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arXiv:1111.3804v1 [astro-ph.HE] 16 Nov 2011 ihadnrsa fB o e pcrltp (Be type binarie spectral Oe) X-ray (or X-1 Be Vela 2010). of 2003a), star al. donor F¨urst et al. a et with 2008; therein), references 1991; al. et end al. (Kreykenbohm pulsars 2003b, et (Angelini al. X-ray et X-1 a Moon SMC “flaring” (e.g. of X-4 best-known top LMC super-or are on the or hours observed outbursts Among several X-ray often to modulation. to is related seconds variation activity flux of Flaring slower tens flares. few X-ray l a – X-ray from their in lasting increases minosity abrupt show X-ra often high-mass (HMXBs), in naries residing mostly pulsars, X-ray Accreting Introduction 1. eRswsehbtdb h 2spla X 2030 EXO pulsar s 42 the by exhibited was BeXRBs e Caballero 2008). 2008; al. al. et et Postnov Reig 1999; 2008; al. fo et 2011, occasio (Finger show Reig flares to the X-ray known e.g. also from (see are systems matter star These review). donor accretes recent the star around neutron disk the equatorial when d several weeks from lasting several outbursts X-ray sporadic characteriz or are periodic (SFXT), BeXRBs class 2011). transients Sidoli another X-ray (e.g. growing fast although rapidly supergiant HMXBs 2006), the of al. pulsars, class et HMXB (Liu numerous pulsars X-ray the with currently are BeXRB) or iso i ogl qiitn ae ihama eidof period a with flares equidistant roughly six of ries nadto oto“ao”otussi 95ad20 iha with 2006 and 1985 in of outbursts luminosity “major” X-ray peak two to In e h in ubrto h orei 95(amre l 198 al. et (Parmar 1985 in source the 2030 EXO af- of months outburst few giant a the satellite EXOSAT ter the with observed was hours oebr9 2018 9, November srnm Astrophysics & Astronomy us-eidcflrsi X 0035osre ihINTEGRA with observed 2030+375 EXO in flares Quasi-periodic rbbytems eakbeflrn ciiyamong activity flaring remarkable most the Probably eevd** cetd*** accepted ***; Received 6 Results. 2030 EXO in observed Methods. Aims. Context. 4 5 2 ciiyhspeiul enosre nEO2030 EXO in observed been previously has activity 3 1 e words. Key activity. flaring the of cause probable most lobe bevdwt te satellites.) other with observed been also nqisec rpirt ubrt.EO2030 EXO outbursts. to prior or quiescence in eisrnpsaeo h ria eouin oeo these of Some revolution. orbital the of passage periastron .Klochkov D. AAMrhl pc lgtCne,Hnsil,A 51,US 35812, AL Huntsville, Center, Flight Marshall NASA E aly DSM Saclay, CEA trbr srnmclIsiue nvriesip.13, pr. Universitetski Institute, Astronomical Sternberg SCDt etrfrAtohsc fteUiest fGenev of University the of Astrophysics for Center Data ISDC uoenSaeAtooyCnr (ESA Centre Astronomy Space European ntttfu srnmeudAtohsk Universit¨at T¨ Astrophysik, und f¨ur Astronomie Institut + h NERLdt eeldsrn us-eidcflrn ac flaring quasi-periodic strong revealed data INTEGRAL The 7 soeo h otrglrymntrdBeXRBs. monitored regularly most the of one is 375 ae nteosrainlpoete fteflrs eargue we flares, the of properties observational the on Based psdcflrn ciiyi omnfaueo -a pulsar X-ray of feature common a is activity flaring Episodic epeetteaayi fteflrn eairo h oreb source the of behavior flaring the of the present We -a iais–nurnsas–accretion – stars neutron – binaries X-ray 1 .Ferrigno C. , / IRFU L + X 7 n1985. in 375 & aucitn.flares no. manuscript / A-M I 75)CNRS (7158) AIM SAp-UMR 10 38 r s erg 2 .Santangelo A. , − 1 suigadistance a assuming , / SC,SineOeain eatet ilnead aCan la de Villanueva Department, Operations Science ESAC), + / 7 saBe a is 375 -a binaries X-ray + + .A Wilson-Hodge A. C. L 7 nyoc,i 95wt XST Sm niain fsing of indications (Some EXOSAT. with 1985 in once, only 375 7.Ase- A 375. 1 te othe to etter .Staubert R. , bne IA) ad1 27 T¨ubingen, Germany 72076 1, Sand (IAAT), ubingen ocw 191 Russia 119991, Moscow, y to ays ∼ ubrt eeocsoal oioe ihteITGA ob INTEGRAL the with monitored occasionally were outbursts dby ed / bi- y CEA bital ABSTRACT four / al. t -a iaysoig“oml ubrt lotevery almost outbursts “” showing binary X-ray nal a r 9). u- of is s / hmnd’ chemin a nvri´ .Diderot–F-91191Universit´e P. A f7kc(isne l 02,tesuc xiie espower less exhibited source with the outbursts 2002), al. “normal” et (Wilson kpc 7 of h ria eido h ytmis system the of period orbital The ic t ao ubrti 06 X 2030 EXO 2006, in outburst major its Since processing data and Observations 2. the since first the flaring for quasi-periodic havior. that, strong INTEGRAL reveal source the observations, EXOSAT present the was we of that Letter unclear phenomenon this observations it In quasi-periodic EXOSAT. make same with du observations the seen short the the is of But this spacing 2008). whether al. broad et Wilson 20 and in the ration after 3 shortly (Fig. outbursts outburst normal pres major apparently the 1998; also between Coe were & and Flares (Reig during 2005). outbursts al. normal et of Arranz rise Camero si the a be during to flare appeared gle generally has it but observations, EXOSAT ntuet:()teiae BSsniiefrom sensitive X-ray IBIS main imager three the has (i) observatory instruments: “Cygn INTEGRAL the The on concentrated Region”. of program observational view the of during field Laboratory the Astrophysics Ray in appeared edly iiy(ernee l 03;ad(i)teXrymntrJE monitor X-ray the (iii) and 2003); much al. with et imagin (Vedrenne but s lower IBIS, bility substantially is as and which range resolution energy resolution spectral SPI, higher same spectral spectrometer roughly the the moderate in (ii) and sitive 2003); spatial al. et high (Ubertini with MeV few iiydrn h iigpr foeo h ytmsoutburst system’s the of one of part rising the during tivity 1 htteisaiiya h ne deo h crto iki disk accretion the of edge inner the at instability the that E nHXs nsm Be some In HMXBs. in s .Kretschmar P. , oeflrn fEO2030 EXO of flaring Some ditor sdo NERLdt n opr twt h flares the with it compare and data INTEGRAL on ased cga 619 esi,Switzerland Versoix, 1290 16 Ecogia, ´ 6 3 .Caballero I. , / L -a iaisflrswr observed were flares binaries X-ray X ITGA,Wnlre l 2003) al. et Winkler (INTEGRAL, ∼ + 7 a enrpre ic the since reported been has 375 10 a a(ard,Spain ˜ada (Madrid), 37 ∼ 4 6d(isne l 2008). al. et (Wilson d 46 .Postnov K. , r s erg nentoa - International − 1 ∼ loteeyorbit. every almost 6dy,near days, 46 eflrshave flares le + 7 a repeat- has 375 servatory.

5 ∼ c and , 0kVt a to keV 20 S 2018 ESO Such . the s capa- g L be- en- M- ent ful 06 us n- 1 - D. Klochkov et al.: Quasi-periodic flares in EXO 2030+375 observed with INTEGRAL

Fig. 2. The INTEGRAL/ISGRI light of EXO2030+375 during the rise of its normal outburst in November–December 2010. The flaring activity is clearly seen. The light curve of the entire outburst is shown in Fig.3. One Crab correspondsto ∼260 / Fig. 1. INTEGRAL observations of EXO2030+375 (vertical cts s in the specified energy range. bars) superposed on the Swift/BAT light curve of the source since its major outburst in June–October 2006.

X with moderate spectral and spatial resolution, operating be- tween ∼3 and ∼35keV (Lund et al. 2003). INTEGRAL obser- vations normally consist of a of pointings called Science Windows, 2 to 4ksec each. Because the of the observations were not optimized for EXO2030+375, the INTEGRAL coverage of the source’s normal outbursts is rather sparse. The Swift/BAT light curve of the pulsar1 with the indicated INTEGRAL observations is shown in Fig. 1. For our analysis we used the ISGRI detector layer of IBIS sensitive in the 20–300keV energy range (Lebrun et al. 2003) and JEM-X. Owing to limited count-rate statistics, no additional information could be gained from SPI data. The standard data processing was performed with version 9 of the Offline Science Analysis (OSA) software provided by the INTEGRAL Science Fig. 3. Top: ISGRI light curve of the EXO2030+375 out- Data Centre (ISDC, Courvoisier et al. 2003). We performed an burst approximated with a polynomial (solid curve) additional gain correction of the ISGRI energy scale based on which represents the averaged evolution of the flux. Bottom: the background spectral lines. Normalized excess variance of the source flux with respect to To search for the flaring activity, we examined the entire the polynomial function. IBIS/ISGRI light curve by combining the publicly available ISGRI data products in the HEAVENS data base2 with the re- sults of our own analysis. We did not find any clear evidence adjacent intervals in the rising part and five equal adjacent inter- of flares in all the data except for the latest INTEGRAL obser- vals in the decay. The normalized excess variance is often used vations of the source in November and December 2010 (MJD as a simple of the intrinsic variability amplitude in light ∼55520–55540), partially covering a normal outburst. The ris- , (see e.g. Nandra et al. 1997): ing part of the outburst is shown in Fig.2. The upper panel of Fig.3 shows the entire outburst as observed with INTEGRAL. 1 σ2 = ΣN [( f − f aver)2 − σ2]. (1) In this we concentrate on the analysis of the source’s flar- NXS Nh f i2 i=1 i i i ing behavior during this outburst. Here N is the number of data points in the corresponding time , fi is the flux of the individual data points, σi – their 3. Timing analysis aver uncertainty, fi is the smoothed evolution of the flux obtained As can be seen in Fig.2, the flux in the rising part of the out- by a polynomial fit to the light curve (upper panel of Fig.3), burst experiences quasi-periodic oscillations/flares that cease as and h f i is the mean value of the flux within the interval. The 2 the averaged flux increases. One can identify at least five subse- normalized variance σNXS in our case represents the amplitude quent flares with a mean period of ∼0.3 days (∼7 hours). of intrinsic flux variations superimposed on the smoothed flux To characterize the level of the flux variability during the out- development. The term σi under the (Σ) eliminates burst we calculated the normalized excess variance in the light the contribution of the Poisson noise. The bottom panel of Fig.3 curve in relatively broad time intervals. We defined four equal represents the normalized variance as a function of time during the outburst of EXO2030+375.It can be seen that the amplitude 1 We used the Swift/BAT transient monitor results provided by the of the variability is high at the rising phase (corresponding to the Swift/BAT team flaring episode). Then it decreases towards the maximum of the 2 http://www.isdc.unige.ch/heavens/heavens outburst and remains low during the decay.

2 D. Klochkov et al.: Quasi-periodic flares in EXO 2030+375 observed with INTEGRAL

16 14 20−80 keV 12 10 12 8 10 6 8 ISGRI cts/s

Power 4 6 4 2 12 0 3−20 keV 0.1 0.2 0.3 0.4 0.5 10 Period [d] 16 8 14

JEM−X cts/s 6 12 4 10

Rate 0.0 0.5 1.0 1.5 2.0 8 Pulse phase 6 4 Fig. 5. Pulse profiles of EXO2030+375 obtained with ISGRI 0.0 0.5 1.0 1.5 2.0 Phase (top) and JEM-X (bottom) instruments during the outburst (MJD 55518.5 – 55533.0). Fig. 4. Top: The Lomb-Scargle periodogram of the “flaring” part of EXO2030+375 light curve (between MJD 55518.5 and 55521.5). The peak around ∼0.3d is clearly seen. Bottom: The “flaring” part of the light curve folded with best period found from the periodogram (0.293d).

To study periodicity of the flares, we performed a formal period search in the rising part of the outburst (between MJD 55518.5 and 55521.5) using the Lomb-Scargle periodogram. The results are presented in Fig.4. The periodogram shown in the top panel indicates a clear peak around ∼0.3days (∼7hours). The averaged profile obtained by folding the light curve with this period is shown in the bottom panel. The profile shape is asym- metric and characterized by a steep rise and a slower decay. Fig. 6. Photon index Γ as a functionof flux duringthe rising (flar- We used the INTEGRAL data to study the pulse period be- ing) part of the outburst (left) and the decay phase (right). One havior and pulse profiles during the outburst. The photon ar- Crab corresponds to ∼260 cts/s in the specified energy range. rival times were converted to the reference frame of the system barycenter and corrected for the binary orbital using the ephemeris by Wilson et al. (2008). Using the pulse- are at the 90% confidence level. The value of nH is substantially phase-connection technique (e.g. Staubert et al. 2009), we found higher than measured in previous observations of the source, in- 22 −2 the pulse period P = 41.31516(2)s at the T0(MJD)= cluding older INTEGRAL measurements (1–3×10 cm , e.g. 55526.056994 and the period P˙ = −1.9(1) × 10−9 s/s, Klochkov et al. 2007; Wilson et al. 2008). We note, however, that indicates significant spin-up. We used the measured pulse that studying nH with JEM-X is generally problematic as the data ephemeris to construct and study the pulse profiles of the source. below ∼3keV are not available. The significance of the mea- We could not find any difference between the profiles obtained sured increase in absorption is therefore questionable. during the rise and decay of the outburst.Figure5 shows the pro- The data quality does not permit spectroscopy of individual files accumulated during the entire outburst. flares. To characterize the spectral behavior of EXO2030+375 during the flaring episode and compare it with the rest of the outburst, we explored the luminosity-dependence of the source 4. Spectral analysis spectrum during the rising (flaring) part and the decay of the For the spectral analysis we used JEM-X data between 3.5 and outburst. We grouped the individual INTEGRAL pointings ac- 35keV, and ISGRI data between 20 and 80keV. We added sys- cording to the measured flux in the 20–80keV range. For each tematic uncertainties at the level of 2% to the JEM-X spectra group we extracted and analyzed the X-ray spectrum using the and 1% to the ISGRI spectra based on the recommendations of spectral model described above. The statistics did not allow us the instrument teams and the Crab observations. The spectrum to explore the dependence of each individual spectral parameter of the source during the outburst (accumulated between MJD on flux. We therefore fixed nH and Efold to their averaged val- 55518.5 and 55533.0) was modeled with the cutoff-powerlaw ues (11 × 1022 cm−2 and 30keV, respectively) and explored the −Γ model (F(E) ∝ E × exp[E/Efold], where E is the photon en- photon index Γ as a function of flux. ergy, Γ and Efold are the photon index and the folding energy, The resulting dependenceis shown in Fig.6 for the rise (left) respectively) modified at lower energies by photoelectric absorp- and decay (right) of the outburst. Higher values of Γ correspond tion. to a softer spectrum (see the model description above). The hori- The best-fit parameters are Γ= 1.6(1), Efold = 30(2) keV,the zontal error bars represent the width of the flux bins. The vertical 22 absorption column density nH = 11(1)×10 hydrogen atoms per error bars indicate 1σ-uncertainties. While the spectrum appar- cm2. The uncertainties in parentheses refer to the last digit and ently tends to get harder with increasing flux during the rising

3 D. Klochkov et al.: Quasi-periodic flares in EXO 2030+375 observed with INTEGRAL

2 2 part of the outburst, it remains roughly constant during the de- be estimated as τc ∼ rc /ν(rc) ∼ 1/[Ωα(H/R) ], where ν(rc) is cay. To quantify this behavior, we performed linear fits to the the viscosity at rc, Ω is the spin frequency of the neutron star, data points and calculated the with the corresponding un- and (H/R) the semithickness of the accretion disk. Using “stan- certainties. The is (−9.0 ± 3.7) × 10−3 (cts/s)−1 in the rising dard” values of α = 0.1, H/R = 0.05, and the known pulse −3 −1 part and (2.8 ± 4.2) × 10 (cts/s) in the decay of the outburst period P ≃ 40s, one gets τc ∼ 7hr, i.e. of several hours, as was (uncertainties at 1σ confidence level). observed. The averaged profile of the flares is characterized by a steep rise and a slower decay (Fig. 4), which is very similar to the 5. Discussion flares observed with EXOSAT in 1985 (Fig.2 of Parmar et al. A direct comparison of the flares presented here with those 1989). According to the authors, such a shape suggests a “drain- observed with EXOSAT in 1985 revealed significant similar- ing reservoir” that is in line with the picture described above ity. The peak fluxes and the relative amplitude of the flares are (matter piling up on the inner edge of the disk). roughly the same in both episodes. Also the average shape – fast The observed difference in the spectral behavior between the rise / slow decay – is similar in the two cases. The mean period flaring part and the rest of the outburst (Sect.4) suggests dif- is, however, different: ∼4hr for the EXOSAT flares and ∼7hr ferent configurations of the region where matter couples to the for the INTEGRAL ones. The location of the EXOSAT flares field lines. Such a difference is indeed expected if the flares are with respect to the nearest “normal” outburst is difficult to re- caused by the oscillating inner edge of the accretion disk. In this construct because no monitoring of the source flux (apart from case, matter from the oscillating inner disk rim would couple to the EXOSAT data themselves) was performed at the time. Since different dipole field lines of the neutron star (and follow them) the orbital phase of “normal” outbursts varies significantly with compared to the decay part of the outburst where the configura- time (Wilson et al. 2002), the extrapolation of the orbital phase tion of the inner disk rim is presumably stable. ephemeris back to the EXOSAT observations would not resolve The difference in the mean period of flares in the the problem. INTEGRAL and EXOSAT observations can also be understood X-ray flares in accreting pulsars are usually attributed to in the described picture. The period must depend on the mass one of the following mechanisms: (1) instabilities of the ac- transfer rate through the accretion disk, i.e. time needed to refill cretion flow around/within the magnetospheric boundary (e.g. the reservoir. This rate could be different between the EXOSAT Moon et al. 2003b; Postnov et al. 2008), (2) highly inhomoge- and INTEGRAL observations due to, e. g., changes in the state neous stellar of the donor star (e.g. Taamet al. 1988; of the Be-disk. In Fig.2 one might also notice some shortening Walter & Zurita Heras 2007), and (3) nuclear burning at the of the the flare separation time as the flux increases (although neutron star (e.g. Levine et al. 2000; Brown & Bildsten this behavior is difficult to quantify with the available statistics). 1998). The nuclear burning scenario is, however, difficult to Such behavior, if real, might reflect shortening of the reservoir reconcile with a relatively high accretion rate before and af- refill time as the mass transfer rate increases towards the - ter a flare, which would suppress the thermonuclear instability mum of the outburst. (Bildsten & Brown 1997). In the case of EXO2030+375, inho- Thus, we argue that the observational appearance of the mogeneities of the companion’s stellar wind are also unlikely to flares in EXO2030+375 suggests that the instability of the in- be a direct cause of the flares for the following reasons. First, the ner disk edge (pile-up/draining of matter) is the most probable viscous time of the accretion disk that is believed to be present cause of the flares. in EXO2030+375 during normal outbursts (Wilson et al. 2002) It is important to note that the rarity of the detected flaring and to even survive during quiescence (Hayasaki & Okazaki episodes (even considering the relatively sparse observational 2006), is at least several days, which would smooth out any coverage) that the range of physical conditions needed variations in the mass accretion rate M˙ caused by inhomogene- to initiate flares could be very narrow, which would lead to the ity of the wind shorter than this time. Second, nonuniform stel- serendipitous character of the phenomenon. lar wind cannot explain the observed quasi-periodic appearance Acknowledgements. The work was supported by the Carl-Zeiss-Stiftung and by of the flares. On the other hand, various kinds of magneto- DLR grant BA5027. This research is based on observations with INTEGRAL, hydrodynamicinstabilities at the inner edge of the accretion disk an ESA project with instruments and science data centre funded by ESA member may easily lead to oscillations in the mass flow towards the po- states. The authors thank the anonymous referee for useful suggestions. lar caps of the neutron star (Apparao 1991; Postnov et al. 2008; D’Angelo & Spruit 2010), leading to the observed flaring activ- ity. For example, D’Angelo & Spruit (2010) have illustrated that References when the magnetospheric radius rm (where magnetic field of the Angelini, L., White, N. E., & Stella, L. 1991, ApJ, 371, 332 neutron star truncates the accretion disk) is larger but close to the Apparao, K. M. V. 1991, ApJ, 375, 701 Bildsten, L. & Brown, E. F. 1997, ApJ, 477, 897 corotation radius rc (where the Keplerian frequency is equal to Brown, E. 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T. 2006, MNRAS, 372, 1140 served in EXO2030+375, we estimated the expected character- Klochkov, D., Horns, D., Santangelo, A., et al. 2007, A&A, 464, L45 istic time scale of M˙ -oscillations. Without going into physical Kreykenbohm, I., Wilms, J., Kretschmar, P., et al. 2008, A&A, 492, 511 Lebrun, F., Leray, J. P., Lavocat, P., et al. 2003, A&A, 411, L141 details of the disk-field coupling at rm one would generally ex- ˙ Levine, A. M., Rappaport, S. A., & Zojcheski, G. 2000, ApJ, 541, 194 pect that the oscillations in M take place on the time scale close Liu, Q. Z., van Paradijs, J., & van den Heuvel, E. P. J. 2006, A&A, 455, 1165 to the local viscous time at rm ∼ rc. Following the standard α- Lund, N., Budtz-Jørgensen, C., Westergaard, N. J., et al. 2003, A&A, 411, L231 viscosity prescription (Shakura & Sunyaev 1973), this time can Moon, D.-S., Eikenberry, S. S., & Wasserman, I. M. 2003a, ApJ, 582, L91

4 D. Klochkov et al.: Quasi-periodic flares in EXO 2030+375 observed with INTEGRAL

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