THE ASTROPHYSICAL JOURNAL, 467 : L81–L84, 1996 August 20 q 1996. The American Astronomical Society. All rights reserved. Printed in U.S.A.
RADIO AND X-RAY VARIABILITY OF THE GALACTIC SUPERLUMINAL SOURCE GRS 19151105 R. S. FOSTER
AND
E. B. WALTMAN
Remote Sensing Division, Naval Research Laboratory, Code 7210, Washington, DC 20375;
[email protected],
[email protected]
M. TAVANI Columbia University, Columbia Astrophysics Laboratory, 538 West 120th Street, New York, NY 10027;
[email protected]
B. A. HARMON
AND
S. N. ZHANG
Marshall Space Flight Center, ES-81, Huntsville, AL 35812;
[email protected],
[email protected]
W. S. PACIESAS Department of Physics, University of Alabama in Huntsville, Huntsville, AL 35899;
[email protected] AND
F. D. GHIGO National Radio Astronomy Observatory, P.O. Box 2, Green Bank, WV 24944;
[email protected] Received 1996 April 1; accepted 1996 June 4
ABSTRACT We report results of radio and hard X-ray monitoring observations of the Galactic superluminal X-ray source GRS 19151105 carried out with the Green Bank Interferometer and the Burst and Transient Source Experiment during the period 1994 September through 1996 March. Both the radio and the hard X-ray light curves show a complex transient behavior. The radio emission monitored at 2.25 and 8.3 GHz is correlated with episodes of enhanced hard X-ray emission. A phenomenological classification of the radio emission indicates two distinct emission modes: plateau and flaring. Plateau radio emission is in general optically thick, with a flat-topped light curve showing a rapid onset and decrease of the flux density. The radio flaring state shows large radio flares that can increase in flux density by 2 orders of magnitude in less than 18 hr, followed by an optically thin exponential decay. These observed large radio flares are consistent with external propagation of plasmoids emitting synchrotron radiation. We comment on the significance of these results, and suggest a scenario for modeling the multiwavelength behavior of GRS 19151105. Subject headings: accretion, accretion disks — radio continuum: stars — X-rays: stars constraints on the plasmoid motion. GRS 19151105 was the first transient X-ray source showing relativistic ejection of superluminal plasmoids in our Galaxy. One additional transient X-ray source, GRO J1655240, is known to produce similar relativistic plasmoid ejections associated with hard X-ray outbursts (Harmon et al. 1995a; Hjellming & Rupen 1995, hereafter HR95). Superluminal Galactic X-ray sources apparently can, on occasion, accelerate radio-emitting material to relativistic speeds in a way that resembles jetlike sources observed in active galactic nuclei or quasars. Motivated by the interest in studying the accretion and radiative processes of a compact Galactic object producing jetlike emission and relativistic plasmoid ejection, we started a systematic radio monitoring program in 1994 September. The main aim of our investigation is to provide high-density time sampling at radio wavelengths with the possibility of correlating, for the first time, the radio and hard X-ray emission for an extended time interval. Of particular interest is the relationship between the accretion process and the relativistic ejection phenomenon. Hard X-ray emission can be a useful diagnostic of plasma heating and possible nonthermal processes occurring in accretion disks, which can relate to instabilities producing relativistic plasmoids.
1. INTRODUCTION
The superluminal X-ray source and black hole candidate GRS 19151105 is one of the most interesting transient sources in our Galaxy. Discovered by the WATCH instrument in 1992 (Castro-Tirado et al. 1994), GRS 19151105 shows erratic hard X-ray outbursts that may reach several hundred millicrabs in the 20 –100 keV band (Harmon et al. 1994; Paciesas et al. 1995). Occasional X-ray outbursts have been detected in the 8 –20 keV band by the WATCH instrument (Sazonov et al. 1994), and two ASCA pointings during low hard X-ray states in 1994 September and 1995 April detected the source near the Eddington limit in the 1–10 keV band (Nagase et al. 1994; Ebisawa, White, & Kotani 1995). A radio monitoring program carried out during the period 1993 September through 1994 April at the Nanc¸ay radio telescope and at the Very Large Array (VLA) was successful in detecting radio flares from GRS 19151105 (Rodriguez et al. 1995, hereafter R95). A remarkable sequence of plasmoid ejections were detected by the VLA in 1994 March–April (Mirabel & Rodriguez 1994, hereafter MR94) showing apparent superluminal motion of two clearly separated radioemitting plasmoids of true speed b [ v/c 5 0.92 H 0.08 for a jet-axis inclined at an angle u 5 708 H 28 to the line of sight. Hydrogen absorption in the 21 cm line along the line of sight gives a kinematic distance d 3 12.5 H 1.5 kpc, consistent with the large optical depth ( AV 1 30), X-ray absorption (NH 1 5 3 10 22 cm 22 ; Ebisawa et al. 1995), and relativistic
2. OBSERVATIONS AND RESULTS
We summarize here the radio observations of GRS 19151105 carried out with the Green Bank Interferometer L81
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(GBI) and at high energies with the Compton Gamma Ray Observatory Burst and Transient Source Experiment (BATSE). GRS 19151105 has been continuously monitored since 1994 September through 1996 March using the GBI. The GBI consists of two 85 foot (25.9 m) diameter antennas separated by a 2.4 km baseline. The antennas each have cryogenically cooled dual-polarization receivers that observe simultaneously at 2.25 and 8.3 GHz with 35 MHz of total bandwidth at each frequency. The telescopes were operated by the National Radio Astronomy Observatory (NRAO) for the Naval Research Laboratory (NRL). The observing and data reduction techniques are described in Fiedler et al. (1987) and Waltman et al. (1991, 1994, 1995). The absolute flux density calibration is ultimately referred to 3C 286, where the assumed values for 3C 286 are 11.85 Jy at 2.25 GHz and 5.27 Jy at 8.3 GHz (Baars et al. 1977). The average integration times for individual scans is approximately 10 minutes with up to 12 scans made on the source per day. Typical errors in the GBI data set are flux density dependent (1 s): 4 mJy (2 GHz) or 6 mJy (8 GHz) for fluxes less than 100 mJy; 15 mJy (2 GHz) or 50 mJy (8 GHz) for fluxes 11 Jy. The limiting sensitivity of the GBI is about =10 mJy. The BATSE monitoring of GRS 19151105 consists of using the Earth occultation technique to measure source fluxes in the 20 keV–2 MeV band. The eight large area sodium iodide detectors provide coverage of about 70% of the sky at any given time, with occultations of a source location occurring twice every 90 minute orbit. The attenuation of the hard X-rays and low-energy gamma rays by the Earth’s atmosphere is modeled as a steplike function, in which the difference in the detector counting rate is determined before and after an occultation. A 1 day limiting sensitivity of approximately 75 mcrab (3 s) in the 20 –100 keV band can be obtained by summing the counting rates measured at occultation. More details of the BATSE instrument and analysis techniques can be found in Fishman et al. (1989) and Harmon et al. (1992). Various properties of GRS 19151105 in the high-energy bands from X-ray to low-energy gamma rays can be found in Harmon et al. (1994), Sazonov et al. (1994), and Paciesas et al. (1995). The radio flux density measurements at 2.25 GHz and the BATSE data are plotted in Figure 1 for the complete time period covered by the GBI monitoring, 1994 September through 1996 March. The daily averaged radio flux measurements at both 2.25 and 8.3 GHz are plotted on a logarithmic scale in Figure 2 for a time period of 100 days preceding and following a 1995 August 10 flare (Harmon et al. 1995b). Additional panels in Figure 2 display the corresponding hard X-ray flux observed by BATSE and the radio spectral index, a (S 5 S0 n 1a , where n is the observed radio frequency). Both the hard X-ray and radio light curves show a complex time variable behavior. When the source is undetected by the GBI (typically for radio flux densities less than 110 mJy at 2.25– 8.3 GHz), the hard X-ray emission is most of the time either below 100 mcrab (0.03 photons cm 22 s 21) or undetectable. Active states of radio emission can be phenomenologically separated into two distinct classes that we have identified as plateau and flaring. Plateau.—The plateau states result in an increase in the total flux density to levels of 1100 mJy, and they always occur in coincidence with hard X-ray outbursts. Both the onset and
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FIG. 1.—Daily averaged BATSE (20 –100 keV) hard X-ray and GBI 2.25 GHz radio emission of GRS 19151105 for the time period 1994 September through 1996 March are plotted vs. MJD (JD 2 2,400,000.5). The lower limit for GBI signal detection is about 10 mJy.
the decay timescales of the radio plateau states can be as short as 11 day. The plateau state is characterized by optically thick radio emission (i.e., a 1 0) in the 2.25– 8.3 GHz band except for episodes of superimposed major radio flares. The duration of the plateau state range from a day or two to many weeks (for example, see the plateau radio states shown in Fig. 2 for the time interval MJD1 49958 – 49970). Flaring.—The flaring states (always preceded by plateau radio states) are characterized by a very rapid rise time of the flare (less than 1 day) and optically thin exponential decay of the radio flux. We note that the radio flaring states are usually related to a decrease of the hard X-ray emission during the radio source exponential decay. Several significant radio flares occurred within the 18 month period encompassed by our observations. The most prominent flare (with peak intensity on 1995 August 10, MJD 49939) showed a large increase of the radio flux density to values over 790 mJy at 2.25 GHz and over 400 mJy at 8.3 GHz. The flare rise time was less than 18 hr (consistent with the time gap between different GBI slews to the source), while the exponential decay was characterized by a 1/e decay time constant of 1.1 H 0.1 day (see Fig. 3) as measured from the optically thin part of the exponential decay. This flare was also characterized by a change in the radio spectral index, which became negative during the rise and decay of the flare. The flare is consistent with optically thin synchrotron emission of energized particles. Other significant flares detected during these observations occurred near MJD 49610 and MJD 49680. Presumably, for the 1995 August 10 event we detected the rapid expansion and decay of a synchrotron bubble arising from the core of the superluminal source after a transition from an optically thick to an optically thin state. The flare had all the characteristics of radio emission from outwardly propagating plasmoids as previously detected from GRS 19151105 (MR94; R95). The very rapid onset of the radio flare, the optically thin decay of the flare within the frequency range 2.25– 8.3 GHz, and the decay time constant of a few days all point to an origin similar to the 1994 April radio flare 1
MJD is a modified Julian date defined as MJD 5 JD 2 2,400,000.5.
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FIG. 3.—Radio data from seven days around the flare of 1995 August 9 –10 are shown at 2.25 ( filled circles) and 8.3 (crosses) GHz. Note that the exponential decay of the source presumably indicates that the source is becoming optically thin as it expands outward.
FIG. 2.—Daily averaged BATSE (20 –100 keV) hard X-ray and daily averaged GBI (2.25 and 8 GHz) radio emission from GRS 19151105 during the 1995 May–November time period vs. MJD (JD 2 2,400,000.5). The final panel shows the spectral index computed from the observed dual frequency radio flux density measurements. The 2.25 GHz panel shows horizontal bars under the regions that are in the plateau state. A straight line is drawn through the exponential decay associated with the 1995 August 10 flare state event. Error bars represent 1 s confidence limits. The lower limit for GBI signal detection is about 10 mJy.
coincident with major plasmoid ejections (R95). However, we notice that the 1/e decay time of this flare is substantially less that the 1994 April event. The fast rise from low-level radio emission to very large flux density emission detected between MJD 49938 and 49939 occurred within =18 hr and gives an upper limit to the spatial size of the emitting region Lu 1 2 3 1015 cm. The actual size within which the process of particle energization and propagation occurs may be less than Lu . We also detected, for the first time in GRS 19151105, erratic radio emission superposed on an underlying radio plateau state. These erratic fluctuations (e.g., those occurring during the period MJD 50010 –50040 of Fig. 2) appear to be caused by rapid flares of very short timescale (=1 day). The erratic state tends to show a slight drop in the radio spectral index. 3. DISCUSSION
In general, we detect correlated hard X-ray and radio emission from GRS 19151105. The hard X-ray outbursts are
correlated with the plateau radio states and occasionally with the (late) occurrence of optically thin strong radio flares (e.g., episodes during MJD 49920 – 49940 and MJD 49990 –50015). We notice that hard X-ray outbursts may occur with no detectable radio flares, as exemplified by the emission episodes during MJD 49950 – 49980 and MJD 49990 –50000 (the same “dual” behavior of the hard X-ray outbursts with and without optically thin radio flares has been detected also in GRO J1655240; see Tavani et al. 1996). Radio flaring was detected in GRO J1655240 12 days after the 1994 July– August X-ray outburst, at a time when the X-ray decreased strongly (Tingay et al. 1995). Harmon et al. (1995a) reported that radio outbursts from GRO J1655240 followed the X-ray outbursts by a few days to about 2 weeks. Harmon et al. (1995a) suggested that this behavior in GRO J1655240 indicates a change of state in the inner disk rather than a simple variation in accretion rate. A possible mechanism for the radio and hard X-ray emission of GRS 19151105 is that the configuration of the magnetic field entrained in the inner part of the accretion disk may act as a “gating” mechanism (Tavani 1996). During the radio events, a disk/magnetic field instability may lead to magnetohydrodynamical acceleration of the plasmoid that freely expands outward (e.g., Blandford & Payne 1992). In our scenario, the very process of particle energization and jet formation uses part or all of the available magnetic energy. The optically thick plateau emission (see the “precursor” optically thick radio emission preceding the major radio flares of MJD 49940 and 50015) may be a signature of the storage of plasma for ejection. The subsequent decay of the hard X-ray flux following major radio flares may reflect the decreased magnetic energy density necessary to energize coronal particles (Tavani 1996). We note that the conditions favoring the magnetic gating and “opening” due to an instability do not always occur in coincidence with major disk energization events producing hard X-ray emission. We also notice that the “gating” behavior of GRS 19151105 resembles that of Cygnus X-3, as proposed by Kitamoto et al. (1994) and Watanabe et al. (1994).
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FOSTER ET AL. 4. CONCLUSIONS
At the moment, the only published measurements of plasmoid proper motion of GRS 19151105 are those from 1994 outbursts (MR94; R95; Mirabel & Rodriguez 1995). Our data strongly indicate that external plasmoid propagation occurred for the 1995 August 10 radio flare. Mirabel et al. (1996) confirm that propagating radio-emitting plasmoids were detected interferometrically following this flare. At the moment, we do not know the composition of the ejected plasmoids, i.e., whether electrons/positrons pairs constitute the major contribution of the ejected plasmoids, or whether a large number of baryons are entrained in the outflow. Additional data on radio flares are necessary to address fundamental questions on the physics of plasmoid acceleration. However, our results indicate that the hard X-ray and radio emissions of GRS 19151105 are correlated, and that a recognizable pattern of energy dissipation/accumulation indicated by enhanced hard X-ray and plateau radio emission precedes strong radio flares. GRS 19151105 is a remarkable accreting source able to “recharge” itself within a timescale of months and to produce strongly correlated hard X-ray and radio emission. Our combined radio/hard X-ray monitoring provides a useful database for multiwavelength studies. Future X-ray and gamma-ray missions (GRO, XTE, SAX) will provide simultaneous information on the soft/hard X-ray spectral properties of GRS 19151105, and pointed observations of these instruments can be “triggered” by episodes of strongly enhanced radio emission as potentially monitored by the GBI. High-quality X-ray spectroscopic observations of GRS 19151105 following epi-
sodes of major radio events will be important in determining the plasmoid composition and ionization state within the GRS 19151105 jets. These observations would be possible following an alert procedure (based on X-ray and radio data) set up for GRS 19151105. The good flux sensitivity and temporal resolution achievable with the GBI is crucial in detecting the short timescale radio flaring of GRS 19151105. We also notice that a prominent X-ray outburst in the 2–12 keV band detected by the All Sky Monitor on board of XTE (Morgan, Remillard, & Greiner 1996) starting at the beginning of 1995 March was not associated with radio emission detectable with the GBI throughout the period ending on 1995 April 2. The lack of detectable radio emission with the onset of an intense soft X-ray outburst of GRS 19151105 reinforces the correlation between the strong radio emission and the hard X-ray outbursts. We acknowledge the outstanding contribution of the staff of the National Radio Astronomy Observatory in maintaining the GBI. Unfortunately, because of budget constraints, the GBI program was interrupted following the acquisition of the last data points shown in Figure 1. No funds are currently available to continue operations of this instrument. Radio astronomy at the Naval Research Laboratory is supported by the Office of Naval Research. M. T. acknowledges interesting discussions with F. Mirabel and R. M. Hjellming at the 1996 Aspen Winter Workshop and support by NASA through grant NAG 5-2729.
REFERENCES Baars, J. W. M., Genzel, R., Pauliny-Toth, I. I. K., & Witzel, A. 1977, A&A, 61, 99 Blandford, R. D., & Payne, D. G. 1992, MNRAS, 199, 883 Castro-Tirado, A., Brandt, S., Lund, N., Lapshov, I., Sunyaev, R. A., Shlyapnikov, A. A., Guziy, S., & Pavlenko, E. P. 1994, ApJS, 92, 469 Ebisawa, K., White, N. E., & Kotani, T. 1995, IAU Circ. 6171 Fiedler, R. L., et al. 1987, ApJS, 65, 319 Fishman, G. J., et al. 1989, in Proc. Gamma Ray Observatory Science Workshop ed. N. Johnson (Greenbelt: Goddard Space Flight Center), 2-39 and 3-47 Harmon, B. A., et al. 1992, in Proc. Compton Observatory Science Workshop, ed. C. R. Shrader, N. Gehrels, & B. Dennis (Washington: NASA) 69 Harmon, B. A., et al. 1994, in AIP Conf. Proc. 304, Second Compton Symp., ed. C. E. Fichtel, N. Gehrels, & J. P. Norris, (New York: AIP), 210 Harmon, B. A., et al. 1995a, Nature, 374, 703 Harmon, B. A., Paciesas, W. S., Fishman, G. J., Ghigo, F. D., Waltman, E. B., & Foster, R. S. 1995b, IAU Circ. 6204 Hjellming, R. M., & Rupen, M. P. 1995, Nature, 375, 464 (HR95) Kitamoto, S., Miyamoto, S., Waltman, E. B., Fiedler, R. L., Johnston, K. J. & Ghigo, F. D. 1994, A&A, 281, L85 Mirabel, I. F., et al. 1996, in preparation Mirabel, I. F., & Rodriguez, L. F. 1994, Nature, 371, 46 (MR94)
———. 1995, Proc. NATO Advanced Institute on Solar and Magnetohydrodynamic Flows, ed. K. Tsinganos (Dordrecht: Kluwer), in press. Paciesas, W., et al. 1995, in Ann. NY Acad. Sci., 759, Seventeenth Texas Symposium on Relativistic Astrophysics, ed. H. Bohringer, G. E. Morfill, & J. E. Tru ¨mper, 308 Morgan, E., Remillard, R., & Greiner, J. 1996, IAU Circ. 6392 Nagase, F., Inoue, H., Kotani, T., & Ueda, Y. 1994, IAU Circ. 6094 Rodriguez, L. F., Gerard, E., Mirabel, I. F., Gomez, Y., & Velasquez, A. 1995, ApJS, 101, 173 (R95) Sazonov, S. Y., et al. 1994, Astron. Lett., 20, 787 Tavani, M. 1996, A&AS, in press Tavani, M., Fruchter, A., Harmon, B. A., Zhang, S. N., Hjellming, R. M., & Livio, M. 1996, in preparation Tingay, S. J., et al. 1995, Nature, 374, 141 Waltman, E. B., Fiedler, R. L., Johnston, K. J., & Ghigo, F. D. 1994, AJ, 108, 179 Waltman, E. B., Fiedler, R. L., Johnston, K. J., Spencer, J. H., Florkowski, D. R., Josties, F. J., McCarthy, D. D., & Matsakis, D. N. 1991, ApJS, 77, 379 Waltman, E. B., Ghigo, F. D., Johnston, K. J., Foster, R. S., Fiedler, R. L., & Spencer, J. H. 1995, AJ, 110, 290 Watanabe, H., Kitamoto, S., Miyamoto, S., Fiedler, R. L., Waltman, E. B., Johnston, K. J., & Ghigo, F. D. 1994, ApJ, 433, 350
