Volume 22, Issue 1
March 2001, pages 1-144
pp 1-8 March 2001
Using the recently completed Giant Meterwave Radio Telescope, we have detected the HI 21 cm-line absorption from the peculiar galaxy C153 in the galaxy cluster Abell 2125. The HI absorption is at a redshift of 0.2533, with a peak optical depth of 0.36. The full width at half minimum of the absorption line is 100 km s−1. The estimated column density of atomic Hydrogen is 0.7×1022(Ts/100) cm−2. The HI absorption is redshifted by ∼400km s−1 compared to the [OIII] emission line from this system. We attribute this to an infalling cold gas or to an out-flowing ionised gas, or to a combination of both as a consequence of tidal interactions of C153 with either a cluster galaxy or the cluster potential.
pp 9-20 March 2001
The black hole candidate Cyg X-1 was observed in ultra low state on march 30, 1997 using Large Area Scintillation counter Experiment (LASE) in the hard X-ray energy region of 20–180 keV. During the 30 minute exposure a combined signal of 68 sigma was obtained, however, the measured flux at 50 keV was lower by a factor of 2 than the minimum flux reported so far. Using the recent orbital ephemeris of the source, our snap-shot observations were made at ϕ5.6 = 0.915, which corresponds to the binary minimum revealed by the ASM light curves. The daily average data from the BATSE detectors give the source intensity level to be higher by a factor of 5. Very low flux values measured in the present experiment suggest that the hard X-ray source may have been partially occulted by the primary companion during its transit near the X-ray minimum.
pp 21-34 March 2001
The redshifted 1420 MHz emission from the HI in unresolved damped Lyman-α clouds at high z will appear as a background radiation in low frequency radio observations. This holds the possibility of a new tool for studying the universe at high-z, using the mean brightness temperature to probe the HI content and its fluctuations to probe power spectrum. Existing estimates of the HI density atz−3 imply a mean brightness temperature of 1 mK at 320 MHz. The cross-correlation between the temperature fluctuation across different frequencies and sight lines is predicted to vary from 10−7 K2 to 10−8 K2 over intervals corresponding to spatial scales from 10 Mpc to 40 Mpc for some of the currently favoured cosmological models. Comparing this with the expected sensitivity of the GMRT, we find that this can be detected with ∼ 10 hrs of integration, provided we can distinguish it from the galactic and extragalactic foregrounds which will swamp this signal. We discuss a strategy based on the very distinct spectral properties of the foregrounds as against the HI emission, possibly allowing the removal of the foregrounds from the observed maps.
pp 35-50 March 2001
Nearby interstellar clouds with high (|ν|≥10km s−1) random velocities although easily detected in NaI and CaII lines have hitherto not been detected (in emission or absorption) in the HI 21cm line. We describe here deep Giant Metrewave Radio Telescope (GMRT) HI absorption observations toward radio sources with small angular separation from bright O and B stars whose spectra reveal the presence of intervening high random velocity CaII absorbing clouds. In 5 out of the 14 directions searched we detect HI 21cm absorption features from these clouds. The mean optical depth of these detections is ∼0.09 and FWHM is ∼10km s−1, consistent with absorption arising from CNM clouds.
pp 51-80 March 2001
We present the results of a search for carbon recombination lines in the Galaxy at 34.5 MHz (C575α) made using the dipole array at Gauribidanur near Bangalore. Observations made towards 32 directions resulted in detections of lines, in absorption at nine positions. Followup observations at 328 MHz (C272α) using the Ooty Radio Telescope detected these lines in emission. A VLA D-array observation of one of the positions at 330 MHz yielded no detection implying a lower limit of 10′ for the angular size of the line forming region.
The longitude-velocity distribution of the observed carbon lines indicate that the line forming regions are located mainly between 4 kpc and 7 kpc from the Galactic centre. Combining our results with published carbon recombination line data near 76 MHz (Erickson, McConnell & Anantharamaiah 1995), we obtain constraintson the physical parameters of the line forming regions. We find thatif the angular size of the line forming regions is ≥ 4°, then the range of parameters that fit the data are:Te=20–40 K,ne ∼ 0.1–0.3 cm−3 and pathlengths ∼ 0.07–0.9 pc which may correspond to thin photodissociated regions around molecular clouds. On the other hand, if the line forming regions are ∼ 2° in extent, then warmer gas (Te ∼ 60–300 K) with lower electron densities (ne ∼ 0.03–0.05 cm−3) extending over several tens of parsecs along the line of sight and possibly associated with atomic HI gas can fit the data. Based on the range of derived parameters, we suggest that the carbon line regions are most likely associated with photo-dissociation regions.
pp 81-119 March 2001
In a previous paper we presented a low-resolution (2°×2°) survey of radio recombination lines (RRLs) at 327 MHz in the longitude rangel=330° to 0° to 89°. In this paper, we present the results of a higher resolution (2°×6′) survey of RRLs from seven 2°-wide fields and two 6°-wide fields in the same longitude range. Observations were made using the Ooty Radio Telescope (ORT). A total of 252 spectra that were obtained are presented. RRLs were detected in almost all the individual positions within the fields withl<35° and at several individual positions within the fields in the longitude rangel=35° to 85°. Detailed analysis of the data towards the field centered at G45.5+0.0, shows that the line emission consists of discrete zones of ionized gas. The angular extent of these zones are likely to be one degree or more corresponding to a linear size of >110 pc at the kinematic distance.
pp 121-130 March 2001
HD 111154 is a member of the Coma Cluster. It is here shown to be a somewhat unequal pair of stars of approximately solar type. They are in an orbit that has a period of just under 27 days and quite a high eccentricity (0.442). Although the minimum masses (1.08 and 1.00M⊙) are large enough to encourage a search for eclipses, no such events have been detected.
pp 131-144 March 2001
The design and performance of a portable three channel photometer installed at the Uttar Pradesh State Observatory (UPSO), Naini Tal is described. The photometer is modular and the whole unit can be disassembled as individual channels such that the system can also be used as a single channel or two channel photometer. The system also has provision to monitor a guide star. The instrument was put into operation since November 1999 on the 1m Sampurnanand telescope at UPSO, Naini Tal. Since then, it is used extensively for the ‘Survey of rapidly oscillating Ap (roAp) stars in the northern sky’ from UPSO. Observational results using this new photometer in its initial phase of operation are discussed. The advantage of having continuous sky measurement is demonstrated.
Volume 41, 2020
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