THE BRIGHTEST OH MASER IN THE SKY: A FLARE OF EMISSION IN W75 N. 1,* 1 1 1 A.V. Alakoz, V.I. Slysh, M.V. Popov, and I.E. Val'tts 1 Astro Spa e Center of the Lebedev Physi al Center of RAS, Profsoyuznaya Str. 84/32, 117997 Mos ow, Russia 5 0 A flare of maser radio emission in the OH-line 1665 MHz has been dis overed in 0 2 the star forming region W75 N in 2003, with the flux density of about 1000 Jy. n At the time it was the strongest OH maser dete ted during the whole history of a J observations sin e the dis overy of osmi masers in 1965. The flare emission is 5 2 linearly polarized with a degree of polarization near 100%. A weaker flare with a 1 v flux of 145 Jy was observed in this sour e in 2000 (cid:21) 2001, whi h was probably a 9 3 pre ursor of the powerful flare. Intensityof two other spe tral features hasde reased 5 1 after beginning of the flare. Su h variation of the intensity of maser ondensation 0 5 emission (in reasing of one and de reasing of the other) an be explained by passing 0 / h of the magneto hydrodynami sho k a ross regions of enhan ed gas on entration. p - o r Key words: interstellar medium, OH masers, variability. t s a : v i X r a * Ele troni address: retttanatos.as .rssi.ru 2 INTRODUCTION W75 N is one of the a tive star formation regions in the Galaxy whi h is a ompanied 2 by the maser emission in lines of OH, methanol and H O. Mapping observations of the maserswithhighangularresolutionhasshownthatthemaseremissionsour es are onne ted with ultra ompa t HII regions (5; 9; 13), and possibly are lo ated in protoplanetary disk surroundingmassivestars. Inthispaperdis overyofthepowerfulflareofemissionisreported bywhi hW75Nbe amethebrightestOH osmi maserforthewholehistoryofobservations. OBSERVATIONS Polarization observations of the maser emission from W75 N in all four Stokes param- eters were ondu ted at the Nan (cid:11)ay radio teles ope (Fran e) (4) in O tober 2003, and in two ir ular polarizations at the 64-m radio teles ope in Kalyazin (Russia) (12) in July and O tober 2004, in OH lines 1665 and 1667 MHz. Digital auto orrelators with spe tral resolu- tion 0.137 km/s were used at both teles opes. The number of hannels at Nan (cid:11)ay was 8192, divided in 8 se tions by 1024 hannels ea h (both frequen ies were observed, and ea h se - tion orresponded to a separate polarization), the bandwidth was 0.781 MHz. At Kalyazin there was only one se tion of 4096 hannels, with the bandwidth 3.125 MHz, so the measure- ments of the right hand and left hand ir ular polarizations had to be made sequentially. Additionally several re ords on the magneti tape were made in the VLBI system S-2 on April 12, 2001 at 64-m radio teles ope in Bear Lakes (Russia) and July 22, 2004 at Kalyazin. (The bandwidth in both ases was 8 MHz). The spe tral analysis was performed with a soft- ware orrelator, whi h alowed us to vary resolution in a wide range. The sensitivity of the Nan (cid:11)ay radio teles ope was 1.0 K/Jy, and that of Bear Lakes and Kalyazin radio teles opes was 0.65 K/Jy. The flux density alibration of Bear Lake and Kalyazin radio teles opes was performed with ontinuum sour es Cyg A and 3C 274. The beam width of the Nan (cid:11)ay radio ′ ′ teles ope was 3.5 in the East-West dire tion and 19 in North-South dire tion. Bear Lakes ′ and Kalyazin radio teles opes had ir ular beams with the width 8. In addition we have analyzed also observational data of this sour e in OH lines from NRAO VLBA ar hive for November 2000 and January 2001 (observations by Fish and Migenes). 3 RESULTS Spe tra of W75 N obtained with the VLBA in 1998 (13) and in 2001 (VLBA ar hive), at Nan (cid:11)ay in 2003, and in Kalyazin in 2004, in the OH line 1665 MHz in right hand ir ular polarization are presented on Fig. 1. Some differen es of spe tra obtained with the VLBA from the rest three spe tra arise be ause they were obtained in the VLBI regime while the single dish spe tra were obtained in the total power regime. We do not show total power spe tra from individual VLBA antennae, be ause they are orrupted by the ontribution from nearby maser W75 S due to a large beam of VLBA antennae. In the first spe trum of 1998 whi h was obtained during VLBA mapping several features were observed whi h were marked as A,C,E,F,G,J and K in 13. They were present also in earlier observations of 1983 (3), approximately with the same intensity ex ept features J and K at radial velo ities 3.0 km/s and 0.65 km/s whi h had variable intensity. Subsequent VLBA observations in November 2000 and January 2001 (VLBA ar hive) have shown that the spe trum of the sour e in the radial velo ity range from 4 km/s to 14 km/s, remained un hanged, while at lower radial velo ities signifi ant hanges took pla e. A strong spe tral feature at the radial velo ity 2.3 km/s and with a flux density 145 Jy (75 Jy in the right hand ir ular polarization whi h is shown in Fig. 1) appeared, with the line width 0.35 km/s as well as − a weaker feature at the radial velo ity 0.2 km/s with a flux density 35 Jy. We marked 1 2 them as P and P , respe tively. At the same time the features J and K have diminished by a fa tor from 2 to 3. Subsequent observations in April 2001 with a 64-m teles ope at Bear 1 2 Lakes (spe trum is not shown) demonstrated that the flared features P and P dimmed by a fa tor from 2 to 5, remaining at the same radio velo ities. One an suppose that the flare 1 2 of the features P and P was a pre ursor of a stillbrighter flare of the maser emission whi h has been first observed by us 2.5 years later, on O tober 24, 2003 with the Nan (cid:11)ay radio teles ope. At the spe trum on Fig. 1 one an see a very bright feature at the radial velo ity 1 1.8 km/ and the flux density 750 Jy and line width 0.35 km/s (N ) as well as two weaker − 2 3 features N at the radial velo ity 0 km/s and N at 1 km/s. Polarization measurements 1 2 3 at Nan (cid:11)ay show that all the features of the flare N , N , N have high degree of linear ≥ polarization ex eeding ( 80%). The differen e between radial velo ities of the new features − 1 2 N , N and those of the pre ursor whi h amounts to about 0.5 km/s, and the de rease of the pre ursor in April 2001 mean that a flare of new spe tral features was observed rather 4 than the brightening of old ones. At the same time the features from A to G have the same flux density and radial velo ity. The features J and K, even if present, have the flux density less than 10 Jy. Subsequent observations ondu ted at Kalyazin in July 2004 onfirmed the existen e of the flared features pra ti ally at the same radial velo ities and with the same flux densities. Later observations at Kalyazin in O tober-De ember 2004 reveal some 1 de ay of the strongest spe tral feature N a ompanied by an in rease of the weaker flare 2 3 features N and N . In the se ond OH line at the frequen y 1667 MHz, observed at Nan (cid:11)ay, new spe tral features have appeared also, but the intensity did not ex eed 20 Jy and was omparable with the intensity of the rest of spe tral feature. DISCUSSION Although the variability of OH masers is a well established phenomenon, there was no reports about OH maser emission flares whi h su h a large flux density. Moreover, at present there is no known OH maser with a flux density equal or ex eeding that of W75 N. A powerful flare of maser emission has been observed twi e in the line of water vapor at 1.35 m in Orion (1; 8; 10) with the flux above 10 MJy. Sin e the flux of water masers 3 ex eeds the flux of OH masers by a fa tor of 10 (cid:22) 10 , there is no surprise that the s ale of the flare is respe tively larger for water maser. In the paper of Slysh et al. (2002) a model of W75 N maser was proposed, in whi h the maser spots are lo ated in disks rotating around two massive stars whi h in turn ex ite ultra ompa t HII-regions VLA 1 and VLA 2 (14) (Fig. 2). Spe tral features from A to I at the radial velo ities from 12.45 km/s to 3.70 km/s are onne ted with VLA 1 (some weaker features are notvisibleon the s aleof our diagramandhave no marks); onlytwofeatures are onne ted withVLA 2: JandKatthe radialvelo ities3.0km/sand0.65 km/s, respe tively. Judging on radial velo ities of new spe tral features of the flare 2 km/s and 0 km/s, they ould be also onne ted with the VLA 2. VLA 1 and VLA 2 are separate enters of maser 2 emission also in lines of H O and, probably, of methanol. These ultra ompa t HII regions ⊙ are ex ited by young massive stars of spe tral type B1 or B2 with a mass 10 M (11). The separation between the stars is about 1400 AU. In the paper of Torrelles et al. (14) a model of the sour e W75 N was proposed, in whi h the stars are sour es of a wind and a sho k whi h ex ite the maser emission. Maser system onne ted with VLA 2 is more ompa t 5 and, apparently, more a tive. The eviden e of this omes from observations of high velo ity 2 masers in the OH 1667 MHz line in 1986 (6) and of the variability of H O maser emission with a period of 11.5 years, whi h are asso iated with VLA 2 (7). The new flare in the OH 1665 MHz line with the flux 1000 Jy, whi h is reported in this paper has o urred also in the maser system onne ted with the VLA 2. In the framework of the model of stellar wind sho k one anassume thatthe flarebegan uponarrivalofthesho k intoa regionofenhan ed 2 abundan e of H O and hydroxyl mole ules and elevation of the kineti temperature in this 2 region, or was triggered by the enhan ement of abundan e of H O and hydroxyl mole ules aused by the sho k. Almost simultaneously with the rising of the flux of spe tral features of the pre ursor − flare spe tral features at the radial velo ity 2.3 km/s ø 0.2 km/s, a drop of the flux from spe tral features J and K at the radial velo ities 3.0 km/s and 0.65 km/s has o ured. One an see from Fig. 1 that these features are not visible in the spe trum of W75 N of 2001, at least at the level of 10 Jy, while in 1998 they had flux omparable with that of the brightest feature A (13). The features J and K were brighter in the past: the maximum flux of feature J was observed in 1993 (6); the feature was weaker in the earlier epo hs of 1983 and 1986 (3; 6), as well as in later epo h 1998 (2; 13). Using this data one an estimate that the duration of feature J emission was t=11 years at the half intensity level, with the maximum in 1993. It is known from the VLBA maps (Fig. 2) (13) that the feature J as × well as feature K is extended and has a shape of a thin strip 10.7 2.4 milliar se in extent, ◦ with the position angle 114 . At the distan e to W75 N 2 kp this orresponds to the linear × × 1 2 size of the maser ondensation l l =(21.4 4.8) AU. If one assumes that the emission from the feature J was aused by passing of a sho k a ross the lump of matter with the above 1 mentioned size, the velo ity of the sho k must be not less than l /t=9.3 km/s, if the lump is in the plane of the sky, or more with an a ount of the proje tion effe t. Normal supersoni sho ks are hardly able to propagate with su h a velo ity, sin e the sound velo ity in mole ular louds is small, less than 1 km/s. However magneto hydrody- nami wavespropagatingwiththeAlfvenvelo itymaywellbetheagentex itingmaseremis- × −3 sion. Itisknownthatinthemaseremissionregionsmagneti fieldmayex eed5 10 Gauss, as follows from measurements of the Zeeman splitting. For example in W75 N two maser × −3 × −3 ondensations have magneti field 5.2 10 Gauss and 7.7 10 Gauss, respe tively ([13℄). × −3 5 H2 Assuming the value of magneti field equal to H=3 10 Gauss and gas density n =10 (cid:21) 6 6 −3 10 m , one gets Alfven velo ity 15 km/s and 5 km/s, respe tively, in agreement with the estimates of the ex itation propagation velo ity whi h ignited the maser emission of om- ponent J. One an suppose that after passing through the omponent J the ex itation with the Alfven velo ity has arrived to the next ondensation whi h gave start to the next flare of the maser emission. Its intensity this time happened to be about 50 times higher than the intensity of omponent J; this an be explaned by a more favorable onditions for the maser emission. More detailed information about temporal evolution of the maser emission flare an be obtained from results of high angular resolution VLBI mapping and from the spa e interferometer "Radioastron". CONCLUSION The brightest flare of emission in OH line sin e the beginning of observations in 1965 was dis overed. The flare is onne ted with the a tivity of a young massive star whi h ex ites ultra ompa t HII region VLA 2 in the star forming region W75 N. The onset of the flare was a ompanied by attenuation of emission from other maser ondensations onne ted with the VLA 2. Su h a behavior an be explained by propagation of magneto hydrodynami sho k through a region of enhan ed gas on entration. ACKNOWLEDGMENTS Authors are grateful to ollaborators from Astro Spa e Center of the Lebedev Physi al Institute B.Z. Kanevskii, A.I. Smirnov, S.V. Logvinenko, V.I. Vasil'kov, S.V. Kalenskii, and also to Ray Es offier (NRAO, USA) for the development of the re eiver and orrelator, and to Jean-Mi hel Martin, Eri Gerard (Meudon Observatory, Fran e), Vi tor Migenes (University of Guanojuato, Mexi o) for the help with observations, and to data analysts of NRAO for the a ess to the ar hive of results of observations. This resear h was ondu ted under a partial support from the Russion Foundation for Basi Resear h (proje t ode N 04-02-17057), CRDF (grant N RP1-2392-MO-02), Program for Fundamental Resear h of the Russian A ademy of S ien e "Extended Obje ts in the Universe", Federal Program "Astronomy", and also in the framework of the preparation the observational program for the spa e interferometer "Radioastron". 7 1. Z. Abraham, N. L. Cohen, R. Opher, et al., Astron. Astrophys. 100, L10 (1981). 2. A. L. Argon, M. J. Reid, and K. M. Menten, Astrophys. J. Suppl. Ser. 129, 159 (2000). 3. E. E. Baart, R. J. Cohen, R. D. Davies et al., MNRAS 219, 145 (1986). 4. 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WWWWW7777755555 NNNNN 1000 8 N N 3 2 0 2004 / X K N J GFE C A 1 1000 N N 3 2 0 2004 / VII K N J GFE C A 1 1000 yyyyy JJJJJ x, x, x, x, x, N3N2 uuuuu FlFlFlFlFl 0 2003 / X K N J GFE C A 1 1000 P P 2 1 0 2000 /XI K J GFE C A 1000 0 1998 / VII K J GFE C A -2 0 2 4 6 8 10 12 14 16 VVVVV ,,,,, kkkkkmmmmm/////sssss lllllsssssrrrrr Figure 1. Spe tra of OH maser W75 N in 1665 MHz line (right hand ir ular polarization), obtained at different epo hs. 1998/VII: VLBA array, July 1, 1998; 2000/XI: VLBA array, November 22, 2000; 2003/X: Nan ay radio teles ope, O tober 24, 2003; 2004/VII: Kalyazin radio teles ope, July 14, 2004; 2004/X: Kalyazin radio teles ope, O tober 20, 2004; Spe tral features marked by letters A (cid:22) K, were 1 2 1 2 3 identified in the paper of Slysh et al. (2002), features marked by letters P , P and N , N N are new spe tral features of the pre ursor (P) and flare (N). VLBA spe tra are shown in ′ interferometri mode, in whi h ontribution from the nearby maser W75 S at the distan e 14 is an eled. 9 1 2 Figure 2. Position of pre ursors P and P (in the vi inity of VLA 2) and the dire tion of the magneti field in them (determined from VLBA ar hive observations in November 2000 and Jan- 1 2 uary 2001, Slysh, private ommuni ation). Features P and P are plotted on the VLBA map of OH maser ondensations in W75 N, taken from Slysh et al. (2002). Cir les show 1665 MHz features, triangles show 1667 MHz features. The dire tion of the magneti field is shown by the ve tors. The position of the ultra ompa t HII regions VLA 1 and VLA 2 are shown by an el- lipse and a large ir le, respe tively. Dashed line shows a disk possibly onne ted with VLA 1.