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HST observations of Middle Aged Isolated Neutron Stars
Adv. Space Rex Vol. 21, No. 112,pp. 197-202, 1998 01998 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/98$19.00 + 0.00 PII: SO273-1177(97)0080&l
HST OBSERVATIONS OF MIDDLE AGED ISOLATED NEUTRON STARS R. Mignani****, P. A. Caraveo** and G. F. Bignamt .**.***.t *Max Planck Institute fir Extraterrestriche Physik, Postfach 1603, 085740 Garching, Germany **lstituto di Fisica Cosmica de1 CNR, via Bassini 1.5, 20133 Milan, Italy ***On leave from University of Cassino, 03403 Cassino. Italy tAS1, Rome, Ital)
ABSTRACT. We report the results of recent HST observations of three Middle Aged Isolated Neutron Stars i.e. PSR1055-52, Geminga and PSRO656+14. For PSR1055-52, a probable optical counterpart has been identified with the FOC. For PSR0656+14, WFPC2 imaging provided a clear detection of the proposed optical counterpart and was used to obtain a tentative measure of its proper motion. For Geminga, new FOC colors lead to a better characterization of the source spectral shape. The results are also discussed in the general framework of the optical emission of Middle Aged Isolated Neutron Stars 01998COSPAR. Published by Elsevier Science Ltd. (MINS).
1
INTRODUCTION.
INSs may be grouped in different families according to their dynamical age. Of particular interests is the class of “Middle Aged” Isolated Neutron Stars (MINS), i.e. . the ones with age in the range lo5 - lo6 yrs. Up to now, only three MINSs (PSR0656+14, Geminga and PSR1055-52) have more than three positive detections in different energy bands (Table 1) and their multiwavelength behaviour is similar. The “family mark” is a thermal soft X-ray emission (Finley, hgelman and Kiziloglu, 1992; Halpern & Ruderman, 1993; Ggelman and Finley, 1993) at T < lo6 K, consistent with the predictions of standard models of neutron star cooling (Nomoto & Tsuruta, 1987). Although thermal in origin, the X-ray emission of the three MINSs is partially pulsed with shallow light curves. MINSs are also high-energy r-ray emitters seen by EGRET (Ramanamurthy et al, 1996; Bertsch et al, 1992; Fierro et al, 1993). In the optical, they are faint objects (V 2 25): Geminga was identified by Bignami et al. (1993) with star G” (mv w 25.5) at a distance of 160 pc (Caraveo et al., 1996), a candidate counterpart (mv N 25) to PSR0656+14 was proposed on the basis of positional coincidence (Caraveo et al., 1994), while for PSR1055-52 only an upper limit of mv w 23 (or worse) was obtained (Bignami, Caraveo and Vacanti, 1988). As to the nature of the optical emission, we note that the Rayleigh-Jeans part of the Planckians seen at soft X-rays cannot account entirely for the data obtained so far. In particular, when multicolor photometry is available, significant deviations from a pure black body are seen (Bignami et al., 1996). 197
R. Mignani er al.
198
PULSAR 0656+14 GEMINGA 1055-52
Logr yrs 5.0
P ms 385
LogP s s-l -13.26
LogE erg s-l 34.58
Gauss
5.5 5.7
237 197
-13.96 -14.23
34.51 34.48
12.17 12.04
Log B 12.61
R
Opt
N/EUV
X
y
P u.1. P
P? [28.11] D [26.8] D [27.9]
D/D
P [5.9] P [5.7] P [5.8]
P? P P
D/P -
Table 1: The table lists the intrinsic parameters of the three IN% discussed in the text and their multiwavelength behaviour. Near and Extreme UV observations are marked in a single column (N/EUV). In columns 7th to llth, D stands for the objects’ detection in a given energy band, P for pulsed emission, u.1 for an upper limit and the hyphen for no data available. In column 8th and l&h, the numbers in brackets (logaritmic units) refer to the V band luminosity and to the soft X ray temperature, respectively.
2
PSR1055-52.
A ground based identification of PSR1055-52 is challenging since the pulsar radio position lies within 4 arcsec of a bright (mv = 14.6) field star (star A in Figure la). Thus, the strong background, induced locally by the seeing dependent PSF of the nearby star, makes it difficult to resolve any faint emission from the pulsar position. This is a case where the seeing-free optics of the HST, coupled with the UV sensitivity of the FOG’, can perform at its best yielding the first deep, high resolution imaging at the pulsar position. The field was actually observed on May 11 ” 1996 (Mignani, Caraveo & Bignami, 1997). The FOC was operated in the nominal F151 mode, with a 7 x 7 arcsec field of view (0.014 arcsec/pixel). Given the sharpness of the instrument PSF (0.42 arcsec FWHM), the flux of star A drops a factor lop5 at the target position. To further reduce any residual background, the observations have been performed with the 342W wide band filter (A = 3402A; AA = 702w). The final image, corresponding to a total exposure of 8900 s, is shown in Figure lb, after cosmic ray cleaning, and smoothing. While star A falls partially outside the frame, a faint point source (rns4zw = 24.88 i 0.1) is present near the field center. According to the HST astrometric solution, the object coordinates (52000) are cy = 10h57m58s.83 b = -52’26’56”.29. These coincide, within 0.1 arcsec, with the target radio position, itself known with an accuracy of 0.3 arcsec. Thus, the good positional coincidence gives us confidence that the detected source is the optical countepart of PSR1055-52.
3
PSR0656+14.
Although a candidate counterpart to PSR0656+14 already exists (Caraveo et al., 1994) the identification is in need of confirmation. As in the case of Geminga (Bignami et al, 1993) one could secure the optical identification taking advantage of the pulsar proper motion (Thompson and Cordova, 1994). Accurate positionings of the candidate counterpart with the WFPC2 would thus allow to search for the angular displacement measured in radio (- 70 mas/yr) at a position angle N 114’. A first observation was perfomed in Jan. 1996 with the wide band 555W filter (X = 5252& Ax = 1222.5A) for an exposure time of N 6200 s. The target was clearly detected in the PC (35 x 35 arcsec for a resolution of 0.0455 arcsec/px) with a magnitude rn555w = 25.1 f 0.1 and its centroid was computed with a precision of few hundreths of pixel (Mignani, Caraveo & Bignami, 1997). We have then compared the present position with the those obtained from our 1989 and 1991 frames. Unfortunately, the difference in pixel size (0”.675 and 0”.44, for the ground-based observations, against 0”.046, for the PC one) translates in a large uncertain on the pulsar relative position. Although this affects the statistical significance of
HST Observations of INSs
199
Figure 1: a) Image of the region around PSR1055-52 taken from the ground with NTT/SUSI (V filter) in a night of good seeing (0.6 arcsec). The cross marks the radio position of the pulsar. b) Image of the same field taken with the HST/FOC (342W filter). The faint point source in the field center is the optical counterpart of PSR1055-52. North is at top East at the left. the result, a hint of proper motion is certainly present. We obtain: p = 0.107 f 0.044 arcsec yr-’ with a position angle of 112” k 9”. These values, although far from being conclusive, are certainly consistent with the radio proper motion of Thompson and Cordova (1994) and, as such, support the proposed identification of PSRO656+14. An independent confirmation has been provided by Sheraer et al (1996) who have detected optical pulsations from a position compatible with our candidate.
4
GEMINGA.
Previous multicolor imaging of Geminga (Bignami et al. 1996) has shown that the flux distribution around 5500 a deviates significantly from the Rayleigh-Jeans extrapolations of the soft X-ray Planckian. To assess the reality of the effect, we obtained on May 1996 two more colors of Geminga using the FOC. One N 8000 s exposure was taken with the 430W filter (X = 3940& Ax = 832A) and yielded a new, more precise, measure in B (mdsow = 25.67 f 0.1) confirming the previous ground-based observation (Bignami et al, 1988), but significantly reducing the error bar. The other one (- 5000 s) was taken with the 195W filter (X = 2110& Ax = 94641) to investigate the spectral trend in the near UV. The magnitude was computed to be mlg5w = 23.9 f 0.1. As in Bignami et al (1996), we have compared the new colors with the low energy extrapolation of the EUVE black body curves (T = 2.0 - 2.5 lo5 K) normalized for the source distance of 157 pc (Caraveo et al., 1996), and a for a 15 km standard neutron star radius (see Bignami, 1997). Correction for interstellar absorption has been applied consistently to the optical/UV points using the NH best-fitting the ROSAT/EUVE data (- 1020). The whole data set is shown in Figure 2. It is evident that while the “U” (195W, 342W) and “B” (430W, B) fluxes, as well a~ the I upper limit, are consistent with the planckian, the 555W, V and R points appear to be well above
200
R. Mignani cr al.
Frequency
(Hz)
PSR01356+ 14
10-m
^ b I
PSR1055-52
I
P
5551 -13OLP
:
1
-tB
E E 10-m 4” I F .%
1.’ /’
.
1.’ ,
: C
/
.’ .’ .’ .’ I
/.’
/ 10-1’
,
,.I
,
,
,
I IO”
Frequency
(Hz)
Frequency
(Hz)
Figure 2: Comparison of multicolor observations with ROSAT soft X-ray Planckians for the three MINS discussed in the text. Spectral distorsions induced by the neutron star atmosphere are not considered in the extrapolation of the X-ray Planckians. Geminga: T = (2.0 - 2.5) x lo5 K 1020), D = 157 PC, R = 15 km; PSR0656+14: T N 8.6 x lo5 K (NH N 1020 cmB2), (NH N D = 500 pc, R = 12 km; PSR1055-52: T N 7.5 x lo5 K (NH N 6 102’), D = 500 pc, R = 15 km. While for Geminga the distance is measured, for both PSR0656+14 and PSR1055-52 it is estimated from the X-ray absorption. The uncertainty on the distance thus reflects on the value of the best fitting radius.
HST Observations of INSs
201
it. Thus, the presence of a spectral feature superimposed on the thermal continuum is now confirmed.
5
DISCUSSION.
The HST observations have improved significantly our knowledge of the optical emission of MINSs. For Geminga, apart the non trivial measure of the distance, we have now a test case. A feature appear to be superimposed to the continuum as extrapolated from soft X-rays and EUV data. For PSRO656+14, a much clear detection yields a better assessment of the magnitude and a hint of proper motion. In the case, of PSR1055-52, we present the discovery of its optical counterpart. For the two latter cases, multicolor photometry is just starting. Following the Geminga recipe, the optical/UV fluxes of PSR0656+14 and PSR1055-52 have been converted into monochromatic fluxes and compared with the optical extrapolation of the corresponding ROSAT soft X-ray spectra. For PSRO656+14 the optical data set consists of our V/555W magnitudes (Caraveo et al, 1994a; Mignani, Caraveo & Bignami, 1996) plus the FOC 130LP wide band detection (Pavlov et al., 1996) and the B magnitude of Shearer et al (1996). In all cases the fluxes lie above the Rayleigh Jeans extrapolation of the ROSAT planckian (Finley, hgelman and Kiziloglu, 1992). For PSR1055-52, only our FOC 342W point is available. However, it appears compatible with the optical extrapolation of the model spectrum (6gelman, & Finley, 19931, thus showing that the optical emission is compatible with a thermal origin.
References PI Bertsch, D.L. et al., 1992 Nature 357, 306 PI Bignami,G.F.,
Caraveo,P.A.,
Paul, J.A., Salotti,L.
& Vigroux, L. 1987, Ap.J.
319, 358
PI Bignami, G.F., Caraveo, P.A. 8z Paul, J.A., 1988 A&A 202, Ll PI Bignami, G.F., Caraveo, P.A. & Vacanti, G., 1988 A&A 196, 191 151Bignami, G.F., Caraveo, P.A. & Mereghetti S., 1993 Nature 361, 704 [Cl Bignami, G.F., Caraveo, P.A., Mignani, R., Edelstein, J. & Bowyer, S. 1996 Ap. J. Lett. 456, Llll [71 Bignami, G.F., 1997, A&J. Sp. Res.-in press. PI Caraveo, P.A., Bignami, G.F. & Mereghetti, S. , 1994 Ap. J. Lett. 422, L87 PI Caraveo, P.A., Bignami, G.F, Mignani, R. & Tti, L., 1996 .4p. J. Lett. 461, L9l PO1 Fierro, J.M. et al, 1993 Ap. J.
413, L27
Pll Finley, J.P., Ggelman, H. & Kiziloglu, U., 1992 Ap. J. 394, LZl PI
Halpern, J.P. & Ruderman, M., 1993 Ap.J.
415, 286
P31 Mignani, R., Caraveo, P.A. & Bignami, G.F., 1997 The Messenger-in press P4 Mignani, R., Caraveo, P.A. & Bignami, G.F., 1997. Ap. J.Lett. 474, L51 1151Nomoto, K. & Tsuruta, S., 1987 Ap.J. 312, 711 P61 dgelman, H. & Finley, J.P., 1993, Ap.J. 413, L31 P71 Pavlov, G.G., Strigfellow, G.S. & Cordova, F.A., 1996, Ap. J. 467, 370
R. Mignani et al.
202
[18] Ramanamurthy,
P.V., Fichtel,
C.E., Kniffen,
D.A., Sreekumar,
458, ‘755 [19] Shrearer,
[20] Thompson,
A. et al 1996 IAUC6502 R.J. & Cordova,
F.A., 1994 Ap. J. 421, L13
P. & Thompson,
D.J. 1996 Ap.J.
HST OBSERVATIONS OF MIDDLE AGED ISOLATED NEUTRON STARS R. Mignani****, P. A. Caraveo** and G. F. Bignamt .**.***.t *Max Planck Institute fir Extraterrestriche Physik, Postfach 1603, 085740 Garching, Germany **lstituto di Fisica Cosmica de1 CNR, via Bassini 1.5, 20133 Milan, Italy ***On leave from University of Cassino, 03403 Cassino. Italy tAS1, Rome, Ital)
ABSTRACT. We report the results of recent HST observations of three Middle Aged Isolated Neutron Stars i.e. PSR1055-52, Geminga and PSRO656+14. For PSR1055-52, a probable optical counterpart has been identified with the FOC. For PSR0656+14, WFPC2 imaging provided a clear detection of the proposed optical counterpart and was used to obtain a tentative measure of its proper motion. For Geminga, new FOC colors lead to a better characterization of the source spectral shape. The results are also discussed in the general framework of the optical emission of Middle Aged Isolated Neutron Stars 01998COSPAR. Published by Elsevier Science Ltd. (MINS).
1
INTRODUCTION.
INSs may be grouped in different families according to their dynamical age. Of particular interests is the class of “Middle Aged” Isolated Neutron Stars (MINS), i.e. . the ones with age in the range lo5 - lo6 yrs. Up to now, only three MINSs (PSR0656+14, Geminga and PSR1055-52) have more than three positive detections in different energy bands (Table 1) and their multiwavelength behaviour is similar. The “family mark” is a thermal soft X-ray emission (Finley, hgelman and Kiziloglu, 1992; Halpern & Ruderman, 1993; Ggelman and Finley, 1993) at T < lo6 K, consistent with the predictions of standard models of neutron star cooling (Nomoto & Tsuruta, 1987). Although thermal in origin, the X-ray emission of the three MINSs is partially pulsed with shallow light curves. MINSs are also high-energy r-ray emitters seen by EGRET (Ramanamurthy et al, 1996; Bertsch et al, 1992; Fierro et al, 1993). In the optical, they are faint objects (V 2 25): Geminga was identified by Bignami et al. (1993) with star G” (mv w 25.5) at a distance of 160 pc (Caraveo et al., 1996), a candidate counterpart (mv N 25) to PSR0656+14 was proposed on the basis of positional coincidence (Caraveo et al., 1994), while for PSR1055-52 only an upper limit of mv w 23 (or worse) was obtained (Bignami, Caraveo and Vacanti, 1988). As to the nature of the optical emission, we note that the Rayleigh-Jeans part of the Planckians seen at soft X-rays cannot account entirely for the data obtained so far. In particular, when multicolor photometry is available, significant deviations from a pure black body are seen (Bignami et al., 1996). 197
R. Mignani er al.
198
PULSAR 0656+14 GEMINGA 1055-52
Logr yrs 5.0
P ms 385
LogP s s-l -13.26
LogE erg s-l 34.58
Gauss
5.5 5.7
237 197
-13.96 -14.23
34.51 34.48
12.17 12.04
Log B 12.61
R
Opt
N/EUV
X
y
P u.1. P
P? [28.11] D [26.8] D [27.9]
D/D
P [5.9] P [5.7] P [5.8]
P? P P
D/P -
Table 1: The table lists the intrinsic parameters of the three IN% discussed in the text and their multiwavelength behaviour. Near and Extreme UV observations are marked in a single column (N/EUV). In columns 7th to llth, D stands for the objects’ detection in a given energy band, P for pulsed emission, u.1 for an upper limit and the hyphen for no data available. In column 8th and l&h, the numbers in brackets (logaritmic units) refer to the V band luminosity and to the soft X ray temperature, respectively.
2
PSR1055-52.
A ground based identification of PSR1055-52 is challenging since the pulsar radio position lies within 4 arcsec of a bright (mv = 14.6) field star (star A in Figure la). Thus, the strong background, induced locally by the seeing dependent PSF of the nearby star, makes it difficult to resolve any faint emission from the pulsar position. This is a case where the seeing-free optics of the HST, coupled with the UV sensitivity of the FOG’, can perform at its best yielding the first deep, high resolution imaging at the pulsar position. The field was actually observed on May 11 ” 1996 (Mignani, Caraveo & Bignami, 1997). The FOC was operated in the nominal F151 mode, with a 7 x 7 arcsec field of view (0.014 arcsec/pixel). Given the sharpness of the instrument PSF (0.42 arcsec FWHM), the flux of star A drops a factor lop5 at the target position. To further reduce any residual background, the observations have been performed with the 342W wide band filter (A = 3402A; AA = 702w). The final image, corresponding to a total exposure of 8900 s, is shown in Figure lb, after cosmic ray cleaning, and smoothing. While star A falls partially outside the frame, a faint point source (rns4zw = 24.88 i 0.1) is present near the field center. According to the HST astrometric solution, the object coordinates (52000) are cy = 10h57m58s.83 b = -52’26’56”.29. These coincide, within 0.1 arcsec, with the target radio position, itself known with an accuracy of 0.3 arcsec. Thus, the good positional coincidence gives us confidence that the detected source is the optical countepart of PSR1055-52.
3
PSR0656+14.
Although a candidate counterpart to PSR0656+14 already exists (Caraveo et al., 1994) the identification is in need of confirmation. As in the case of Geminga (Bignami et al, 1993) one could secure the optical identification taking advantage of the pulsar proper motion (Thompson and Cordova, 1994). Accurate positionings of the candidate counterpart with the WFPC2 would thus allow to search for the angular displacement measured in radio (- 70 mas/yr) at a position angle N 114’. A first observation was perfomed in Jan. 1996 with the wide band 555W filter (X = 5252& Ax = 1222.5A) for an exposure time of N 6200 s. The target was clearly detected in the PC (35 x 35 arcsec for a resolution of 0.0455 arcsec/px) with a magnitude rn555w = 25.1 f 0.1 and its centroid was computed with a precision of few hundreths of pixel (Mignani, Caraveo & Bignami, 1997). We have then compared the present position with the those obtained from our 1989 and 1991 frames. Unfortunately, the difference in pixel size (0”.675 and 0”.44, for the ground-based observations, against 0”.046, for the PC one) translates in a large uncertain on the pulsar relative position. Although this affects the statistical significance of
HST Observations of INSs
199
Figure 1: a) Image of the region around PSR1055-52 taken from the ground with NTT/SUSI (V filter) in a night of good seeing (0.6 arcsec). The cross marks the radio position of the pulsar. b) Image of the same field taken with the HST/FOC (342W filter). The faint point source in the field center is the optical counterpart of PSR1055-52. North is at top East at the left. the result, a hint of proper motion is certainly present. We obtain: p = 0.107 f 0.044 arcsec yr-’ with a position angle of 112” k 9”. These values, although far from being conclusive, are certainly consistent with the radio proper motion of Thompson and Cordova (1994) and, as such, support the proposed identification of PSRO656+14. An independent confirmation has been provided by Sheraer et al (1996) who have detected optical pulsations from a position compatible with our candidate.
4
GEMINGA.
Previous multicolor imaging of Geminga (Bignami et al. 1996) has shown that the flux distribution around 5500 a deviates significantly from the Rayleigh-Jeans extrapolations of the soft X-ray Planckian. To assess the reality of the effect, we obtained on May 1996 two more colors of Geminga using the FOC. One N 8000 s exposure was taken with the 430W filter (X = 3940& Ax = 832A) and yielded a new, more precise, measure in B (mdsow = 25.67 f 0.1) confirming the previous ground-based observation (Bignami et al, 1988), but significantly reducing the error bar. The other one (- 5000 s) was taken with the 195W filter (X = 2110& Ax = 94641) to investigate the spectral trend in the near UV. The magnitude was computed to be mlg5w = 23.9 f 0.1. As in Bignami et al (1996), we have compared the new colors with the low energy extrapolation of the EUVE black body curves (T = 2.0 - 2.5 lo5 K) normalized for the source distance of 157 pc (Caraveo et al., 1996), and a for a 15 km standard neutron star radius (see Bignami, 1997). Correction for interstellar absorption has been applied consistently to the optical/UV points using the NH best-fitting the ROSAT/EUVE data (- 1020). The whole data set is shown in Figure 2. It is evident that while the “U” (195W, 342W) and “B” (430W, B) fluxes, as well a~ the I upper limit, are consistent with the planckian, the 555W, V and R points appear to be well above
200
R. Mignani cr al.
Frequency
(Hz)
PSR01356+ 14
10-m
^ b I
PSR1055-52
I
P
5551 -13OLP
:
1
-tB
E E 10-m 4” I F .%
1.’ /’
.
1.’ ,
: C
/
.’ .’ .’ .’ I
/.’
/ 10-1’
,
,.I
,
,
,
I IO”
Frequency
(Hz)
Frequency
(Hz)
Figure 2: Comparison of multicolor observations with ROSAT soft X-ray Planckians for the three MINS discussed in the text. Spectral distorsions induced by the neutron star atmosphere are not considered in the extrapolation of the X-ray Planckians. Geminga: T = (2.0 - 2.5) x lo5 K 1020), D = 157 PC, R = 15 km; PSR0656+14: T N 8.6 x lo5 K (NH N 1020 cmB2), (NH N D = 500 pc, R = 12 km; PSR1055-52: T N 7.5 x lo5 K (NH N 6 102’), D = 500 pc, R = 15 km. While for Geminga the distance is measured, for both PSR0656+14 and PSR1055-52 it is estimated from the X-ray absorption. The uncertainty on the distance thus reflects on the value of the best fitting radius.
HST Observations of INSs
201
it. Thus, the presence of a spectral feature superimposed on the thermal continuum is now confirmed.
5
DISCUSSION.
The HST observations have improved significantly our knowledge of the optical emission of MINSs. For Geminga, apart the non trivial measure of the distance, we have now a test case. A feature appear to be superimposed to the continuum as extrapolated from soft X-rays and EUV data. For PSRO656+14, a much clear detection yields a better assessment of the magnitude and a hint of proper motion. In the case, of PSR1055-52, we present the discovery of its optical counterpart. For the two latter cases, multicolor photometry is just starting. Following the Geminga recipe, the optical/UV fluxes of PSR0656+14 and PSR1055-52 have been converted into monochromatic fluxes and compared with the optical extrapolation of the corresponding ROSAT soft X-ray spectra. For PSRO656+14 the optical data set consists of our V/555W magnitudes (Caraveo et al, 1994a; Mignani, Caraveo & Bignami, 1996) plus the FOC 130LP wide band detection (Pavlov et al., 1996) and the B magnitude of Shearer et al (1996). In all cases the fluxes lie above the Rayleigh Jeans extrapolation of the ROSAT planckian (Finley, hgelman and Kiziloglu, 1992). For PSR1055-52, only our FOC 342W point is available. However, it appears compatible with the optical extrapolation of the model spectrum (6gelman, & Finley, 19931, thus showing that the optical emission is compatible with a thermal origin.
References PI Bertsch, D.L. et al., 1992 Nature 357, 306 PI Bignami,G.F.,
Caraveo,P.A.,
Paul, J.A., Salotti,L.
& Vigroux, L. 1987, Ap.J.
319, 358
PI Bignami, G.F., Caraveo, P.A. 8z Paul, J.A., 1988 A&A 202, Ll PI Bignami, G.F., Caraveo, P.A. & Vacanti, G., 1988 A&A 196, 191 151Bignami, G.F., Caraveo, P.A. & Mereghetti S., 1993 Nature 361, 704 [Cl Bignami, G.F., Caraveo, P.A., Mignani, R., Edelstein, J. & Bowyer, S. 1996 Ap. J. Lett. 456, Llll [71 Bignami, G.F., 1997, A&J. Sp. Res.-in press. PI Caraveo, P.A., Bignami, G.F. & Mereghetti, S. , 1994 Ap. J. Lett. 422, L87 PI Caraveo, P.A., Bignami, G.F, Mignani, R. & Tti, L., 1996 .4p. J. Lett. 461, L9l PO1 Fierro, J.M. et al, 1993 Ap. J.
413, L27
Pll Finley, J.P., Ggelman, H. & Kiziloglu, U., 1992 Ap. J. 394, LZl PI
Halpern, J.P. & Ruderman, M., 1993 Ap.J.
415, 286
P31 Mignani, R., Caraveo, P.A. & Bignami, G.F., 1997 The Messenger-in press P4 Mignani, R., Caraveo, P.A. & Bignami, G.F., 1997. Ap. J.Lett. 474, L51 1151Nomoto, K. & Tsuruta, S., 1987 Ap.J. 312, 711 P61 dgelman, H. & Finley, J.P., 1993, Ap.J. 413, L31 P71 Pavlov, G.G., Strigfellow, G.S. & Cordova, F.A., 1996, Ap. J. 467, 370
R. Mignani et al.
202
[18] Ramanamurthy,
P.V., Fichtel,
C.E., Kniffen,
D.A., Sreekumar,
458, ‘755 [19] Shrearer,
[20] Thompson,
A. et al 1996 IAUC6502 R.J. & Cordova,
F.A., 1994 Ap. J. 421, L13
P. & Thompson,
D.J. 1996 Ap.J.