X-ray quasi-periodic eruptions from the galactic nucleus of RX J1301.9+2747
AAstronomy & Astrophysics manuscript no. final c (cid:13)
ESO 2020March 23, 2020 L etter to the E ditor X-ray quasi-periodic eruptions from the galactic nucleus ofRX J1301.9+2747
Margherita Giustini , Giovanni Miniutti , and Richard D. Saxton Centro de Astrobiología (CSIC-INTA), Camino Bajo del Castillo s / n, Villanueva de la Cañada, E-28692 Madrid, Spain Telespazio-Vega UK for ESA, Operations Department; European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s / n,Villanueva de la Cañada, E-28692 Madrid, SpainReceived / Accepted
ABSTRACT
Following the recent discovery of X-ray quasi-periodic eruptions (QPEs) coming from the nucleus of the galaxy GSN 069, here wereport on the detection of QPEs in the active galaxy named RX J1301.9 + + ff erence between the consecutive recurrence times is significantly smaller in GSN 069. Longer X-ray observations will betterclarify the temporal pattern of the QPEs in RX J1301.9 + ffi cient active galaxies. PreviousX-ray observations of RX J1301.9 + Key words. galaxies: active — galaxies: nuclei — quasars: general — quasars: super massive black holes — X-rays: individuals:RX J1301.9 +
1. Introduction
The phenomena known as X-ray quasi-periodic eruptions(QPEs) have recently been detected in the nucleus of the galaxyGSN 069 (Miniutti et al. 2019) as high-amplitude, recurrent X-ray flares over a generally stable flux level (quiescent level). InGSN 069, QPEs last about one hour and recur about every ninehours, with a subtle alternating pattern of long-short recurrencetimes and strong-weak amplitudes. The QPE amplitude in GSN069 is energy-dependent and up to two orders of magnitude inthe 600 −
800 eV band. The X-ray spectrum of GSN 069 in thequiescent level is super-soft (most of the emission is at E < kT ∼
50 eV. During the overall X-ray decaybetween December 2010 and January 2019, the long-term evo-lution of the quiescent emission is consistent with the L ∝ T relation expected from a constant-area emitting accretion disk.This allowed Miniutti et al. (2019) to estimate a black hole massof M BH ∼ × M (cid:12) , associated with an uncertainty factor of afew due to the unknown black hole spin and observer inclination.During QPEs, the X-ray spectrum of GSN 069 smoothly evolvesinto a warmer state with kT ∼
120 eV and back to the tem-perature preceding the QPE onset. The QPEs detected in GSN069 are a new phenomenon whose physical origin is under in-vestigation: they might be related, among other possibilities, toradiation- or magnetic-pressure instabilities of the inner accre- tion flow or to the orbital motion of a secondary body (Miniuttiet al. 2019).Motivated by this discovery, we scanned the literature forcosmic sources that could be potentially analogous to GSN 069in order to search for similar events. The properties that makeGSN 069 stand out among the general active galactic nuclei(AGN) population are: (i) small black hole mass; (ii) high Ed-dington ratio; (iii) pure thermal disk spectrum with little or nohard X-ray power law emission; and (iv) lack of broad opticalor UV emission lines (Miniutti et al. 2013, 2019). We selectedRX J1301.9 + + z = . ∼ (cid:48) away from the center of the Comacluster. It had already been detected by EXOSAT in the 80s( f . − . ∼ . × − erg cm s − , Branduardi-Raymont et al.1985) but it was only after the ROSAT observations performedin the 90s that the source was identified as an active galaxy (De-wangan et al. 2000).A rapid flare lasting (cid:38) ∼ . + Article number, page 1 of 9 a r X i v : . [ a s t r o - ph . H E ] M a r & A proofs: manuscript no. final in June 1991 (see Figure 4 of Dewangan et al. 2000). RXJ1301.9 + ∼ + kT ∼
55 eV (Dewangan et al. 2000).The flux-resolved spectral analysis performed on the XMM-Newton and Chandra data by Sun et al. (2013) and Shu et al.(2017) revealed a spectrum that is well-fitted by a thermal diskcomponent with kT ∼ −
50 eV in the low-flux state, and kT ∼ −
300 eV in the high-flux state, plus a weak hard powerlaw emission. The Eddington ratio of RX J1301.9 + m ∼ .
14, and its black hole massat M BH ∼ . − . × M (cid:12) , from the UV / X-ray analysis per-formed by Shu et al. (2017), which also revealed an absence ofbroad optical / UV emission lines. Middleton & Ingram (2015)noted the peculiar behaviour of the XMM-Newton 2000 X-raylight curve of RX J1301.9 + + σ confidence level throughout the pa-per. A flat cosmology ( Λ = . q = H =
70 km s − Mpc − ) is assumed for the computation of the source intrinsicluminosity.
2. Data reduction and analysis
We reduced and analyzed the new data collected by XMM-Newton in May 2019, taking place during a DDT observa-tion pointed at RX J1301.9 + + . / N) drops above 2 keV, therefore, for our analysis we discardthe signal above this energy. We focus our analysis on the EPIC- The actual amplitude and duration of the event are unknown, as onlythe decaying phase was caught by the satellite due to orbital constraints. pn data because of superior S / N with respect to the EPIC-MOScameras. Details of the observations are reported in Table 1.
The RX J1301.9 + . − ∼ ∼ ∼
40 ks after the beginning of the scientific exposure, withthe count rate sharply increasing and decreasing with respect toa stable X-ray emission. During the 2000 observation, one anda half events were detected by the EPIC-MOS cameras, whileonly one was detected by the EPIC-pn due to the later start ofits scientific exposure. In both epochs of observation, the shapeof the X-ray light curve of RX J1301.9 + ff erentfrom the typical AGN, in addition to being remarkably similarto the one displayed by GSN 069 from December 2018 onward(Miniutti et al. 2019). Although the events in RX J1301.9 + + + + E > / N drops. Forcompleteness, we also plot in the bottom panels of Fig. A.1 the1.3-2 keV light curves for the two epochs of observation. Re-sults of the analysis are shown in Fig. 2, where the 2000 QPEproperties are plotted with black squares, the three 2019 QPEs(hereafter QPE1, QPE2, and QPE3) with red, green, and bluecircles, respectively. In both 2000 and 2019, the X-ray QPEs inRX J1301.9 + (cid:28)
10 at E (cid:46) . (cid:38)
50 at E (cid:38) . / N dropsat E > / N up to E ∼ . ∼ −
800 s at the highest energies probed. In-deed, the duration of the QPEs in RX J1301.9 + ff erence in Gaussianpeaks used to model the QPE between the 0 . − >
500 s at E (cid:46) . ff erent amplitudes,with QPE3 being more intense than QPE 1, which, in turn, ismore intense than QPE2 at all energies. QPE2 is the one withthe lowest amplitude and also the one with the longest durationat all energies. Article number, page 2 of 9iustini, Miniutti & Saxton: X-ray QPEs in RX J1301.9 + Table 1: Observation log for the XMM-Newton EPIC-pn observations of RX J1301.9 + OBSID Start / End date exposure (src + bkg) . − (bkg) . − (src + bkg) − (bkg) − (yyyy-mm-dd hh:mm:ss) (s) counts s − counts s − counts s − counts s − / . ± .
002 0 . ± . . ± . . ± . / . ± .
003 0 . ± . . ± . . ± . EPIC-MOS2EPIC-MOS1EPIC-pn C oun t r a t e ( s - ) A m p li t ude Time (ks)
Time (ks)
Fig. 1: Background-corrected light curves of RX J1301.9 + . − We divide the data in a low-flux and a high-flux state, using athreshold of < . − and > . − applied to the 0 . − E (cid:38) . E ∼ . χ statis-tics is used as a measure of the goodness of fit of the model tothe data. Spectral analysis results are reported in Table A.1.In all our fits, we model the absorption along the line of sightwith the tbabs model in Xspec (Wilms et al. 2000), leavingthe column density value N H free to vary. First, we fit the datato a simple thermal disk model (model 0: diskbb into Xspec),leaving all the parameters free to vary during the fit: the resultingstatistics are very poor, giving a reduced chi squared χ r ∼ . ν ). The addition of a power law com-ponent improves the fit statistics significantly ( ∆ χ / ∆ ν = / Γ = . powerlaw + diskbb in Table A.1) is marginally ac-ceptable ( χ r ∼ .
15) for a disk temperature of about 60 eV inthe low-flux state at both epochs, and ∼ −
150 eV in thehigh-flux state.We then assume a scenario where the quiescent emissionstays constant during each observation and a further emissioncomponent is only added to the high-flux state to represent thespectral contribution of the QPE. In practice, the model pa-rameters for the quiescent emission (modeled with powerlaw+ diskbb ) are tied between the low-flux and high-flux spec-tra of each epoch, while they are allowed to vary between 2000and 2019; the model parameters of the spectral component rep-resenting the QPE emission are fixed to zero in the low-fluxspectra, and are left free to vary in the high-flux spectra. Weuse three di ff erent models to represent the emerging QPE duringthe high-flux state: a blackbody emission (Model 2: bbody ), abremsstrahlung emission (model 3: bremss ), and a Comptoniza-tion emission where the disk photon temperature is used as input Article number, page 3 of 9 & A proofs: manuscript no. final
Fig. 2: RX J1301.9 + . − + ff erent models for theComptonized disk emission: nthcomp (model 4a) by Zdziarskiet al. (1996) and ˙Zycki et al. (1999), and comptt (model 4b)by Titarchuk (1994). Model 2 does not give a fair representa-tion of the data ( χ r ∼ . χ r < . + kT disk ∼
50 eV and a constant flux between epochs, f disk . − ∼ × − erg cm − s − , corresponding to an unabsorbedluminosity L disk . − ∼ . × erg s − . The flux of the hard X-ray power law in quiescence is instead found to be significantly Article number, page 4 of 9iustini, Miniutti & Saxton: X-ray QPEs in RX J1301.9 + higher in 2019 ( f pow . − ∼ . × − erg cm − s − , correspondingto L pow . − ∼ . × erg s − ) than in 2000 ( f pow . − ∼ − ergcm − s − , or L pow . − ∼ . × erg s − ). The QPE emergent spec-trum (i.e., the di ff erence between the high- and low-flux state)is well-represented by either a thermal bremsstrahlung or by aComptonized emission with an observed flux f QPE . − ∼ . × − erg cm − s − and a corresponding luminosity (corrected for ab-sorption along the line of sight) of L QPE . − ∼ . × erg s − . Inmodel 3, the bremsstrahlung temperature is found to increase be-tween 2000 and 2019, from ∼
220 to ∼
300 eV. In model 4a, it isthe Comptonisation asymptotic power law slope to significantlychange, hardening from Γ nth ∼ . ∼ ∼ ∼
14 from 2000 to 2019. In Model 3, amarginal improvement of the fit ( ∆ χ = kT bre = + − eV) to 2019 ( kT bre = + − eV).No improvement of the fit to models 4a and 4b is found insteadwhen allowing the power law normalization to vary during thehigh-flux state, because the power law normalization is degener-ate with the slope of the asymptotic power law of the nthcomp component (model 4a) and with the optical depth of the comptt component (model 4b). We point out that no spectral model canexplain the low-flux and high-flux spectra with only a change inoverall normalization because of the di ff erent spectral shape atthe two flux levels. We conclude that the 2019 QPE spectrum isharder and hotter than the 2000 QPE spectrum, regardless of theadopted best-fitting model.The amount of intrinsic neutral absorption at the redshift ofRX J1301.9 + / Argentine / Bonn survey ( N H = . × cm − , Kalberla et al. 2005) in model 3, while it is significantlylarger in Model 4, which allows for a softer intrinsic spectralshape. With the present data quality, we cannot exclude the pres-ence of more substantial columns of gas along the line of sight,especially if this is either ionized or only partially covering thecontinuum source.
3. Discussion
RX J1301.9 + + + + S of the X-ray emitting region is given bythe distance that light can travel in a given time ∆ t , and yields: S <
200 ( ∆ t / M (cid:12) / M BH ) r g , where r g = GM BH / c is the gravitational radius. Since, at the highest probed energies,the count rate doubling time during QPEs in RX J1301.9 + + − E (cid:46)
300 eV (Fig. 2) and, as in GSN 069, it drops at lowerenergies: in RX J1301.9 + E ∼ . E ∼ . + r g around a 10 M (cid:12) black hole is t dyn ∼
160 s. The threeQPEs detected in the 2019 observation of RX J1301.9 + ∼
20 ks between QPE1and QPE2, and one short of ∼ . t vis = ( H / R ) − t th . Here H / R is the scale height of the accre-tion flow and t th = t dyn /α is the thermal timescale, that canbe associated to the rising and decaying times of the QPEs.Given the short rising and decaying times in RX J1301.9 + α ∼ .
15 would beinferred. Adopting this value for the viscosity parameter yields H / R ∼ .
25, possibly indicating a geometrically thick or mag-netically elevated inner accretion flow (Noda & Done 2018;Dexter & Begelman 2019).The quiescent X-ray spectra of both RX J1301.9 + . − erg s − . The typical AGN X-raysignatures are very weak (e.g., the hard X-ray power law) orcompletely absent (e.g., a reflection component, a soft X-ray ex-cess) in both sources in the quiescent level. In GSN 069, the hardX-ray power law emits a very low luminosity ( (cid:46) erg s − )compared to typical AGN and has a roughly constant amplitudein di ff erent epochs of observation. A similar luminosity levelis inferred for the hard X-ray power law of RX J1301.9 + ∼
3. The RXJ1301.9 + E (cid:38) . L . − ∼ . − . × erg s − emerges in the spectra ofRX J1301.9 + + −
300 eV, or by Comptonization of theseed disk photons into a warm gas with a similar tempera-ture of the bremsstrahlung model. From 2000 to 2019, the RXJ1301.9 + nthcomp .This hardening would correspond, for a fixed temperature, to anincrease of the Comptonizing region optical depth from τ ∼ τ ∼
15 from 2000 to 2019 (Petrucci et al. 2018), compat-ible with the results of our spectral analysis with comptt . TheComptonizing region in RX J1301.9 + E (cid:46) . Article number, page 5 of 9 & A proofs: manuscript no. final is slightly larger when measured at E ∼ . − / N drops in 2000, while in 2019 there is good S / N until E ∼ . / N su ffi ciently to assess whether the di ff erences in prop-erties between QPEs are significant and whether there exists acorrelation between the QPE amplitude and duration. Such fu-ture observations should also clarify the pattern of variability ofthe QPEs of RX J1301.9 +
4. Conclusions
During a 48 ks XMM-Newton observation performed in May2019, three strong and rapid X-ray QPEs have been detected inthe nucleus of the galaxy RX J1301.9 + + + . − erg s − , aboutone order of magnitude higher than the luminosity of the quies-cent level. There are also clear di ff erences between the QPEs ob-served in RX J1301.9 + + + ff erent ( ∼
20 and ∼ . ff erencebetween long and short recurrence times . On the other hand,if the two sources do share the same physical phenomenon, asstrongly suggested by our analysis, longer observations of RXJ1301.9 + + . − erg s − , constant betweenthe 18.5 years between the two XMM-Newton observations; plusa weak hard X-ray power law, whose 0 . − < × erg s − . Also the spectrum of the QPE changedbetween 2000 and 2019, having become harder: this might meanthat in the time elapsed between the two observations the tem- The maximum di ff erence between consecutive recurrence times ob-served so far in GSN 069 is ∼ . perature of the QPE has increased or that in 2019, the power lawemission was also contributing to the QPE, which is contrary to2000.While in GSN 069, the QPEs are detectable during the over-all ∼
10 year-long (so far) decay following an outburst first de-tected in 2010, the QPEs of RX J1301.9 + ∼ . ffi cient AGN. Thequestion whether QPEs are directly associated with accretionflow variability or instabilities or whether they are due insteadto extrinsic phenomena (such as interactions with a secondaryorbiting body) remains to be studied (see King 2020; Coughlin& Nixon 2020, for possible interpretations). Future X-ray ob-servations of both sources will enable us to constrain possibletheoretical models taking advantage of the di ff erent propertiesand timescales in the two sources, which need to be consistentwith a similar theoretical framework.The detection of X-ray QPEs in RX J1301.9 + Acknowledgements.
We thank the XMM-Newton Project Scientist N. Schartelfor approving the DDT observation, and the XMM-Newton sta ff for performingit. We also thank the referee for useful comments and suggestions. MG is sup-ported by the “Programa de Atracción de Talento” of the Comunidad de Madrid,grant number 2018-T1 / TIC-11733. GM is supported by the Spanish State Re-search Agency (AEI) Project No. ESP-2017-86582-C4-1-R. This research hasbeen partially funded by the AEI Project No. MDM-2017-0737 Unidad de Ex-celencia “María de Maeztu” - Centro de Astrobiología (INTA-CSIC).
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Article number, page 6 of 9iustini, Miniutti & Saxton: X-ray QPEs in RX J1301.9 + Appendix A: Supplementary figures and tables
Article number, page 7 of 9 & A proofs: manuscript no. final
Fig. A.1: EPIC-pn light curves of RX J1301.9 + ff erent energy bandslabeled in units of keV in the top right corner of each panel. We note the di ff erent y-scales in di ff erent energy bands. In the bottompanels, we plot instead the light curve in the 1.3-2 keV band for the two epochs of observation. Article number, page 8 of 9iustini, Miniutti & Saxton: X-ray QPEs in RX J1301.9 + Table A.1: RX J1301.9 + − s − . Errors are quoted at 1 σ confidence level. Model 1: tbabs*(pow + diskbb) : χ /ν = / N H < × cm − Spectrum f pow . − kT disk f disk . − [erg cm s − ] [eV] [erg cm s − ]2000 low 8 . + . − . × − + − . + . − . × − . + . − . × − ± . + . − . × − . ± . × − + − . + . − . × − . + . − . × − + − . + . − . × − Model 2: tbabs*(pow + diskbb + bbody) : χ /ν = / N H < × cm − Spectrum f pow . − kT disk f disk . − kT bb f bb . − [erg cm s − ] [eV] [erg cm s − ] [eV] [erg cm s − ]2000 low 1 . ± . × − ± . + . − . × − − − ± . ± . × − . + . − . × − + − . + . − . × − − − ± . + . − . × − Model 3: tbabs*(pow + diskbb + brems) : χ /ν = / N H = . ± . × cm − Spectrum f pow . − kT disk f disk . − kT bre f bre . − f tot . − [erg cm s − ] [eV] [erg cm s − ] [eV] [erg cm s − ] [erg cm s − ]2000 low 9 . ± . × − + − . + . − . × − − − . + . − . × − ±
17 7 . + . − . × − . + . − . × − . + . − . × − ± . + . − . × − − − . + . − . × − ±
17 7 . + . − . × − . + . − . × − Model 4a: tbabs*(pow + diskbb + nthcomp) : χ /ν = / N H = ± × cm − cm − Spectrum f pow . − kT disk f disk . − kT nth Γ nth f nth . − f tot . − [erg cm s − ] [eV] [erg cm s − ] [eV] [erg cm s − ] [erg cm s − ]2000 low 9 . ± . × − + − . + . − . × − − − − . + . − . × − >
250 4 . + . − . . ± . × − . + . − . × − . + . − . × − ± . + . − . × − − − − . + . − . × − + − . ± . . + . − . × − . + . − . × − Model 4b: tbabs*(pow + diskbb + comptt) : χ /ν = / N H = ± × cm − cm − Spectrum f pow . − kT disk f disk . − kT com τ com f com . − f tot . − [erg cm s − ] [eV] [erg cm s − ] [eV] [erg cm s − ] [erg cm s − ]2000 low 1 . ± . × − + − . + . − . × − − − − . + . − . × − + − ± . + . − . × − . + . − . × − . + . − . × − + − . + . − . × − − − − . + . − . × − + − + − . + . − . × − . + . − . × −13