spectral analysis, we generated images in 30 energy intervals logarithmically spaced between 25 and 450 keV.

For a fast timing analysis and the analysis of JEM-X data, we used the INTEGRAL Offline Scientific Analysis version 11.2. Before constructing the ISGRI light curves, we first recalculated the photon energies in the instrument’s event files using up-to-date calibration data. To this end, we launched the osa procedure ibis_science_analysis to the energy correction (COR) level. Next, using the ii_pif procedure, we calculated the ‘open part’ of each pixel (i.e. pixel-illumination-fraction, PIF) of the detector for the direction to the source. Then, we constructed a light curve in a broad 30–60 keV energy band with a 0.01 s time resolution for ‘fully open’ pixels subtracted from it a light curve obtained from the ‘fully closed’ pixels with the same time resolution, taking into account the number of pixels used for each light curve. In our case, when there is only one bright source in the field of view of the IBIS telescope, this is the simplest and most correct way to reconstruct a light curve.

2.2 SRG/ART-XC observations

Shortly after the discovery of Swift J1727.8−1613, we observed it with the Mikhail Pavlinsky ART-XC telescope (Pavlinsky et al. 2021) on board the SRG observatory (Sunyaev et al. 2021). The first observation, conducted on 2023 August 27, caught the source during the rise stage. Three later observations, on 2023 August 31, September 5 and 8, fell on the plateau phase on the light curve. ART-XC data were processed using the artproducts v1.0 software with the latest CALDB v20220908. Spectra and light curves were extracted from a circular region of radius |$R = 225\,$| arcsec centred at the source position. For the extraction of the light curve, a wide energy range of 4–25 keV was used. The spectral analysis was performed in a slightly different energy band of 5–25 keV, where the spectral response of the telescope is known better. Additionally, we decided to exclude 11–14 keV range for spectral analysis, where significant residuals arise from inaccuracies of effective area calibration near Ir edges. 2 per cent systematic errors were added to spectra.

3 OUTBURST LIGHT CURVE AND EARLY OPTICAL OBSERVATIONS

Fig. 1 shows the X-ray light curve of the outburst based on MAXI (2–20 keV) and IBIS (30–80 and 80–150 keV) data. The first detection of the source in these bands happened at a flux level of ≈0.1 Crab. In hard X-rays, the flux peaked approximately at MJD 60181.6 and then started to decline. The 2–20 keV light curve shows no obvious peak. After the initial rise, the source reached a plateau of ≈7 Crab and stayed at this level for about a week. Afterwards, the plateau changed to a modest decline, seen also in the hard band. Later on, the source started to exhibit soft X-ray flares, indicating a transition to a disc dominated soft-intermediate state. The overall light curve is typical of LMXBs in outburst, before transition to the soft state.

In recent years, monitoring observations of the optical sky have become widespread, with several projects covering significant parts of the sky with daily cadence. In conjunction with space-based X-ray observatories, this makes it possible to trace the evolution of outbursts of newly discovered X-ray transients simultaneously in the optical and X-ray bands. Together with dedicated programmes (e.g. XB-NEWS; Russell et al. 2019), this monitoring shows that optical emission often precedes X-ray outburst.

The optical counterpart of Swift J1727.8−1613 was detected by the ATLAS project (Tonry et al. 2018) as early as MJD 60176.368 (Wang & Bellm 2023) during the rapid rise. This brightening was bracketed by serendipitous INTEGRAL observations taken between MJD 60176.095 and MJD 60176.626. This allowed us to obtain a strict 4σ upper limit of 18 mCrab (or 3.5 × 10⁻¹¹ erg cm⁻² s⁻¹) on the average 30–60 keV flux of the source during this period. The optical flux at the same epoch was ≈100 μJy in the ATLAS o-filter. Using the value of the neutral hydrogen column density in the direction of Swift J1727.8−1613, N_H = 4 × 10²¹ cm⁻², determined from X-ray data (Draghis et al. 2023) and adopting A_V/N_H = 0.48 (Güver & Özel 2009), we estimate the total extinction in the o-filter at ≈1.4 mag. Hence, the monochromatic optical flux from Swift J1727.8−1613 was ≈2 × 10⁻¹² erg cm⁻² s⁻¹. We can thus place a conservative upper limit on the X-ray to optical luminosity ratio:L_X/L_opt ≲ 20.

Both the MAXI and INTEGRAL data show that the rapid brightening of the source started shortly before MJD 60180.5. Yet, as it was said earlier, MAXI data for Swift J1727.8−1613 is contaminated by the nearby bright X-ray binary GX 9 + 9 which was at ≈0.2 Crab (in 2–20 keV band) flux level in an August, right before the Swift J1727.8−1613 outburst. On the other hand, thanks to its better angular resolution, IBIS data is unaffected by nearby sources. For that reason we chose to use it to determine the epoch of onset of the X-ray outburst more precisely. We fitted a power law to the IBIS measurements of the flux on the rise (the first eight observations) and obtained F_X ∝ (t − 60180.2)^2.4. Therefore, the X-ray outburst started with a delay of at least 3.8 d with respect to the optical one.

Such an X-ray-to-optical lag, previously observed in many LMXBs (e.g. Orosz et al. 1997; Jain et al. 2001; Wren et al. 2001; Buxton & Bailyn 2004), can be explained in the disc-instability model (see section 5.1 in Dubus, Hameury & Lasota 2001). Using the scaling relation from Bernardini et al. (2016), we can estimate the radius R_o at which the optical outburst started. Assuming the mid-plane disc temperature to be T = 40 000 K, adopting the viscosity parameter α = 0.2 and assuming that the X-ray outburst starts when the inner radius of the thin disc reaches R_X = 5 × 10⁸ cm (as in Dubus et al. 2001), we get R_o ≈ 10⁹ cm or about 700R_g for a |$10\, {\rm M}_{\odot }$| black hole.

4 TIMING FEATURES

Long hard X-ray observations by INTEGRAL are well suited to search for LF QPOs in the initial stages of bright LMXB outbursts (e.g. Mereminskiy et al. 2018). We examined all available data from the JEM-X and IBIS telescopes in order to search for prominent features in the power spectra of Swift J1727.8−1613. We also searched through ART-XC  and Swift/XRT data in order to get as many measurements of the QPO fundamental frequency as possible. We fitted each power spectrum (in Leahy normalization, 0.01–50 Hz) by a model consisting of four components: a constant for Poisson noise, a wide zero-centred Lorentzian for low-frequency noise and two narrow Lorentzians with tied frequencies to describe the fundamental QPO and its harmonic. The observed power-spectra observed with Swift/XRT, IBIS, and ART-XC telescopes at the beginning of the plateau (MJD60185−60186) are shown in Fig. 2 We provide all reliable QPO measurements in Table 1 along with Fig. 1. During the flaring activity after MJD 60205, the QPO switched to the type-B with values up to ∼10 Hz (see Fig. 1). Since these QPO are beyond the scope of this paper we excluded them from the analysis.

The first secure detection of type-C QPO in IBIS was at F_QPO ≈ 80 mHz on MJD 60181.46, one day after the first detection of the source. Then QPOs were detected at frequencies F_QPO = 140...240 mHz on several occasions with JEM-X, when the source was in its field of view. It should be noted that these observations were a part of a regular Galactic Centre region survey (PI: Sunyaev), therefore they were not optimized for studying Swift J1727.8−1613. Later on, the QPO frequency continued to grow until it reached a plateau of 1.2 Hz a week after the start of the outburst. This plateau lasted for another week. Fig. 3 summarizes the early evolution of the QPO frequency. The initial frequency growth is nearly linear, whereas the later evolution can be roughly described by a logistic function with a characteristic width of 0.8 d. After the plateau frequency continued to grow, while the hard flux receded.

It is interesting to see how the QPO frequency behaves during the plateau stage. Since both the soft and hard X-ray fluxes are nearly constant during this period, we can assume that the accretion disc was stable so that the QPO frequency drifted around some central value. After subtraction of the linear trend (estimated from the IBIS data alone, ≈0.03 Hz d⁻¹), it can be clearly seen (Fig. 3, lower panel) that the frequency shows little variability on short time-scales (as evident from the dense ART-XC  measurements) while oscillating around the mean value on a time-scale of ≈10 h with a standard deviation of 0.09 Hz. It should be noted that simultaneous ART-XC and INTEGRAL measurements of QPO frequency do agree well with respect to uncertainties.

Using equation (2) from Ingram et al. (2009) and assuming that M_BH = 10, a = 0.9, ζ = 0, and the inner radius of the precessing flow depends on the black hole spin (Lubow, Ogilvie & Pringle 2002), we can infer the outer radius of the hot flow and trace its evolution with time. We thus find that during the initial fast-rise phase, the precessing flow shrinks rapidly at a rate of |$\approx 40\, \mathrm{\mathit{ R}_{g}}\, \mathrm{d^{-1}} \approx 700$| cm s⁻¹. At the plateau stage, the characteristic peak-to-peak speeds are lower: |$15 \, \mathrm{\mathit{ R}_{g}}\, \mathrm{d^{-1}}$|⁠.

5 SPECTRUM AND HARD X-RAY EMISSION

A detailed spectral analysis of the Swift J1727.8−1613 X-ray emission at different phases of the outburst is beyond the scope of this paper and will be carried out elsewhere, using data from all INTEGRAL instruments, including the SPI telescope. Below, we only present the results obtained by the ART-XC telescope and the INTEGRAL observatory during the plateau phase in order to characterize the general shape of the spectrum and to search for hard X-ray – soft gamma-ray radiation.

The source was observed four times by the ART-XC telescope in the period from 2023 August 27 to September 8. For spectral analysis we used data from second, third, and fourth observations, which were conducted during the plateau phase. Since the source shown little variability during the plateau (see Fig. 1), we decided to average all ART-XC  spectra and use them along with the spectrum obtained from simultaneous observations by the IBIS/INTEGRAL telescope in harder X-rays. The joint spectrum were fit in the xspec package (Arnaud 1996) using phenomenological model consisting of two major components. First component is power law with exponential cutoff at high energies with additional reflection from ionized accretion disc (xillver; García et al. 2013), which is expected from the presence of strong, broadened Fe-line (Draghis et al. 2023). Given the limited energy resolution of ART-XC, we decided not to use more sophisticated models, which rely on exact shape of Fe-line. Second component is responsible for significant part of the emission above 100 keV and is also a power law with cutoff. We also included the absorption, fixed at N_H = 4 × 10²¹ cm⁻² (Draghis et al. 2023) and multiplicative constant to account for cross-calibration differences. Regarding the parameters of an ionized disc we chose to fix relative Fe-abundance at unity and irradiation parameter logξ = 4. This simple model allows us to reproduce the general shape of the spectrum (see Fig. 4, |$\chi ^{2}_{\mathrm{ red}.}\approx 0.9$|⁠). Best-fitting parameters hint that the first component is reflection dominated, with fraction of direct emission less than 1 per cent and produced by hard spectrum (Γ = 1.40 ± 0.05, E_cut  = 30 ± 2 keV). High-energy component is characterized by a moderate |$\Gamma =1.86^{+0.18}_{-0.41}$| and cutoff energy E_cut = |$144^{+60}_{-44}$| keV, total 5–200 keV flux of the hard component is |$2.8^{+0.4}_{-0.5}\times 10^{-8}$| erg cm⁻² s⁻¹.

Such high-energy components are often observed by INTEGRAL in transient black-hole X-ray binaries (see e.g. Jourdain, Roques & Rodi 2017; Zdziarski et al. 2021; Cangemi et al. 2023) and are typically attributed to either synchrotron emission from jet (Laurent et al. 2011) or presence of non-thermal particles in corona (Coppi 1999; Zdziarski et al. 2001). However, multiwavelength data are needed to make a decisive conclusion on nature of this component. Given the detection of the compact jet emission in radio and mm bands (Bright et al. 2023; Miller-Jones et al. 2023; Vrtilek et al. 2023) it will be interesting to compare properties of radio-to-near-infrared spectra with hard X-rays to see if they belong to single synchrotron continuum.

From Fig. 4 it can be seen that even on a limited data set, hard X-ray emission from Swift J1727.8−1613 is significantly detected up to ≃230 keV. This, as well as the presence of a power-law hard X-ray tail in the spectrum, suggests the possibility of detecting even more energetic photons from the source. To this end, we used all available IBIS/INTEGRAL data obtained in the period from August 24 to September 26 and constructed images of the sky area around Swift J1727.8−1613 at energies above 250 keV. We found that Swift J1727.8−1613 was significantly registered up to energies ≃400 keV as could be seen from Fig. 5. In particular, the source flux in the 350–400 keV energy range is 0.60 ± 0.13 Crab. Although, no signal was detected from the source in the harder 400–500 keV range with the corresponding upper limit of 200 mCrab (1σ).

6 CONCLUSION

Thanks to both serendipitous and dedicated INTEGRAL observations, we have managed to trace the evolution of the hard X-ray flux and the growth of the type-C QPO frequency during the initial stages of the outburst of Swift J1727.8−1613. Assuming that the QPO frequency traces the boundary between the disc and the hot flow, we used INTEGRAL and SRG/ART-XC observations to measure the characteristic speed of the inward motion of this boundary during the first two weeks of the outburst. We have also observed switching from type-C to type-B QPO that happened on MJD60205, when the source began the flaring activity.

We have constructed a broad-band 5–230 keV spectrum of Swift J1727.8−1613 and demonstrated the presence of a hard spectral component, that dominates emission above ≈100 keV. Finally, we have obtained an upper limit on the hard X-ray flux at the beginning of the optical outburst F_{30–60 keV} < 3.5 × 10⁻¹¹ erg cm⁻² s⁻¹ and estimated the delay of the X-ray outburst with respect to the optical one to be greater than 3.8 d.

Acknowledgement

This work is based on observations with the Mikhail Pavlinsky ART-XC telescope, hard X-ray instrument on board the SRG observatory. The SRG observatory was created by Roskosmos in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI) in the framework of the Russian Federal Space Program, with the participation of Germany. This work is based on observations with INTEGRAL, an ESA project with instruments and the science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), and Poland, and with the participation of Russia and the USA. The work was supported by the RSF grant 19-12-00423.

The authors are grateful to anonymous referees for critical remarks and useful comments.

DATA AVAILABILITY

The INTEGRAL, MAXI, and Swift data are accessible through the corresponding web pages. At the time of writing, the SRG/ART-XC data and the corresponding data analysis software have a private status. We plan to provide public access to the ART-XC scientific archive in the future.

References