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Timing of accreting neutron stars with the ART-P telescope aboard GRANAT
Adv. Space Rcs. Vol. 16. No. 3. pp. (3)135-(3)138, 1995
Copyright Q 1995 COSPAR Printedin Great Britain. All rights reserved.
Pergamon
0273-I 177i95 $9.50 + 0.00 0273-1177(95)00062-3
TIMING OF ACCRETING NEUTRON STARS W ITH THE ART-P TELESCOPE ABOARD GRANAT A. Lutovinov, S. Grebenev, R. Sunyaev and M. Pavlinsky Spuce Research Institute, Russian Academy of Sciences, Profsoyuznaya 84132, Moscow 117810, Russia
ABSTRACT We report here some results obtained with the ART-P telescope aboard the GRANAT observatory under the program of X-ray pulsars timing. In 1990-1992 pulse periods and secular trends of 16 X-ray pulsars were measured with the instrument, and their pulse profiles in different el $rgy bands were determined. Among the most interesting results of ART-P th ere were: discovery of a transition from spin-up to spin-down in 4U1626-67, observations of a change in the pulse profile shape of this source associated with this transition, determination of binary parameters of 4UO115+63 and the upper limits of the mass function and the projected major semiaxis of 4Ul626-67. INTRODUCTION In total 16 X-ray binary pulsars were observed with the ART-P telescope aboard GRANAT during 1990-1992. This coded-mask instrument is able to measure the arrival times of photons detected by the 625 cm2 position-sensitive detector in the energy band from 3 to 60 keV with time resolution of 3.9 ms. Absolute timing of the data coming from ART-P is derived from reading of the GRANAT onboard clock which has the accuracy better than N 2 . 10e7 ss-l. The instrument dead time is about 580 ps. The telescope was described in detail in /7/. The observed photon arrival times had been converted to those at the barycenters of the solar system and the binary system for all the sources whose orbital parameters were known. Thereafter the pulse period have been determined by the standard epoch folding technique. Observations of the Crab pulsar had been performed once every year for calibration of instrument. Some of the most interesting results obtained with ART-P are presented below. SECULAR
VARIATIONS
OF PULSE PERIOD
There is a large variety in the pulse periods of the X-ray pulsars. Results of pulse period measurements with for some of them are presented in Table 1. The investigation of secular variation of the rotation period for each pulsar is especially important as it provide a valuable information regarding the torque exerted on a neutron star by the accreting matter. The histories for 6 X-ray pulsars are shown in Figure 1. Her X-l is a well-known X-ray pulsar exhibiting three types of periodicity: spin, orbital and 35days cycle “on-off’. Pulse period of this source measured on 6 Mar 1990 with ART-P and shown in Figure 1 and Table 1 testifies the continuation of the quasi-uniform acceleration of the neutron star rotation. We estimated the mean rate of this spin-up between the observations of the KVANT and GRANAT observatories to be equal to k/P = (-5.8 f 0.6) . 10v6 yr-’ . Unfortunately, in other sessions of observations this source was in “off’-state and the upper limit for the pulsations was equal to 2.7~. SMC X-l is the sole X-ray pulsar exibiting secular spin-up at the approximately (3)135
constant rate of
(3)136
A. Lutovinov c-f al.
TABLE1 Source SMC X-l HER X-l 4UO115+63 CEN X-3 4Ul626-67 GX1+4 GX301-2
date ’ JD 22.04.90 22.04.91 06.03.90 19.02.90 10.02.90 19.08.90 25.09.90 21.09.91 07.10.90 08.09.91 09.01.91 13.08.91 07.02.92
2440000+ 8004.4 8369.4 7957.2 7942.3 7933.2 8123.4 8160.0 8521.0 8172.2 8508.2 8266.4 8482.1 8660.4
Period+ 0.70957531 0.70925399 1.23775454 3.61461 4.821536 4.820909 7.65989 7.66170 114.657 116.158 683.92 679.21 677.29
Error(la),s 5.9 * 1o-8 1.5.10-r 4.8 . 1O-7 1.0. 1o-5 1.6 . 1O-5 1.4.10-s 4.5 .10-4 6.8 . 1o-5 1.4 .10-2 5.7.10-2 0.06 0.22 0.24
P/P N -6. 10e4 yr-‘. Recently it was suspected that the rate of this spin-up is no longer constant but decreasing /4/. The observations with ART-P confirm this point of view. We estimated the mean 1990-1991 rate to be equal to p/P = (-4.61 f 0.12) . 10e4 yr-’ .The figure shows that there is another X-ray pulsar, 4U1626-67, which demonstrated steady spin-up during more than a decade. However the measurements of its period performed with ART-P in 1991 indicated that a transition of the source from spin-up to spin-down had taken place just before the date. The period value of 4U1626-67 obtained during the previous ART-P observation in 1990 Sept was still consistent with the source spin-up at the approximately constant rate of P/P N -2. 10e4 yr-’ /4/.
IWP
c 7 ’
’
c ’ ” CX1+4
t
110
't
’ 3 L 0
’ -KvANI - CRANAT
.
i
JD 2440000+
Fig.1. Pulse period histories for 6 X-ray pulsars. References to papers containing the results of other experiments are given in /4/ and /a/. Cen X-3 is a massive binary system with a well determined rate cf the orbital
period change
GRANATIART-P
Timing of Accreting Neutron Stars
(3)137
GRANAT/ART-P
4U1626-6’7
Pulse
phase
Fig.2. Pulse profile dependence on time obtaining with ART-P for 4U1626-67. Hollow circles show the profiles with higher phase resolution. ko,.b/Po,.b = (-1.738 f 0.004) . 10e6 yr-’ /5/. Th e source has rather interesting history of the period variability: initially the pulsar exhibits secular spin-up at approximately constant rate, then the pulse period is practically constant, thereafter this cycle is reproduced again. Note that among 6 pulse period histories presented in Figure 1 only one shows a steady pulsar spin-down during a long time interval. This history belongs to the disk-fed pulsar GXl+4. PULSE PROFILES For each of the pulsars observed with ART-P there were pulse profiles produced in different energy bands and were all the changes in the profile shape versus energy and time revealed and analysed. 4Ul626-67 gives a good example of such complex behaviour demonstrating the dependence of its pulse profile shape on both the energy band and time (Figure 2). Pulse profiles of the source measured in 1990 Sept and 1991 Sept are shown in the left and right panels of the figure, respectively. In the left panel there drop (at the confidence level of 2.60 in the soft, energy band near the pulse phase of 0.7 whereas in the hard energy band there is a narrow peak in this place. In the right panel such peculiarities are absent. Apparently such a drastic change of the pulse profile of this source correlate with its transition from spin-up to spin-down. DETERMINATION
OF BINARY
PARAMETERS
The value of the pulse period observed from an X-ray pulsar is affected by its orbital movement. For example, in Figure 3a pulse periods of 4UO115+63 measured with ART-P are presented as functions of its orbital phase (filled circles). Variation in the period visible in the figure can be used to determine the orbital parameters of the source. In particular, the time of periastron passage by 4UO115+63, r(JD)= 2447942.224 f 0.004, and its orbital period, PO,+ = 24.31643 f 0.00007 days, were determined on the basis of ART-P data (other parameters were taken from /6/). The best-fit orbital curve with is shown in the figure by a solid line. The value of orbital period P&b = 2491.06s was determined from the optical observations /3/. We obtained the upper limits (99% confidence) JASR 16:3-J
(3)138
A. Lutovinov et al.
GRANAT/ART-P 4uo115+634
16-22
Feb,lQQO
4U1626-67/KZ
TrA
Fig.3. (a) Doppler shift of the pulse period for 4UO115+63 according with the ART-P measurements. Solid curve represents the period shift predicted for the best-fit binary orbit. (b) Upper limits the mass of the optical star in 4171626-67 as a function of the assumed orbital inclination angle for three values of the neutron star mass: 1.0, 1.4 and 1.8 Mm. Dark circles are obtained assuming values I<, z 280 kms-’ /l/. for the values of projected major semiaxis of its orbit, a,sini 5 13.3 It-ms, and the mass function, of 4U1626-67. The relationship between the mass of the companion star f(M) 5 3.1*10-%@, and the inclination angle for three various values of the neutron star mass are shown in Figure 3b by solid lines. REFERENCES 1. P.C.Joss, S.Rappaport, Ann. Rev. Astron. and Astrophys., 22, 537 (1984). 2. A.Lutovinov, 3. J.Middleditch,
S.Grebenev, R.Sunyaev and M.Pavlinsky, Sov. Astron. Letters, 20, 631 (1994). K.Mason, J.Nelson, N.White, ApJ, 244, 1001 (1981).
4. F.Nagase , Publ.Astron.Soc.Japan,
41,l (1989).
5. F.Nagase, R.Corbet, C.Day et al., ApJ, 396, 147 (1992). 6. S.Rappaport, G.W.Clark.,
L.Cominsky et al., ApJ Lett, 224, Ll (1978).
7. R.Sunyaev et al., Adv. Space. Res., 10, (2)223 (1990).
Copyright Q 1995 COSPAR Printedin Great Britain. All rights reserved.
Pergamon
0273-I 177i95 $9.50 + 0.00 0273-1177(95)00062-3
TIMING OF ACCRETING NEUTRON STARS W ITH THE ART-P TELESCOPE ABOARD GRANAT A. Lutovinov, S. Grebenev, R. Sunyaev and M. Pavlinsky Spuce Research Institute, Russian Academy of Sciences, Profsoyuznaya 84132, Moscow 117810, Russia
ABSTRACT We report here some results obtained with the ART-P telescope aboard the GRANAT observatory under the program of X-ray pulsars timing. In 1990-1992 pulse periods and secular trends of 16 X-ray pulsars were measured with the instrument, and their pulse profiles in different el $rgy bands were determined. Among the most interesting results of ART-P th ere were: discovery of a transition from spin-up to spin-down in 4U1626-67, observations of a change in the pulse profile shape of this source associated with this transition, determination of binary parameters of 4UO115+63 and the upper limits of the mass function and the projected major semiaxis of 4Ul626-67. INTRODUCTION In total 16 X-ray binary pulsars were observed with the ART-P telescope aboard GRANAT during 1990-1992. This coded-mask instrument is able to measure the arrival times of photons detected by the 625 cm2 position-sensitive detector in the energy band from 3 to 60 keV with time resolution of 3.9 ms. Absolute timing of the data coming from ART-P is derived from reading of the GRANAT onboard clock which has the accuracy better than N 2 . 10e7 ss-l. The instrument dead time is about 580 ps. The telescope was described in detail in /7/. The observed photon arrival times had been converted to those at the barycenters of the solar system and the binary system for all the sources whose orbital parameters were known. Thereafter the pulse period have been determined by the standard epoch folding technique. Observations of the Crab pulsar had been performed once every year for calibration of instrument. Some of the most interesting results obtained with ART-P are presented below. SECULAR
VARIATIONS
OF PULSE PERIOD
There is a large variety in the pulse periods of the X-ray pulsars. Results of pulse period measurements with for some of them are presented in Table 1. The investigation of secular variation of the rotation period for each pulsar is especially important as it provide a valuable information regarding the torque exerted on a neutron star by the accreting matter. The histories for 6 X-ray pulsars are shown in Figure 1. Her X-l is a well-known X-ray pulsar exhibiting three types of periodicity: spin, orbital and 35days cycle “on-off’. Pulse period of this source measured on 6 Mar 1990 with ART-P and shown in Figure 1 and Table 1 testifies the continuation of the quasi-uniform acceleration of the neutron star rotation. We estimated the mean rate of this spin-up between the observations of the KVANT and GRANAT observatories to be equal to k/P = (-5.8 f 0.6) . 10v6 yr-’ . Unfortunately, in other sessions of observations this source was in “off’-state and the upper limit for the pulsations was equal to 2.7~. SMC X-l is the sole X-ray pulsar exibiting secular spin-up at the approximately (3)135
constant rate of
(3)136
A. Lutovinov c-f al.
TABLE1 Source SMC X-l HER X-l 4UO115+63 CEN X-3 4Ul626-67 GX1+4 GX301-2
date ’ JD 22.04.90 22.04.91 06.03.90 19.02.90 10.02.90 19.08.90 25.09.90 21.09.91 07.10.90 08.09.91 09.01.91 13.08.91 07.02.92
2440000+ 8004.4 8369.4 7957.2 7942.3 7933.2 8123.4 8160.0 8521.0 8172.2 8508.2 8266.4 8482.1 8660.4
Period+ 0.70957531 0.70925399 1.23775454 3.61461 4.821536 4.820909 7.65989 7.66170 114.657 116.158 683.92 679.21 677.29
Error(la),s 5.9 * 1o-8 1.5.10-r 4.8 . 1O-7 1.0. 1o-5 1.6 . 1O-5 1.4.10-s 4.5 .10-4 6.8 . 1o-5 1.4 .10-2 5.7.10-2 0.06 0.22 0.24
P/P N -6. 10e4 yr-‘. Recently it was suspected that the rate of this spin-up is no longer constant but decreasing /4/. The observations with ART-P confirm this point of view. We estimated the mean 1990-1991 rate to be equal to p/P = (-4.61 f 0.12) . 10e4 yr-’ .The figure shows that there is another X-ray pulsar, 4U1626-67, which demonstrated steady spin-up during more than a decade. However the measurements of its period performed with ART-P in 1991 indicated that a transition of the source from spin-up to spin-down had taken place just before the date. The period value of 4U1626-67 obtained during the previous ART-P observation in 1990 Sept was still consistent with the source spin-up at the approximately constant rate of P/P N -2. 10e4 yr-’ /4/.
IWP
c 7 ’
’
c ’ ” CX1+4
t
110
't
’ 3 L 0
’ -KvANI - CRANAT
.
i
JD 2440000+
Fig.1. Pulse period histories for 6 X-ray pulsars. References to papers containing the results of other experiments are given in /4/ and /a/. Cen X-3 is a massive binary system with a well determined rate cf the orbital
period change
GRANATIART-P
Timing of Accreting Neutron Stars
(3)137
GRANAT/ART-P
4U1626-6’7
Pulse
phase
Fig.2. Pulse profile dependence on time obtaining with ART-P for 4U1626-67. Hollow circles show the profiles with higher phase resolution. ko,.b/Po,.b = (-1.738 f 0.004) . 10e6 yr-’ /5/. Th e source has rather interesting history of the period variability: initially the pulsar exhibits secular spin-up at approximately constant rate, then the pulse period is practically constant, thereafter this cycle is reproduced again. Note that among 6 pulse period histories presented in Figure 1 only one shows a steady pulsar spin-down during a long time interval. This history belongs to the disk-fed pulsar GXl+4. PULSE PROFILES For each of the pulsars observed with ART-P there were pulse profiles produced in different energy bands and were all the changes in the profile shape versus energy and time revealed and analysed. 4Ul626-67 gives a good example of such complex behaviour demonstrating the dependence of its pulse profile shape on both the energy band and time (Figure 2). Pulse profiles of the source measured in 1990 Sept and 1991 Sept are shown in the left and right panels of the figure, respectively. In the left panel there drop (at the confidence level of 2.60 in the soft, energy band near the pulse phase of 0.7 whereas in the hard energy band there is a narrow peak in this place. In the right panel such peculiarities are absent. Apparently such a drastic change of the pulse profile of this source correlate with its transition from spin-up to spin-down. DETERMINATION
OF BINARY
PARAMETERS
The value of the pulse period observed from an X-ray pulsar is affected by its orbital movement. For example, in Figure 3a pulse periods of 4UO115+63 measured with ART-P are presented as functions of its orbital phase (filled circles). Variation in the period visible in the figure can be used to determine the orbital parameters of the source. In particular, the time of periastron passage by 4UO115+63, r(JD)= 2447942.224 f 0.004, and its orbital period, PO,+ = 24.31643 f 0.00007 days, were determined on the basis of ART-P data (other parameters were taken from /6/). The best-fit orbital curve with is shown in the figure by a solid line. The value of orbital period P&b = 2491.06s was determined from the optical observations /3/. We obtained the upper limits (99% confidence) JASR 16:3-J
(3)138
A. Lutovinov et al.
GRANAT/ART-P 4uo115+634
16-22
Feb,lQQO
4U1626-67/KZ
TrA
Fig.3. (a) Doppler shift of the pulse period for 4UO115+63 according with the ART-P measurements. Solid curve represents the period shift predicted for the best-fit binary orbit. (b) Upper limits the mass of the optical star in 4171626-67 as a function of the assumed orbital inclination angle for three values of the neutron star mass: 1.0, 1.4 and 1.8 Mm. Dark circles are obtained assuming values I<, z 280 kms-’ /l/. for the values of projected major semiaxis of its orbit, a,sini 5 13.3 It-ms, and the mass function, of 4U1626-67. The relationship between the mass of the companion star f(M) 5 3.1*10-%@, and the inclination angle for three various values of the neutron star mass are shown in Figure 3b by solid lines. REFERENCES 1. P.C.Joss, S.Rappaport, Ann. Rev. Astron. and Astrophys., 22, 537 (1984). 2. A.Lutovinov, 3. J.Middleditch,
S.Grebenev, R.Sunyaev and M.Pavlinsky, Sov. Astron. Letters, 20, 631 (1994). K.Mason, J.Nelson, N.White, ApJ, 244, 1001 (1981).
4. F.Nagase , Publ.Astron.Soc.Japan,
41,l (1989).
5. F.Nagase, R.Corbet, C.Day et al., ApJ, 396, 147 (1992). 6. S.Rappaport, G.W.Clark.,
L.Cominsky et al., ApJ Lett, 224, Ll (1978).
7. R.Sunyaev et al., Adv. Space. Res., 10, (2)223 (1990).