GMRT monitoring of Cyg X-1 and Cyg X-3

Advances in Space Research 38 (2006) 2820–2823 www.elsevier.com/locate/asr GMRT monitoring of Cyg X-1 and Cyg X-3 M. Pandey a,* , A.P. Rao b, R. Manchanda c, P. Durouchoux d, C.H. Ishwara-Chandra b a c Department of Physics, Mumbai University, Mumbai-400 098, India b National Center for Radio Astrophysics, TIFR, India Department of Astronomy and Astrophysics, TIFR, Colaba, Mumbai-400 005, India d CEA Saclay, Service d’Astrophysique, 91191, Gif sur Yvette, France Received 15 October 2005; received in revised form 16 February 2006; accepted 16 February 2006 Abstract We present the results of low frequency radio observations of the X-ray binaries, Cygnus X-1 and Cygnus X-3, during different X-ray states. The low frequency observations were made for the first time for these sources at 0.61 and 1.28 GHz using the Giant Meter-wave Radio Telescope (GMRT) between 2003 and 2004. Both Cyg X-1 and Cyg X-3 are highly variable at low radio frequencies. We also compare our data with the observations at 15 GHz conducted by the Ryle telescope. Spectral turnover is seen for both the sources below 2 GHz. The data suggest that the change in the radio flux density in both the sources is correlated to the X-ray hardness ratio and follows a similar behavior pattern. Ó 2006 Published by Elsevier Ltd on behalf of COSPAR. Keywords: Binaries, Close; Stars, Cygnus X-1 and Cygnus X-3; ISM, Jets and outflows; Radio continuum stars 1. Introduction Radiative processes in microquasars are extremely complex due to the presence of accretion-ejection episodes and the nature of the underlying compact source. The observed temporal variations and the energy spectra of these sources at different wavelengths represent the behavior of the compact object and its accretion geometry. The hard X-rays and c-rays are produced by the thermal electrons in the accretion disc. This agrees with the model of the formation of hard X-rays due to Comptonisation of seed photons present inside the corona surrounding the accretion disc . The radio emission is due to synchrotron mechanism within the jet medium. The mm-radio emissions from the accretion disc is mostly due to thermal emission whereas the cm- and m-radio emissions are from the radio jets and are produced due to non-thermal emission. The X-ray binaries exist in two different radio states: the radio flaring and radio quiescent state. Thus, the spectral and * Corresponding author. E-mail address: [email protected] (M. Pandey). 0273-1177/$30 Ó 2006 Published by Elsevier Ltd on behalf of COSPAR. doi:10.1016/j.asr.2006.02.065 timing variability studies from the radio to c-ray wavelengths are necessary to understand the underlying physics in these objects. The main objective of present observation is to study the low radio frequency spectral evolution, the radio and X-ray variability, and the mechanism of radio emissions at different frequencies from these sources. 2. Observations The radio snapshot observations for 30 min were carried out at 0.61 and 1.28 GHz with a bandwidth of 16/32 MHz using the Giant Meter-wave Radio Telescope, GMRT. The flux density scale was set by observing the primary calibrators 3C286 and 3C48. Phase calibrators were interleaved with 25 min scans on each of Cyg X-1 and Cyg X-3. The integration time was 16 s. The data recorded from GMRT were converted into FITS files and analyzed using the Astronomical Image Processing System (AIPS). A self calibration on the data was performed to correct for phase related errors and to improve the image quality. Cyg X-3 was detected in each observation and Cyg X-1 during 2821 M. Pandey et al. / Advances in Space Research 38 (2006) 2820–2823 about one-half of them. For the observations performed during maintenance/test time of the telescope the background noise was high due to the reduced number of antennas available. For each field there are some six background sources detected.The near-constancy of the measured flux densities of these ‘control’ sources give us the overall confidence in the reliability of the overall system. The flux variability at a given frequency, observed for the control sources is mainly due to the large systematic errors caused by less number of antennas during some observations. To supplement the data from GMRT, we have also used the available radio data on Cyg X-1 and Cyg X-3 with the Ryle Telescope at 15 GHz. The radio light curve at 15 GHz was obtained with 10 and 5 min averaging time for Cyg X-1 and Cyg X-3, respectively. The typical uncertainty is 2 mJy + 3% in the flux scaling. 3. Results 3.1. Cyg X-1 Cyg X-1 is a bright X-ray binary system containing a black hole candidate (BHC) and an O type companion star identified at a distance of 2.5 kpc (Massey et al., 1995). It shows weak persistent radio emission at high radio frequencies with flat spectrum and the mean radio flux density of 14 mJy from 2 to 220 GHz (Fender et al., 2000). No high energy cut off is known in the case of Cyg X-1. Jet emissions have been observed for the source in the lowhard state at 8.4 GHz (Stirling et al., 2001) during VLBA observations. In the X-ray and the c-ray region, the photon spectrum of the source extends up to MeV energies in the high-soft X-ray state and up to 100 keV in the low-hard state. It spends 90% of its time in low-hard X-ray state as seen is Fig. 1. From the RXTE/ASM X-ray light curve of the source shown in Fig. 1, it is seen that transition between the low-hard to the high-soft state and back to low-hard is very gradual between our observation from 6th June 2003 (MJD 52796) to 2nd May 2004 (MJD 53128), thereby suggesting a stable accretion disc and the absence of instability in the accretion flow. The radio observations were performed with GMRT during the high-soft, intermediate and lowhard states of the source. Out of 13 observations the source was detected only on 6 days thereby suggesting its variable nature. It is interesting to note that Cyg X-1 is always detected at low radio frequencies with GMRT when the 15 GHz flux density exceeds 15 mJy limit. The mean radio flux density detected during each positive detection with GMRT was 8 mJy irrespective of the observation frequency. Table 1 shows the flux density variability trend of Cyg X-1 between June 2003 and May 2004. It can be seen that the source is highly variable at low frequencies. Cyg X-1 is a persistent but variable flat-spectrum source over the range 2–220 GHz with a mean flux density of 15 mJy (Fender et al., 2000). When compared with the GMRT data, the spectral measurements are subject to the additional uncertainty from the lack of precisely simultaneous observations. However, we conclude that the radio spectrum rises from the GMRT observing frequencies to 3 Cyg X-1 2.5 HS ll VV Int ll lV l l V V V V LH l V HS Int lVV l HR2 2 1.5 1 0.5 0 52800 52850 52900 52950 53000 MJD (52790=01-06-2003) 53050 53100 Fig. 1. Hardness ratio, HR2 = (5–13)keV/(13–60)keV for Cyg X-1 during all the GMRT observations marked with arrows. l V 2822 M. Pandey et al. / Advances in Space Research 38 (2006) 2820–2823 Table 1 Radio variability trend of Cyg X-1 in quiescent phase between June 2003 and May 2004 plasmoids in three dimensions within the jet medium (Hjellming and Johnston, 1988). Frequency (GHz) Mean flux density Variability in flux density for Cyg X-1 (%) Variability in flux density for Control source (%) 3.2. Cyg X-3 0.61 1.28 15 8 8 15 47 44 19 12 17 – the regime where it is essentially flat (2–220 GHz). Part of the variation of radio emission from Cyg X-1 is associated with the X-ray state of the system, and is therefore intrinsic. A further part may be associated with propagation through the interstellar medium, in particular, refractive interstellar scintillation (RISS) may introduce significant amplitude variations at low frequencies as seen from Table 1 (Rickett, 1990). The observed behavior of the source at radio frequencies can be explained by adiabatically expanding plasmoids in conical jets (Hjellming and Johnston, 1988), with the radio emissions at different frequencies originating in different regions within the jet medium (Ishwar-Chandra et al., 2004). The radiative lifetime of the electrons emitting at lower radio frequencies is long, as the source should be visible for longer duration at these frequencies. However, the synchrotron self absorption can result in the spectral turnover of the radio emission from the compact component in the optically thick medium. This can be explained by the model of adiabatically expanding The X-ray binary, Cyg X-3 is located in the Galactic plane at a distance of 10 kpc (Predehl et al., 2002; Dickey, 1983). The jet structure in Cyg X-3 has knots and perhaps corresponds to discrete bullets of matter ejected from the central object in quiescent state with frequent flaring modes (Stirling et al., 2001). The source occasionally undergoes huge radio outbursts where the flux density increases to a level of up to 20 Jy in high radio frequency range. Two-sided jets have also been seen on arc-second scales in a N–S orientation (Marti et al., 2001). Whereas a highly-relativistic (b P 0.81) one-sided jet with the same orientation has been reported on milli-arcsec scales with the VLBA (Mioduszewski et al., 2001). Fig. 2 shows the X-ray states of the source between 6th June 2003 (MJD 52796) and 2nd May 2004 (MJD 53128). The light curve of the source shows frequent changes in the X-ray state from low-hard to high-soft, which is believed to arise from the instability within the accretion disc. During the high accretion episodes in the case of Cyg X-3, the radiation pressure instability develops within the accretion disc giving rise to radio flares. During our observations performed with GMRT, the source was positively detected suggesting its persistent nature. The flux density measurement made during our observation also suggests that source is highly variable at low frequencies Cyg X-3 8 HS LH II I VV V I I V V I I V II II V VV 7 HR2 6 5 4 3 I V 2 1 52800 52850 52900 52950 53000 MJD (52790=01-06-2003) 53050 53100 Fig. 2. Hardness ratio, HR2 = (13–60)keV/(5–13)keV for Cyg X-3 during all the GMRT observations marked with arrows. M. Pandey et al. / Advances in Space Research 38 (2006) 2820–2823 Table 2 Radio variability trend of Cyg X-3 in quiescent phase between June 2003May 2004 Frequency (GHz) Mean flux density Variability in flux density for Cyg X-3 (%) Variability in flux density for Control source (%) 0.61 1.28 15 25 55 117 60 53 38 14 8 – as seen in Table 2. The high variability at low frequencies can be attributed to RISS. Cyg X-3 is a persistent radio source at all wavelengths. Cyg X-3 is more luminous at higher frequencies. The data in Table 2 clearly indicates a low frequency turnover in the source spectrum below 1 GHz. As discussed earlier, such behavior can arise due to synchrotron self absorption of the compact radio emitting plasma in an optically thick medium. The observed variability of the flux density is consistent with the assumption of a discrete ejection/plasmoids in adiabatic expansion. The ejection rate and/or lifetime of plasmoids in Cyg X-3 are probably lower/shorter than in Cyg X-1 with little or no overlap of the spectra associated to the single discrete ejection. 4. Discussion The low frequency radio observations are necessary to understand and constrain the geometry of the microquasars. In this paper, we have presented the first ever low frequency monitoring and temporal behavior of these sources over two years at various frequencies. There is a continuous radio emission from 2 to 15 GHz in the case of Cyg X-1 during flaring and quiescence state. The source shows variable behavior from 0.61 to 1.28 GHz with a mean radio flux density being 8 mJy. However, Cyg X-3 is persistent in nature from 0.61 to 1.28 GHz during both the state. The source is also persistent in nature at 15 GHz. The low frequency turnover of 1.28 GHz was determined for the first time for Cyg X-1 and Cyg 2823 X-3 via our observations. Rigorous calculations were performed to look for the variability in the radio flux density at low frequency on the data of these two sources. It is clear that the low frequency data are affected by refractive interstellar scintillation. However, the variability at high frequencies is intrinsic to the source and least affected by interstellar scintillation. Finally, we have discussed the emission model for both the sources in which synchrotron radiation-emitting plasmoids originating at the base of the conical jet and the observed spectral features are due to the superposition of many such plasmoids with different age profile; however a detailed modelling will be presented elsewhere. 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