Bright X-ray Emission Research Articles

X-raying the ζ Tau binary system

Context. The Be star ζ Tau was recently reported to be a γ Cas analog; that is, it displays an atypical (bright and hard) X-ray emission. The origin of these X-rays remains debated. Aims. The first X-ray observations indicated a very large absorption of the hot plasma component (NH ~ 1023 cm−2). This is most probably related to the edge-on configuration of the ζ Tau disk. If the X-ray emission arises close to the companion, an orbital modulation of the absorption could be detected as the disk comes in and out of the line of sight. Methods. New XMM-Newton data were obtained to characterize the high-energy properties of ζ Tau in more detail. They are complemented by previous Chandra and SRG/eROSITA observations as well as by optical spectroscopy and TESS photometry. Results. The high-quality XMM-Newton data reveal the presence of a faint soft X-ray emission, which appears in line with that recorded for non-γ Cas Be stars. In addition, ζ Tau exhibits significant short-term variability at all energies, with larger amplitudes at lower frequencies (“red noise”), as is found in X-ray data of other γ Cas stars. Transient variability (softness dip, low-frequency signal) may also be detected at some epochs. In addition, between X-ray exposures, large variations in the spectra are detected in the 1.5–4 keV energy band. They are due to large changes in absorption toward the hottest (9 keV) plasma. These changes are not correlated with either the orbital phase or the depth of the shell absorption of the Hα line. These observed properties are examined in the light of proposed γ Cas models.

Mixing of Hot Shocked Plasma with Cold Gas in Nova YZ Ret 2020

The origin of bright X-ray emission lines that appear late in a nova eruption remains largely a puzzle. We present two high-resolution X-ray grating spectra of the classical nova YZ Ret, observed 77 and 115 days post-eruption, using XMM-Newton and Chandra, respectively. Both spectra feature resolved emission lines blueshifted by v = –1500 km s−1 and broadened by σ v = 500 km s−1. The two spectra are well described by a collisionally ionized plasma of kT ∼ 70 eV that dimmed by a factor of ∼40 between the two exposures. The spectra also show narrow radiative recombination continua (RRCs) of C+4, C+5, and N+5, indicating the interaction of the hot ionized plasma with cold electrons of kT ∼ 2 eV. The high-n Rydberg series of C+4 is anomalously bright, allowing us to measure the electron density through continuum lowering, which is in agreement with the He-like N+5 density diagnostic of n e = (1.7 ± 0.4) × 1011 cm−3. The high population of these high-n levels constitutes the best evidence to date of charge exchange (CX) with neutral H in an astrophysical ionized plasma. The remarkable fact that the velocity and plasma temperature are the same after 38 days, despite the high density and decreasing flux, is evidence for ongoing heating. We suggest the heating is due to a reverse shock in the nova ejecta, which forms a thin X-ray shell. The narrow RRCs and CX are attributed to direct mixing with cold gas, which overtakes the hot plasma either from the shock front or through the contact discontinuity.

Chandra X-Ray Observations of PSR J1849-0001, Its Pulsar Wind Nebula, and the TeV Source HESS J1849-000

We obtained a 108 ks Chandra X-ray Observatory (CXO) observation of PSR J1849-0001 and its pulsar wind nebula (PWN) coincident with the TeV source HESS J1849-000. By analyzing the new and old (archival) CXO data, we resolved the pulsar from the PWN, explored the PWN morphology on arcsecond and arcminute scales, and measured the spectra of different regions of the PWN. Both the pulsar and the compact inner PWN spectra are hard with power-law photon indices of 1.20 ± 0.07 and 1.49 ± 0.20, respectively. The jet-dominated PWN has a relatively low luminosity, lack of γ-ray pulsations, relatively hard and nonthermal spectrum of the pulsar, and sine-like pulse profile, which indicates a relatively small angle between the pulsar’s spin and magnetic dipole axis. In this respect, it shares similar properties with a few other so-called MeV pulsars. Although the joint X-ray and TeV spectral energy distribution can be roughly described by a single-zone model, the obtained magnetic field value is unrealistically low. A more realistic scenario is the presence of a relic PWN, no longer emitting synchrotron X-rays but still radiating in TeV via inverse-Compton upscattering. We also serendipitously detected surprisingly bright X-ray emission from a very wide binary whose components should not be interacting.

Searching for X-ray counterparts of degree-wide TeV halos around middle-aged pulsars with SRG/eROSITA

Context. Extended gamma-ray TeV emission (TeV halos) has been detected around middle-aged pulsars. A proposed model to explain these TeV halos is that electrons from a degree-wide pulsar wind nebula (PWN) get up-scattered by cosmic microwave background photons through inverse Compton processes. However, no X-ray degree-wide faint diffuse PWNe have been found around these middle-aged pulsars in previous X-ray observations. Aims. We performed a search for degree-wide PWNe around Geminga, PSR B0656+14, B0540+23, J0633+0632, and J0631+1036 using data from the first four consecutive Spectrum Roentgen Gamma/eROSITA all-sky surveys. In order to better understand the mechanisms underlying the formation of TeV halos, we investigated the magnetic field strength within an area of 1° in radius centred on those pulsars. Methods. To achieve our goals, we selected a list of suitable candidate pulsars in the eROSITA-DE part of the sky and applied data reduction techniques to process the eROSITA data. We then performed a spatial analysis of the regions around selected pulsars. Results. We do not detect any degree-wide diffuse emission driven by rotation-powered pulsars around Geminga, PSR B0656+14, B0540+23, J0633+0632, or J0631+1036. Indeed, close inspection of the data shows that the pulsars of interest are all embedded in diffuse emission from supernova remnants such as the Monogem Ring or the Rosetta Nebula, while PSR B0540+23 is located ~2.5° away from the Crab pulsar, which is exceptionally bright, causing its X-ray emission to spread over a significant area up to the position of PSR B0540+23 and therefore strongly diminishes our capacity to search for degree-wide bright diffuse X-ray emission. Conclusions. Despite the non-detection of any degree-wide PWN surrounding the analysed pulsars, we set flux upper limits to provide useful information on magnetic field strength and its spatial distribution around those pulsars, providing additional constraints on the proposed theory for the formation of TeV halos around pulsars.

Fluorescent Fe K line emission of γ Cas stars

Context. γ Cas stars are early-type Be stars that exhibit an unusually hard and bright thermal X-ray emission. One of the proposed scenarios to explain these properties postulates the existence of a neutron star companion in the propeller stage, during which the magnetosphere of a rapidly rotating neutron star repels infalling material. Aims. To test this model, we examined the fluorescent Fe Kα emission line at ~6.4keV in the X-ray spectra of γ Cas stars, which offers a powerful diagnostic of both the primary source of hard X-rays and the reprocessing material. Methods. We computed synthetic line profiles of the fluorescent Fe Kα emission line in the framework of the propelling neutron star scenario. Two reservoirs of material contribute to the fluorescence in this case: the Be circumstellar decretion disk and a shell of cool material that surrounds the shell of X-ray-emitting plasma around the putative propelling neutron star. Results. We analysed the synthetic line profiles and expected equivalent widths of the lines for three well-studied γ Cas stars. The predicted line strengths fall short of the observed values by at least an order of magnitude. Pushing the model parameters to reproduce the observed line strengths led to column densities towards the primary X-ray source that exceed the observationally determined values by typically a factor of 20, and would further imply a higher X-ray luminosity than observed. Conclusions. The strengths of the observed Fe Kα fluorescent emission lines in γ Cas stars are inconsistent with the expected properties of a propeller scenario as proposed in the literature.

SRG/eROSITA survey of Be stars

ABSTRACT Massive stars are known X-ray emitters and those belonging to the Be category are no exception. One type of X-ray emission even appears specific to that category, the γ Cas phenomenon. Its actual incidence has been particularly difficult to assess. Thanks to four semesters of sky survey data taken by the Spectrum-Roentgen-Gamma (SRG)/extended ROentgen Survey with an Imaging Telescope Array (eROSITA), we revisit the question of the X-ray properties of Be stars. Amongst a large catalogue of Be stars, eROSITA achieved 170 detections (20 per cent of the sample), mostly corresponding to the earliest spectral types and/or close objects. While X-ray luminosities show an uninterrupted increasing trend with the X-ray-to-bolometric luminosity ratios, the X-ray hardness was split between a large group of soft (and fainter on average) sources and a smaller group of hard (and brighter on average) sources. The latter category gathers at least 34 sources, nearly all displaying early spectral types. Only a third of them were known before to display such X-ray properties. The actual incidence of hard and bright X-rays amongst early-type Be stars within 100–1000 pc appears to be ∼12 per cent, which is far from negligible. At the other extreme, no bright supersoft X-ray emission seems to be associated with any of our targets.

On the Tidal Capture of White Dwarfs by Intermediate-mass Black Holes in Dense Stellar Environments

Intermediate-mass black holes (IMBHs) are the missing link between stellar-mass and supermassive black holes, widely believed to reside in at least some dense star clusters, but not yet observed directly. Tidal disruptions of white dwarfs (WDs) are luminous only for black holes less massive than ∼105 M ⊙, therefore providing a unique smoking gun that could finally prove the existence of IMBHs beyond any reasonable doubt. Here, we investigate the tidal captures of WDs by IMBHs in dense star clusters, and estimate upper limits to the capture rates of ∼1 Myr−1 for galactic nuclei and ∼0.01 Myr−1 for globular clusters. Following the capture, the WD inspirals onto the IMBH, producing gravitational waves detectable out to ∼100 Mpc by LISA for ∼104 M ⊙ IMBHs. The subsequent tidal stripping/disruption of the WD can also release bright X-ray and gamma-ray emission with luminosities of at least ≳1040 erg s−1, detectable by Chandra, Swift, and upcoming telescopes, such as the Einstein Probe.

Deep X-Ray and Radio Observations of the First Outburst of the Young Magnetar Swift J1818.0−1607

Swift J1818.0−1607 is a radio-loud magnetar with a spin period of 1.36 s and a dipolar magnetic field strength of B ∼ 3 × 1014 G, which is very young compared to the Galactic pulsar population. We report here on the long-term X-ray monitoring campaign of this young magnetar using XMM-Newton, NuSTAR, and Swift from the activation of its first outburst in 2020 March until 2021 October, as well as INTEGRAL upper limits on its hard X-ray emission. The 1–10 keV magnetar spectrum is well modeled by an absorbed blackbody with a temperature of kT BB ∼ 1.1 keV and apparent reduction in the radius of the emitting region from ∼0.6 to ∼0.2 km. We also confirm the bright diffuse X-ray emission around the source extending between ∼50″ and ∼110″. A timing analysis revealed large torque variability, with an average spin-down rate −2.3 × 10−11 Hz2 that appears to decrease in magnitude over time. We also observed Swift J1818.0−1607 with the Karl G. Jansky Very Large Array on 2021 March 22. We detected the radio counterpart to Swift J1818 measuring a flux density of S v = 4.38 ± 0.05 mJy at 3 GHz and a half-ringlike structure of bright diffuse radio emission located at ∼90″ to the west of the magnetar. We tentatively suggest that the diffuse X-ray emission is due to a dust-scattering halo and that the radio structure may be associated with the supernova remnant of this young pulsar, based on its morphology.

Revision of the γ-Ray Emission from SNR CTB 109 with the Fermi Large Area Telescope

CTB 109 is a middle-aged shell-type supernova remnant (SNR) with bright thermal X-ray emission. We reanalyze the GeV γ-ray emission from CTB 109 using 13 yr of Pass 8 data recorded by the Fermi Large Area Telescope. The γ-ray emission of CTB 109 shows a center bright morphology, which is well consistent with its thermal X-ray emission rather than the shell-type structure in the radio band. The spectral analysis shows an evident spectral curvature at ∼several GeV for the GeV γ-ray spectrum, which can naturally explain the lack of TeV γ-ray emission from CTB 109. Although either a leptonic or a hadronic model could fit the multiwavelength observations of CTB 109, the hadronic model is favored considering its γ-ray morphology and the spectral curvature of the GeV spectrum. The unusual γ-ray spectrum of CTB 109 with other SNRs and the luminosity-diameter squared relation make CTB 109 distinguished both from the young-aged SNRs with hard GeV γ-ray spectra and several old-aged SNRs interacting with molecular clouds.

Gamma Cas Stars as Be+White Dwarf Binary Systems

The origin of the bright and hard X-ray emission flux among the γ Cas subgroup of B-emission line (Be) stars may be caused by gas accretion onto an orbiting white dwarf (WD) companion. Such Be+WD binaries are the predicted outcome of a second stage of mass transfer from a helium star mass donor to a rapidly rotating mass gainer star. The stripped donor stars become small and hot white dwarfs that are extremely faint compared to their Be star companions. Here we discuss model predictions about the physical and orbital properties of Be+WD binaries, and we show that current observational results on γ Cas systems are consistent with the expected large binary frequency, companion faintness and small mass, and relatively high mass range of the Be star hosts. We determine that the companions are probably not stripped helium stars (hot subdwarf sdO stars), because these are bright enough to detect in ultraviolet spectroscopy, yet their spectroscopic signatures are not observed in studies of γ Cas binaries. Interferometry of relatively nearby systems provides the means to detect very faint companions including hot subdwarf and cooler main-sequence stars. Preliminary observations of five γ Cas binaries with the CHARA Array interferometer show no evidence of the companion flux, leaving white dwarfs as the only viable candidates for the companions.

Detection of the Extended γ-Ray Emission from the High Galactic Latitude Calvera’s SNR Candidate

We report the extended GeV γ-ray emission that is spatially associated with the high Galactic latitude supernova remnant (SNR) candidate—Calvera’s SNR with the Pass 8 data recorded by the Fermi Large Area Telescope. The γ-ray spectrum of Calvera’s SNR between 100 MeV and 1 TeV shows an evident (∼3.4σ) spectral curvature at several tens of GeV. The multiwavelength data can be fitted with either a leptonic model or a hadronic one. However, the leptonic model exhibits inconsistency between the flat radio spectrum and the hard GeV γ-ray spectrum of Calvera’s SNR. For the hadronic model, the spectral index of protons should be harder than 1.6 and the total energy of protons is fitted to be more than one order of magnitude higher than the explosion energy of a typical supernova, which also challenges the hadronic model. The evident spectral curvature and the absence of nonthermal X-ray emission from Calvera’s SNR makes it to be an interesting source bridging young-aged SNRs with bright nonthermal X-ray emission and old-aged SNRs interacting with molecular clouds.

Particle Acceleration Controlled by Ambient Density in the Southwestern Rim of RCW 86

The physics of particle acceleration at supernova remnant (SNR) shocks is one of the most intriguing problems in astrophysics. SNR RCW 86 provides a suitable environment for understanding the physics of particle acceleration because one can extract the information on both accelerated particles and acceleration environment in the same regions through the bright X-ray emission. In this work, we study X-ray proper motions and spectral properties of the southwestern region of RCW 86. The proper motion velocities are found to be ∼300–2000 km s−1 at a distance of 2.8 kpc. We find two inward-moving filaments, which are more likely reflected shocks rather than reverse shocks. Based on the X-ray spectroscopy, we evaluate thermal parameters such as the ambient density and temperature, and nonthermal parameters such as the power-law flux and index. From the decrease in flux over time of several nonthermal filaments, we estimate the magnetic field amplitudes to be ∼30–100 μG. Gathering the physical parameters, we then investigate parameter correlations. We find that the synchrotron emission from thermal-dominated filaments is correlated with the ambient density n e as (power-law flux) and (power-law index) , and not or only weakly correlated with the shock velocity and shock obliquity. As an interpretation, we propose a shock–cloud interaction scenario, where locally enhanced magnetic turbulence levels have a great influence on local acceleration conditions.

Candidate Tidal Disruption Event AT2019fdr Coincident with a High-Energy Neutrino.

The origins of the high-energy cosmic neutrino flux remain largely unknown. Recently, one high-energy neutrino was associated with a tidal disruption event (TDE). Here we present AT2019fdr, an exceptionally luminous TDE candidate, coincident with another high-energy neutrino. Our observations, including a bright dust echo and soft late-time x-ray emission, further support a TDE origin of this flare. The probability of finding two such bright events by chance is just 0.034%. We evaluate several models for neutrino production and show that AT2019fdr is capable of producing the observed high-energy neutrino, reinforcing the case for TDEs as neutrino sources.

TESS light curves of γ Cas stars

ABSTRACT γ Cas stars constitute a subgroup of Be stars showing unusually hard and bright X-ray emission. In search for additional peculiarities, we analysed the TESS light curves of 15 γ Cas analogues. Their periodograms display broad frequency groups and/or narrow isolated peaks, often superimposed over red noise. The detected signals appear at low frequencies, with few cases of significant signals beyond 5 d−1 (and all of them are faint). The signal amplitudes, and sometimes the frequency content, change with time, even in the absence of outburst events. On the basis of their optical photometric variability, γ Cas stars reveal no distinctive behaviour and thus appear similar to Be stars in general.

Fast Blast Wave and Ejecta in the Young Core-collapse Supernova Remnant MSH 15-52/RCW 89

Abstract One of the youngest known remnants of a core-collapse supernova (SN) in our Galaxy is G320.4−1.2/MSH 15-52, containing an energetic pulsar with a very short (1700 yr) spindown age and likely produced by a stripped-envelope SN Ibc. Bright X-ray and radio emission north of the pulsar overlaps with an Hα nebula RCW 89. The bright X-rays there have a highly unusual and quite puzzling morphology, consisting of both very compact thermally emitting knots and much more diffuse emission of nonthermal origin. We report new X-ray observations of RCW 89 in 2017 and 2018 with Chandra that allowed us to measure the motions of many knots and filaments on decade-long time baselines. We identify a fast blast wave with a velocity of km s−1 ( is the distance in units of 5.2 kpc) with a purely nonthermal spectrum, and without any radio counterpart. Many compact X-ray emission knots are moving vary fast, with velocities as high as 5000 km s−1, predominantly radially away from the pulsar. Their spectra show that they are Ne- and Mg-rich heavy-element SN ejecta. They have been significantly decelerated upon their recent impact with the dense ambient medium north of the pulsar. We see fast evolution in brightness and morphology of knots in just a few years. Ejecta knots in RCW 89 resemble those seen in Cas A at optical wavelengths in terms of their initial velocities and densities. They might have the same origin, still not understood but presumably related to stripped-envelope SN explosions themselves.

Hyperluminous starburst gives up its secrets

Abstract HATLAS J084933.4 + 021443 was identified as a dusty starburst via its rest-frame far-infrared (far-IR) emission. Multifrequency imaging and spectroscopy revealed a cluster of four dusty galaxies at z = 2.41, covering 80 kpc. Here, we use Atacama Large Millimetre Array to confirm a more distant, fifth protocluster member, and present X-ray and rest-frame optical imaging spectroscopy of the brightest, an unlensed hyperluminous IR galaxy (HyLIRG). The data reveal broad Hα and bright [N ii] lines, and bright X-ray emission, characteristics that betray a Type-1 active galactic nucleus (AGN), strengthening evidence that AGN are ubiquitous amongst HyLIRGs. The accreting black hole is supermassive, Mbh ≈ 2 × 109 M⊙, with little intrinsic absorption, NH ≈ 5 × 1021 cm−2. The X-ray properties suggest the accretion luminosity rivals that of the starburst, yet it is not obvious where this emerges in its panchromatic spectral energy distribution. We outline three scenarios that could give rise to the observed characteristics, and how we might distinguish between them. In the first, we see the AGN through the host galaxy because of the cavity it excavates. In the others, the AGN is not cospatial with the starburst, having been ejected via asymmetric gravitational radiation, or having evolved towards the naked quasar phase in an unseen companion.

H.E.S.S. andSuzakuobservations of the Vela X pulsar wind nebula

Context.Pulsar wind nebulae (PWNe) represent the most prominent population of Galactic very-high-energy gamma-ray sources and are thought to be an efficient source of leptonic cosmic rays. Vela X is a nearby middle-aged PWN, which shows bright X-ray and TeV gamma-ray emission towards an elongated structure called the cocoon.Aims.Since TeV emission is likely inverse-Compton emission of electrons, predominantly from interactions with the cosmic microwave background, while X-ray emission is synchrotron radiation of the same electrons, we aim to derive the properties of the relativistic particles and of magnetic fields with minimal modelling.Methods.We used data from theSuzakuXIS to derive the spectra from three compact regions in Vela X covering distances from 0.3 to 4 pc from the pulsar along the cocoon. We obtained gamma-ray spectra of the same regions from H.E.S.S. observations and fitted a radiative model to the multi-wavelength spectra.Results.The TeV electron spectra and magnetic field strengths are consistent within the uncertainties for the three regions, with energy densities of the order 10−12erg cm−3. The data indicate the presence of a cutoff in the electron spectrum at energies of ~ 100 TeV and a magnetic field strength of ~6μG. Constraints on the presence of turbulent magnetic fields are weak.Conclusions.The pressure of TeV electrons and magnetic fields in the cocoon is dynamically negligible, requiring the presence of another dominant pressure component to balance the pulsar wind at the termination shock. Sub-TeV electrons cannot completely account for the missing pressure, which may be provided either by relativistic ions or from mixing of the ejecta with the pulsar wind. The electron spectra are consistent with expectations from transport scenarios dominated either by advection via the reverse shock or by diffusion, but for the latter the role of radiative losses near the termination shock needs to be further investigated in the light of the measured cutoff energies. Constraints on turbulent magnetic fields and the shape of the electron cutoff can be improved by spectral measurements in the energy range ≳ 10 keV.

An Enormous Molecular Gas Flow in the RX J0821+0752 Galaxy Cluster

Abstract We present recent Chandra X-ray observations of the RX J0821.0+0752 galaxy cluster, in addition to ALMA observations of the CO(1–0) and CO(3–2) line emission tracing the molecular gas in its central galaxy. All of the CO line emission, originating from a molecular gas reservoir, is located several kiloparsecs away from the nucleus of the central galaxy. The cold gas is concentrated into two main clumps surrounded by a diffuse envelope. They form a wide filament coincident with a plume of bright X-ray emission emanating from the cluster core. This plume encompasses a putative X-ray cavity that is only large enough to have uplifted a small percent of the molecular gas. Unlike other brightest cluster galaxies, stimulated cooling, where X-ray cavities lift low-entropy cluster gas until it becomes thermally unstable, cannot have produced the observed gas reservoir. Instead, the molecular gas has likely formed as a result of sloshing motions in the intracluster medium induced by a nearby galaxy. Sloshing can emulate uplift by dislodging gas from the galactic center. This gas has the shortest cooling time, so it will condense if disrupted for long enough.

Stormy Weather in 3C 196.1: Nuclear Outbursts and Merger Events Shape the Environment of the Hybrid Radio Galaxy 3C 196.1

Abstract We present a multiwavelength analysis based on archival radio, optical, and X-ray data of the complex radio source 3C 196.1, whose host is the brightest cluster galaxy of a z = 0.198 cluster. HST data show Hα+[N ii] emission aligned with the jet 8.4 GHz radio emission. An Hα+[N ii] filament coincides with the brightest X-ray emission, the northern hotspot. Analysis of the X-ray and radio images reveals cavities located at galactic and cluster scales. The galactic-scale cavity is almost devoid of 8.4 GHz radio emission and the southwestern Hα+[N ii] emission is bounded (in projection) by this cavity. The outer cavity is cospatial with the peak of 147 MHz radio emission, and hence we interpret this depression in X-ray surface brightness as being caused by a buoyantly rising bubble originating from an active galactic nuclei outburst ∼280 Myr ago. A Chandra snapshot observation allowed us to constrain the physical parameters of the cluster, which has a cool core with a low central temperature ∼2.8 keV, low central entropy index ∼13 keV cm2 and a short cooling time of ∼500 Myr, which is < 0.05 of the age of the universe at this redshift. By fitting jumps in the X-ray density, we found Mach numbers between 1.4 and 1.6, consistent with a shock origin. We also found compelling evidence of a past merger, indicated by a morphology reminiscent of gas sloshing in the X-ray residual image. Finally, we computed the pressures, enthalpies E cav and jet powers P jet associated with the cavities: E cav ∼ 7 × 1058 erg, P jet ∼ 1.9 × 1044 erg s−1 for the inner cavity and E cav ∼ 3 × 1060 erg, P jet ∼ 3.4 × 1044 erg s−1 for the outer cavity.

MAHALO Deep Cluster Survey II. Characterizing massive forming galaxies in the Spiderweb protocluster at z = 2.2

This paper is the second in a series presenting the results of our deep H$\alpha$-line survey towards protoclusters at $z>2$, based on narrow-band imaging with the Subaru Telescope. This work investigates massive galaxies in a protocluster region associated with a radio galaxy (PKS 1138$-$262), the Spiderweb galaxy, at $z=2.2$. Our 0.5 mag deeper narrow-band imaging than previous surveys collects a total of 68 H$\alpha$ emitters (HAE). 17 out of the 68 are newly discovered protocluster members. First, a very high characteristic stellar mass of M$_\star^\ast=10^{11.73}$ M$_\odot$ is measured from a Schechter function fit to the mass distribution of HAEs. Together with the Chandra X-ray data, we find that four out of six massive HAEs (M$_\star>10^{11}$ M$_\odot$) show bright X-ray emission, suggesting that they host active galactic nuclei (AGNs). Their mass estimates, therefore, would be affected by the nuclear emission from AGNs. Notably, the X-ray detected HAEs are likely positioned near the boundary between star-forming and quiescent populations in the rest-frame $UVJ$ plane. Moreover, our deep narrow-band data succeed in probing the bright H$\alpha$ (+[Nii]) line nebula of the Spiderweb galaxy extending over $\sim100$ physical kpc. These results suggest that the massive galaxies in the Spiderweb protocluster are on the way to becoming the bright red sequence objects seen in local galaxy clusters, where AGNs might play an essential role in their quenching processes. Though a more statistical database is needed to build a general picture.