arXiv subjects

Md Rashid

Publications and source records attributed to Md Rashid.

5 recordsLinked to original sources

arXiv2026

Detection of non-thermal radio emission components from the Orion Nebula: stellar jets, cloud collision or feedback from stellar winds?

The Orion Nebula is the closest high-mass star-forming region, making it an ideal laboratory to investigate physical processes in complex star-forming environments. At radio frequencies, the dominant emission mechanisms are thermal bremsstrahlung and non-thermal synchrotron. HII regions typically emit thermal radiation tracing the ionised gas; however, detecting and characterising non-thermal emission can provide insights into magnetic fields and the energy distribution of relativistic particles in star-forming regions. We have utilised the upgraded Giant Metrewave Radio Telescope (uGMRT) to study radio emission in the Extended Orion Nebula (EON) region. We present results from wide-band interferometric observations using uGMRT bands 3 and 4, probing a frequency range not covered by other sensitive radio interferometers. We produced deep continuum images with RMS noise levels of $\sim400\,\mu$Jy~beam$^{-1}$ in band 3 and $\sim200\,\mu$Jy~beam$^{-1}$ in band 4. We further generated in-band and broad-band spectral index maps using these images. To establish the robustness of the spectral index measurements, we conducted a detailed analysis using simulated uGMRT data. From the continuum spectral index analysis, we report the unambiguous presence of non-thermal radio emission in the EON region. To investigate its plausible origin, we correlated our results with multiwavelength observations, identifying a strong association between non-thermal emission and outflows from young stellar objects, while also exploring alternative explanations. In future, reliable broad-band radio spectral index measurements, together with dedicated multiwavelength observations, will be invaluable for resolving the origin of non-thermal emission in the Orion Nebula and other star-forming regions.

astro-ph.GA↗

arXiv2025

Deep uGMRT observations for enhanced calibration of 21 cm arrays -- I. First image and source catalogue

Radio-interferometric arrays require very precise calibration to detect the Epoch of Reionization 21-cm signal. A remarkably complete and accurate sky model is therefore needed in the patches of the sky used to perform the calibration. Instruments such as HERA, which use a redundant calibration strategy, also require a reference sky model to fix degenerate gain solutions. We have carried out a deep (20 hours) observation using the upgraded GMRT to make a high-fidelity sky model of one of the HERA calibration fields GLEAM 02H (J0200$-$3053). Here, we present the results from a $16.7\,\rm{MHz}$ bandwidth data centred at $147.4\,\rm{MHz}$. Using multiple GMRT pointings, we have made a $6.9^\circ\times6.9^\circ$ mosaic, which yields a median rms of $3.9^{+3.7}_{-1.4}\,{\rm mJy/beam}$ that reduces to $\sim2\,{\rm mJy/beam}$ at source-free regions. In the overlapping patch, this rms is deeper than the GLEAM catalogue, which is used for HERA calibration. We produce a catalogue of $640$ sources ($26\%$ extended) in the flux range $0.01-19.08\,{\rm Jy}$. The catalogue has a sub-arcsec positional accuracy, and the estimated fluxes are consistent with existing catalogues. The differential source counts are found to be deeper than GLEAM and consistent with LoTSS. Preliminary simulations of the sky models from GLEAM and our catalogue show $\sim 10-25\%$ difference in the visibility amplitude, with relatively small phase difference ($\approx 2^\circ$). Future work is planned for larger survey areas and wider bandwidth to reduce the rms and measure the in-band source spectral indices, which are expected to enhance the fidelity of the HERA calibration model.

astro-ph.CO↗

arXiv2024

Reliability of in-band and broadband spectral index measurement: systematic study of the effect of signal to noise for uGMRT data

Low radio frequency spectral index measurements are a powerful tool to distinguish between different emission mechanisms and, in turn, to understand the nature of the sources. Besides the standard method of estimating the ``broadband" spectral index of sources from observations in two different frequency ``bands", if the observations were made with large instantaneous bandwidth, the ``in-band" spectral index can be determined, either using images of emission at multiple frequency ranges within a band or using the novel Multi Term-Multi Frequency Synthesis (MT-MFS) imaging algorithm. Here, using simulated upgraded Giant Metrewave Radio Telescope (uGMRT) data, we have systematically studied the reliability of various methods of spectral index estimation for sources with a wide range of signal-to-noise ratio (SNR). It is found that, for synthetic uGMRT point source data, the MT-MFS imaging algorithm produces in-band spectral indices for SNR~$\lesssim100$ that have errors $\gtrsim 0.2$, making them unreliable. However, at a similar SNR, the sub-band splitting method produces errors $\lesssim 0.2$, which are more accurate and unbiased in-band spectral indices. The broadband spectral indices produce errors $\lesssim 0.2$ even for SNR $\gtrsim 15$, and hence, they are most reliable if there are no higher-order variations in the spectral index. These results may be used to improve the uGMRT observation and data analysis strategies depending on the brightness of the target source.

astro-ph.IM↗

arXiv2024

Exploring neutral hydrogen in the radio MOlecular Hydrogen Emission Galaxies (MOHEGs) and prospects with the SKA

The empirical studies of cold gas content serve as an essential aspect in comprehending the star formation activities and evolution in galaxies. However, it is not straightforward to understand these processes because they depend on various physical properties of the Interstellar Medium. Massive FRI/II type radio galaxies rich in molecular hydrogen with less star formation activities are known as radio Molecular Hydrogen Emission Galaxies (MOHEGs). We present a study of neutral hydrogen gas-associated radio MOHEGs at redshifts <0.2 probed via the HI 21-cm absorption line. Neutral hydrogen is detected in 70% of these galaxies, which are located at a distance of 8 - 120 kiloparsec from the neighboring galaxies. These galaxies show a scarcity of HI gas as compared to merging galaxies at similar redshifts. We found no strong correlation between N(HI), N(H), and galaxy properties, independent of whether the HI is assumed to be cold or warm, indicating that the atomic gas is probably playing no important role in star formation. The relationship between total hydrogen gas surface density and star formation surface density deviates from the standard Kennicutt-Schmidt law. Our study highlights the importance of HI studies and offers insights into the role of atomic and molecular hydrogen gas in explaining the properties of these galaxies. In the upcoming HI 21-cm absorption surveys with next-generation radio telescopes such as the Square Kilometre Array (SKA) and pathfinder instruments, it may be possible to provide better constraints to such correlations.

astro-ph.GA↗

arXiv2023

Metrewave Galactic Plane with the uGMRT (MeGaPluG) Survey: Lessons from the Pilot Study

Context. The advent of wide-band receiver systems on interferometer arrays enables one to undertake high-sensitivity and high-resolution radio continuum surveys of the Galactic plane in a reasonable amount of telescope time. However, to date, there are only a few such studies of the first quadrant of the Milky Way that have been carried out at frequencies below 1 GHz. The Giant Metrewave Radio Telescope (GMRT) has recently upgraded its receivers with wide-band capabilities (now called the uGMRT) and provides a good opportunity to conduct high resolution surveys, while also being sensitive to the extended structures. Aims. We wish to assess the feasibility of conducting a large-scale snapshot survey, the Metrewave Galactic Plane with the uGMRT Survey (MeGaPluG), to simultaneously map extended sources and compact objects at an angular resolution lower than $10''$ and a point source sensitivity of 0.15 mJy/beam. Methods. We performed an unbiased survey of a small portion of the Galactic plane, covering the W43/W44 regions ($l=29^\circ-35^\circ$ and $|b|<1^\circ$) in two frequency bands: 300$-$500 MHz and 550$-$750 MHz. The 200 MHz wide-band receivers on the uGMRT are employed to observe the target field in several pointings, spending nearly 14 minutes on each pointing in two separate scans. We developed an automated pipeline for the calibration, and a semi-automated self-calibration procedure is used to image each pointing using multi-scale CLEAN and outlier fields. Results. We produced continuum mosaics of the surveyed region at a final common resolution of $25''$ in the two bands that have central frequencies of 400 MHz and 650 MHz, with a point source sensitivity better than 5 mJy/beam. We plan to cover a larger footprint of the Galactic plane in the near future based on the lessons learnt from this study. (Abridged)

astro-ph.GA↗