Astronomy Letters, Vol. 27, No. 8, 2001, pp. 501–506. Translated from Pis’ma v Astronomicheski

œ

Zhurnal, Vol. 27, No. 8, 2001, pp. 586–593.Original Russian Text Copyright © 2001 by Lutovinov, Grebenev, Pavlinsky, Sunyaev.

X-ray Bursts from the Source A1742–294 in the Galactic-Center Region

A. A. Lutovinov1*, S. A. Grebenev1, M. N. Pavlinsky1, and R. A. Sunyaev1, 2

1 Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117810 Russia2 Max-Plank-Institute für Astrophysik, Karl Schwarzschild Strasse 1, 86740 Garching bei München, Germany

Received March 3, 2001

Abstract—We present observations of the X-ray burster A1742–294 near the Galactic center with the ART-Ptelescope onboard the Granat observatory. The shape of its persistent spectra was described well by the modelof bremsstrahlung from optically thin plasma, and it remained essentially unchanged over ~2.5 years of obser-vations. We show that the mean interval between X-ray bursts from the source is several times shorter thanassumed previously and that the burst profile itself depends on the flux during the burst. We analyze in detailthe strong X-ray burst detected from this source on October 18, 1990, and construct the evolution curves of itsluminosity and radiation temperature. © 2001 MAIK “Nauka/Interperiodica”.

Key words: bursters, neutron stars, X-ray sources, and Galactic center

INTRODUCTION

Because of the high concentration of sources of dif-ferent nature, the Galactic-center region is one of themost interesting and most commonly observed regionsin X-rays. Although the first sources in this region werediscovered back in the 1970s (Lewin et al. 1976; Proc-tor et al. 1978), its detailed map was first obtained bythe EINSTEIN observatory in the soft X-ray band(hν ≤ 4.5 keV). These observations revealed more thanten hitherto unknown X-ray sources and an extendeddiffuse source (Watson et al. 1981; Hertz and Grindlay1984). Subsequently, soft X-ray observations of theGalactic center were carried out by the ROSAT (Pre-dehl and Trümper 1994), ASCA (Maeda et al. 1996),and BeppoSAX (Sidoli et al. 1999) observatories. Inthe hard energy band (hν ≥ 10 keV), this region wasfragmentarily observed with the XRT telescopeonboard the Spacelab-2 observatory (Skinner et al. 1987)and with instruments of the Spartan-1 mission (Kawaiet al. 1988). A more detailed study of the hard X-rayemission from the center Galactic was carried out withthe TTM telescope onboard the Kvant module (Sunyaevet al. 1991a) and, in particular, with the ART-P andSIGMA (in gamma-rays) telescopes onboard the Granatobservatory (Pavlinsky et al. 1992a, 1992b, 1994; Su-nyaev et al. 1991b).

Persistent X-ray sources of different nature, tran-sients, the millisecond pulsar SAX J1808.4–3658, thebursting pulsar GRO J1744–28, and sources in globularclusters are observed in the Galactic-center region. X-raybursters, i.e., neutron stars with weak magnetic fields in

* E-mail address for contacts: aal@hea.iki.rssi.ru

1063-7737/01/2708- $21.00 © 20501

low-mass binary systems on whose surfaces thermonu-clear explosions of the accreted matter occasionallyoccur, make up a sizeable proportion. The X-ray burstsproduced by explosions are difficult to observe,because the burst source cannot always be accuratelylocalized and identified with any persistent X-raysource due to the high concentration of sources in thisfield. The ART-P/Granat 2.5–30-keV observations ofthis sky region are unique not only because of their longduration (more than 8 × 105 s) but also because of thetelescope’s technical potentialities. ART-P can imagethe sky with a high angular resolution and can performspectral and timing analyses of any sources in its fieldof view, irrespective of their number.

Here, we present our observations of the X-rayburster A1742–294 in the immediate vicinity (~1°) ofthe Galactic center. The emphasis is on the study ofpeculiarities of bursts from this source.

OBSERVATION

The ART-P X-ray telescope, which is part of the sci-entific payload of the Granat international astrophysicalobservatory, can image a selected region of the sky byusing a coded-aperture technique. It consists of fourcoaxial, completely independent modules; each mod-ule includes a position-sensitive detector with a geo-metric area of 625 cm2 and a coded mask based onURA sequences. The instrument can image the skywithin a 3 4 × 3 6 field of view (FWHM) with a nom-inal resolution of ~5 arcmin (the angular size of themask element). Because of the detector’s higher spatialresolution (~1.25 arcmin), the accuracy of localizingdiscrete sources is several times higher. The telescopeis sensitive to photons in the energy range 2.5–60 keV

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and has an energy resolution of ~22% in the 5.9-keViron line. Readings of the star tracker, which determinesthe instantaneous spacecraft orientation with an accu-racy of 1.5 arcmin, are used in imaging and spectralanalysis. A more detailed description of the telescope isgiven in Sunyaev et al. (1990).

The observations were carried out in the photon-by-photon mode, in which, for each photon, its coordinateson the detector, energy (1024 channels), and arrivaltime (the photon arrival time is accurate to within 3.9ms, and the dead time is 580 µs) were written into theART-P buffer memory. This mode allows both thetiming and spectral analyses of the emission from eachX-ray source within the ART-P field of view. Datatransfer to the main memory was made after the tempo-rary buffer was filled (once in 150–200 s) during ~30 s,which resulted in breaks in the information; therefore,the data were in the form of individual exposures.

The Galactic center was observed by the Granatobservatory in series twice a year, in spring and fall(this was caused by the restrictions imposed on the sat-ellite in-orbit orientation with respect to the Sun). Fiveseries of Galactic-center observations were carried outin 2.5 years of the ART-P operation, with the totalobserving time being ~830 000 s. This allowed us toanalyze in detail the persistent emission from sourcesin this region, their spectra and variability on varioustime scales, to discover several new X-ray sources, andto detect more than 100 X-ray bursts (Pavlinsky et al.1992a, 1992b, 1994; Grebenev et al. 2001).

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Fig. 1. The persistent spectrum of A1742–294 obtainedwith the ART-P telescope during the Galactic-center obser-vation on September 9, 1990. The dots indicate the mea-sured (pulse-height) spectrum, and the histogram representsits best fit by the bremsstrahlung model. The model (pho-ton) spectrum is indicated by the solid line.

It should be noted that the first and fourth ART-Pmodules were used during the first two series of obser-vations (in the spring and the fall of 1990). The subse-quent observations were performed by the third modulewith a lower sensitivity in the soft energy band (3–8 keV),which hampered and, in several cases, prevented thelocalization and spectral analysis of the X-ray burstsdetected by this module [see Grebenev et al. (2001) formore details].

PERSISTENT EMISSION

The X-ray burster A1742–294 is the brightest per-sistent X-ray source among those located near (within~1°) the Galactic center; it is responsible for ~1/3 of thetotal flux from this region in the standard X-ray band(2–20 keV). In this energy band, the source has beenrepeatedly observed from various satellites. Based onSIGMA/Granat data, Churazov et al. (1995) firstshowed that A1742–294 also radiates in the hard energyband (hν ≥ 35 keV).

X-ray emission from the source was observed everytime it fell within the ART-P field of view (Pavlinskyet al. 1994). Its 3–20-keV flux was 30–50 mCrab duringthe 1990 series of observations and slightly decreasedto 20–30 mCrab by the spring of 1992. The shape of theburster’s persistent spectrum, which is described wellby the model of bremsstrahlung from optically thinplasma, remained essentially unchanged. The tempera-ture kTbr of this model determined by fitting the spectra ofthe source in individual observing sessions was within therange 6.4–10.5 keV. When modeling the spectra, wetook into account their distortion in the soft energy bandby strong interstellar absorption, which is characterized bya hydrogen column density NH . 6 × 1022 cm–2.

As an illustration, Fig. 1 shows the typical persistentspectrum of A1742–294 measured with the ART-P tele-scope on September 9, 1990. The dots indicate thepulse-height spectrum (in counts s–1 cm–2 keV–1), andthe solid line represents the corresponding model spec-trum (in phot. s–1 cm–2 keV–1); the temperature that wasdetermined by fitting the spectrum with the model ofbremsstrahlung from optically thin plasma is shown inthe lower left corner.

X-RAY BURSTS

Lewin et al. (1976) discovered that most burstsdetected by them from the Galactic-center region orig-inated from three sources, A1742–294, A1742–289,and A1743–28; bursts from the first two sources wereobserved regularly. Our analysis of the ART-P datarevealed no burst from A1742-289 and A1743-28 (in thosecases where we managed to perform the localization).

Recurrence

Of more than 100 X-ray bursts detected over theentire period of ART-P observations of the Galactic-

ASTRONOMY LETTERS Vol. 27 No. 8 2001

X-RAY BURSTS FROM THE SOURCE A1742–294 503

The X-ray bursts detected by the ART-P telescope from A1742–294 during 1990–1992

Date T0,a UT kTbb, keV Lbb,b 1037 erg s–1 Flux,c mCrab Duration, s

Mar. 20, 1990 18h05m00s 1.64 ± 0.35 2.20 ± 0.65 168 ± 50 26

20 01 29 1.61 ± 0.53 1.38 ± 0.69 88 ± 44 18

Mar. 24, 1990 18 32 42 2.78 ± 0.73 3.32 ± 0.92 253 ± 70 24

21 42 00 1.87 ± 0.61 2.35 ± 0.97 199 ± 74 17

Apr. 8, 1990 14 06 38 2.45 ± 0.34 4.26 ± 0.71 281 ± 47 24

Sept. 9, 1990 14 07 48 1.65 ± 0.41 1.81 ± 0.92 115 ± 67 18

15 53 21 1.89 ± 0.30 3.43 ± 0.86 238 ± 58 17

Sept. 29, 1990 13 32 38 –d –d 372 ± 90 30

Oct. 5, 1990 15 31 44 2.02 ± 0.54 1.60 ± 0.69 147 ± 55 32

19 43 33 1.43 ± 0.48 3.36 ± 1.52 234 ± 125 21

Oct. 6, 1990 22 10 36 1.49 ± 0.23 4.91 ± 1.17 374 ± 89 22

Oct. 9, 1990 17 20 52 2.21 ± 0.59 2.21 ± 0.75 169 ± 57 11

19 06 11 1.60 ± 0.28 2.59 ± 0.70 162 ± 49 15

Oct. 10, 1990 14 35 48 1.97 ± 0.42 2.39 ± 0.83 183 ± 64 12

16 06 25 1.55 ± 0.24 2.48 ± 0.76 418 ± 96 9

Oct. 18, 1990 9 50 31 2.15 ± 0.19 7.97 ± 0.96 607 ± 73 17

Feb. 22, 1991 14 07 59 0.97 ± 0.17 8.26 ± 2.53 709 ± 179 13

Feb. 23, 1991 22 24 36 1.98 ± 0.27 7.21 ± 1.90 550 ± 145 16

Feb. 26, 1991 11 00 20 4.79 ± 2.67 2.97 ± 2.51 312 ± 181 15

Apr. 1, 1991 13 39 44 3.51 ± 1.39 3.30 ± 1.19 252 ± 91 28

15 28 57 1.71 ± 0.84 4.46 ± 1.77 343 ± 136 19

Apr. 8, 1991 13 16 25 1.30 ± 0.25 2.29 ± 2.00 326 ± 152 22

Oct. 15, 1991 19 59 08 –d –d –d 14

Oct. 18, 1991 11 44 04 –d –d –d 15

Feb. 21, 1992 12 52 53 1.86 ± 0.43 4.51 ± 1.53 283 ± 115 21

Mar. 2, 1992 1 43 22 2.01 ± 0.33 10.06 ± 3.18 963 ± 239 15

a The burst peak time (UT).b The source’s mean 6–20-keV luminosity during the burst at a distance of 8.5 kpc.c The source’s mean 6–20-keV flux during the burst.d The source’s spectrum cannot be reconstructed for technical reasons.

center region, 26 bursts with a duration of ~15 s wereidentified with the burster A1742–294 (see the table).Criteria for the detection of bursts and their identifica-tion with persistent sources can be found in Grebenevet al. (2001), whence some of the data in this table weretaken. At the same time, note that the burst parametersin the table were obtained from a spectral analysis ofthe emission observed during bursts; i.e., it is a contin-uation and expansion of the study of bursts initiated byGrebenev et al. (2001). Two successive bursts at a timewere observed during several sessions, which allowedus to directly measure the characteristic burst recur-rence time tr. The measured values of tr lie in the range1.5 to 4.2 h with a mean of ~2.4 h, which is several-foldsmaller than the value found by Lewin et al. (1976).The mean energy spectra of the detected bursts are sat-isfactorily described by a blackbody model whose

ASTRONOMY LETTERS Vol. 27 No. 8 2001

temperature kTbb changes from burst to burst in the range~1–3 keV (see the table) with the mean kTbb = 1.81 ±0.39 keV.

The characteristic burst recurrence time tr can beestimated if the source’s persistent and burst luminosi-ties are known: tr . α∆tLb/Lp, where Lp is the persistentluminosity, Lb is the mean burst luminosity, ∆t is theburst duration, and α ~ 100 is the ratio of energy releaseduring accretion to that during thermonuclear heliumburning. The derived value, tr ~ 3 h, is in satisfactoryagreement with direct measurements.

Burst Profiles

When analyzing the time profiles of the X-ray burstsdetected from A1742–294, we found them to depend onthe mean 3–20-keV flux measured during the burst

504 LUTOVINOV et al.

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Fig. 2. The average profiles of bursts with different intensities observed from A1742–294 in various energy bands.

(Grebenev et al. 2001) at a mean flux of ~100–300 mCrab, the profile is nearly triangular; i.e., the risetime of the burst (~5 s) is close to its decay time,whereas the burst profile becomes classical; i.e., it is char-acterized by a sharp rise (~1–2 s) followed by a smoothdecay, when the flux increases to ~600–1000 mCrab.Bursts of the second type occur much more rarely thanbursts of the first type, so two such events in a row havenever been observed during a single session. To analyzethe above dependence in more detail, we constructedaverage profiles for the bursts of each type. Because ofthe spectral peculiarities of the third ART-P module, weused only data from the first and fourth modules. In oursubsequent analysis, we rejected bursts that partiallyoccurred at the beginning or at the end of the exposures.Each of the twelve remaining bursts was reduced to adimensionless intensity scale in the band 3–20 keV byits normalization to the difference between the maxi-mum burst count rate and the persistent count ratedetermined from the exposure that preceded the expo-sure during which the burst was detected. Subse-quently, the dimensionless bursts were added by match-ing the peaks and averaged. The results of this averag-ing for triangular bursts in two energy bands are shownin Fig. 2a. For comparison, Fig. 2b shows the profile ofthe classical burst detected by ART-P on October 18,1990, in the same energy bands. This was the only burstof this shape and intensity detected by the telescope’sfourth module. Three more such events were observedby the third module.

The differences in the burst profiles are clear:whereas the burst in Fig. 2a has a triangular shape in thesoft and hard energy bands similar to the shape of theburst in Fig. 2b at high energies, the profile of the latterin the 3–8-keV energy band is strictly classical inshape: a sharp (~1 s) rise and a smooth exponentialdecay. This fact leads us to conclude that low-energy

X-ray emission is responsible for the profile shape ofthe X-ray bursts observed from A1742–294. Note alsothat a softening of the source’s spectrum during theburst clearly shows up in both figures: the decay time inthe hard energy band is considerably shorter than thatin the soft energy band, which is one of the characteris-tic features of type I bursts.

The Burst of October 18, 1990

On this day, the ART-P telescope detected the stron-gest and most interesting burst from A1742–294, dur-ing which the peak X-ray 3–20-keV flux was ~1.5 Crab.Figure 3 shows the source’s measured light curves invarious energy bands during this burst. The time profileof the burst suggests that it belongs to type I bursts,which are caused by thermonuclear explosions on theneutron-star surface. We see that the peak flux in thehard energy bands is reached later than that in the softenergy bands; i.e., the burst rise time in the 12–16-keVenergy band (~5 s) is considerably longer than the risetime in the 4–8-keV energy band (1–2 s). Given that thee-folding exponential decay time of the burst texpdecreases from 8.7 s in the 4–8-keV band to 3.9 s in the8–12-keV band, to 2.3 s in the 12–16-keV band, and to1.5 s in the 16–20-keV band, the total burst duration inthe hardest energy band turns out to be several-foldshorter than that in the softest band (Fig. 3). The energydependence of texp indicates that the source’s spectrumhad softened appreciably by the end of the burst, whichis one of the characteristic features of type I X-raybursts. Note that no statistically significant excess ofthe signal over the background was found during theburst in the harder energy band (hν ≥ 20 keV).

To trace the evolution of the source’s parametersduring the burst, we divided it into seven time intervals,reconstructed the photon spectrum for each of them,

ASTRONOMY LETTERS Vol. 27 No. 8 2001

X-RAY BURSTS FROM THE SOURCE A1742–294 505

and fitted it by a blackbody model. Variations of thesource’s temperature and luminosity in the 3–20-keVenergy band during the burst determined in the abovemodel are shown in Fig. 4. The source’s burst-averagedspectrum is described well by the radiation law of ablackbody with a temperature kTbb . 2.15 ± 0.19 keV,a radius Rbb . 6.4 ± 1.4 km, and a 3–20-keV flux Fbb .(1.35 ± 0.13) × 10–8 erg cm–2 s–1, which corresponds toa luminosity Lbb . 1.2 × 1038 erg s–1 at a distance of8.5 kpc. In Fig. 5, this spectrum is compared with thepersistent spectrum of A1742–294. The inferred tem-perature, which corresponds to the super-Eddingtonflux, and the small blackbody radius suggest that thespectrum is distorted by Comptonization, while the dipclearly seen in the burst profile at high energies appar-ently points to the photospheric expansion in the sourcethat took place at the initial burst stage.

DISCUSSION

Long-term observations of the Galactic center withinstruments of the Granat observatory have allowed us

100

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8–12 keVtexp = 3.9 s

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16–20 keVtexp = 1.5 s

Fig. 3. The profiles of the X-ray burst detected on October 18,1990, from A1742–294 in various energy bands with a timeresolution of 1 s: texp is the source’s e-folding decay time ineach band. The errors correspond to one standard deviation.

ASTRONOMY LETTERS Vol. 27 No. 8 2001

not only to analyze in detail the timing and spectralparameters of persistent sources in this region, but alsoto detect more than 100 X-ray bursts; we managed toidentify 26 of these bursts with the X-ray bursterA1742–294 in the immediate vicinity of Galactic-cen-ter region. Long continuous observing sessions (morethan 10 h) made it possible to directly measure the burstrecurrence time. The derived mean value, tr ~ 2.4 h, provedto be several-fold shorter than assumed previously.

The X-ray bursts detected by ART-P from thissource can be arbitrarily divided into ordinary (with amean burst flux of 100–300 mCrab) and strong (a meanflux of 600–1000 mCrab). The burst profiles differ sig-nificantly: ordinary bursts are triangular (the rise timeis comparable to the decay time), while strong burstsare classical in shape (a sharp rise followed by a smoothdecay).

The mean energy release during the burst did notdepend on the source’s pre-burst luminosity and wasE . 9 × 1038 ergs for ordinary bursts and E . 2 × 1039 ergsfor strong bursts. To provide such energy yields, M .E/eHe . 5.0 × 1021 and 1.1 × 1020 g of matter, respec-tively, must be accreted onto the neutron-star surface(here, eHe . 0.002c2 is the helium burning efficiency).The rate of accretion onto the neutron star in quies-

cence can be represented as = LX Rns/GMns, whereLX, Mns . 1.4M(, and Rns . 10 km are the luminosity,mass, and radius of the neutron star, respectively.Assuming that the burster’s mean luminosity in quies-cence is LX . 8 × 1036 erg s–1, we can estimate the char-acteristic times τ in which the required amount of mat-

M

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037 e

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Fig. 4. Evolution of the source’s luminosity (a) and temper-ature (b) determined by fitting the spectra of A1742–294with a blackbody model during the strong burst detected onOctober 18, 1990. The dotted line indicates the source’s per-sistent luminosity.

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506 LUTOVINOV et al.

ter will be accreted onto the neutron-star surface: τ . 3.2and 7.2 h for ordinary and strong bursts, respectively.

The observation of both ordinary (weak) and strongX-ray bursts from A1742–294 and the fact that the lat-ter are observed much more rarely can be naturallyexplained in terms of the above estimates. The differ-ence in the burst time profiles may stem from the factthat the thermonuclear explosion responsible for theburst takes place not instantaneously over the entireneutron-star surface, but only in its region where localconditions for thermonuclear burning were created, andit subsequently spreads over the entire neutron-star sur-face. Depending on the amount and density of accumu-lated matter, the burning front propagates in subsonicmode, deflagration (weak bursts), or in supersonicmode, detonation (strong bursts). The criteria for real-ization of these modes were studied in detail by Fryxelland Woosley (1982). That is why the profiles of ordi-nary bursts have a longer rise compared to strongbursts.

ACKNOWLEDGMENTS

This study was supported by the Russian Founda-tion for Basic Research (projects nos. 99-02-18178 and00-15-99297). We wish to thank K.G. Sukhanov; flightdirector; the staffs of the Lavochkin Research and Pro-duction Center, RNIIKP; and the Deep Space Commu-nications Center in Evpatoria; the Evpatoria team of the

Fig. 5. The photon spectrum of A1742–294 averaged overthe entire October 18, 1990 burst. The solid line representsthe model blackbody spectrum that provides the best fit tothe data. For comparison, the dotted line indicates thesource’s persistent spectrum.

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Space Research Institute (Russian Academy of Sci-ences); the team of I.D. Tserenin, and B.S. Novikov,S.V. Blagiœ, A.N. Bogomolov, V.I. Evgenov, N.G. Kha-venson, and A.V. D’yachkov from the Space ResearchInstitute, who operated the Granat Observatory, pro-vided the scientific planning of the mission, and per-formed a preliminary processing of telemetry data. Wealso wish to thank the team of M.N. Pavlinsky (SpaceResearch Institute) and the staff of the former Researchand Development Center of the Space Research Insti-tute in Bishkek who designed and manufactured theART-P telescope.

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Translated by V. Astakhov

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