arXiv subjects

Nirupam Roy

Publications and source records attributed to Nirupam Roy.

At least 55 records · Page 3Linked to original sources

arXiv2024

The temperature of the neutral Interstellar Medium in the Galaxy

Atomic Hydrogen-21 cm transition (HI) is an excellent tracer to study and understand the properties of the atomic gas in the Galaxy. Using the Westerbork Synthesis Radio Telescope (WSRT), we observed 12 quasar sightlines to detect galactic HI in absorption. We achieve an optical depth RMS of $\sim 1-2 \times 10^{-3}$, essential to detect the Warm Neutral Medium (WNM). We detect HI absorption in all our sightlines except along 1006+349, for which we set a strict upper limit on the spin temperature as $\langle T_s \rangle > 570$ K. We find around 50\% of our sightlines have $\langle T_s \rangle > 500$ K, indicating a WNM dominance. Further, we calculate an upper limit of the CNM fraction along our sightlines and find a median CNM fraction of $\sim 0.12$. With our observations, we reconfirm the existence of a threshold column density of $\sim 2 \times 10^{20} \ cm^{-2}$ to form CNM in the ISM. Using a two-temperature model of the HI disk, we explore the distribution of spin temperature in the Galactic ISM. We find that a simple fixed axisymmetric two-temperature model could not produce either the observed column density or the integral optical depth. This indicates the existence of a more complex distribution of spin temperatures in the Galaxy.

astro-ph.GA↗

arXiv2024

Towards $21$-cm intensity mapping at $z=2.28$ with uGMRT using the tapered gridded estimator -- IV. Wideband analysis

We present a Wideband Tapered Gridded Estimator (TGE), which incorporates baseline migration and variation of the primary beam pattern for neutral hydrogen (${\rm H\hspace{0.5mm}}{\scriptsize {\rm I}}$) 21-cm intensity mapping (IM) with large frequency bandwidth radio-interferometric observations. Here we have analysed $394-494 \, {\rm MHz}$ $(z = 1.9 - 2.6)$ uGMRT data to estimate the Multi-frequency Angular Power Spectrum (MAPS) $C_\ell(Δν)$ from which we have removed the foregrounds using the polynomial fitting (PF) and Gaussian Process Regression (GPR) methods developed in our earlier work. Using the residual $C_\ell(Δν)$ to estimate the mean squared 21-cm brightness temperature fluctuation $Δ^2(k)$, we find that this is consistent with $0 \pm 2 σ$ in several $k$ bins. The resulting $2σ$ upper limit $Δ^2(k) < (4.68)^2 \, \rm{mK^2}$ at $k=0.219\,\rm{Mpc^{-1}}$ is nearly $15$ times tighter than earlier limits obtained from a smaller bandwidth ($24.4 \, {\rm MHz}$) of the same data. The $2σ$ upper limit $[Ω_{\rm H\hspace{0.5mm}{\scriptsize {\rm I}}} b_{\rm H\hspace{0.5mm}{\scriptsize {\rm I}}}] < 1.01 \times 10^{-2}$ is within an order of magnitude of the value expected from independent estimates of the ${\rm H\hspace{0.5mm}}{\scriptsize {\rm I}}$ mass density $Ω_{\rm H\hspace{0.5mm}{\scriptsize {\rm I}}}$ and the ${\rm H\hspace{0.5mm}}{\scriptsize {\rm I}}$ bias $b_{\rm H\hspace{0.5mm}{\scriptsize {\rm I}}}$. The techniques used here can be applied to other telescopes and frequencies, including $\sim 150 \, {\rm MHz}$ Epoch of Reionization observations.

astro-ph.CO↗

arXiv2024

IMUOptimize: A Data-Driven Approach to Optimal IMU Placement for Human Pose Estimation with Transformer Architecture

This paper presents a novel approach for predicting human poses using IMU data, diverging from previous studies such as DIP-IMU, IMUPoser, and TransPose, which use up to 6 IMUs in conjunction with bidirectional RNNs. We introduce two main innovations: a data-driven strategy for optimal IMU placement and a transformer-based model architecture for time series analysis. Our findings indicate that our approach not only outperforms traditional 6 IMU-based biRNN models but also that the transformer architecture significantly enhances pose reconstruction from data obtained from 24 IMU locations, with equivalent performance to biRNNs when using only 6 IMUs. The enhanced accuracy provided by our optimally chosen locations, when coupled with the parallelizability and performance of transformers, provides significant improvements to the field of IMU-based pose estimation.

cs.LG↗

arXiv2024

Observational evidence for local vertical constraining of \HI by molecular cloud complexes

A massive molecular cloud complex represents local gravitational potential that can constrain the vertical distribution of surrounding stars and gas. This pinching effect results in the local corrugation of the scale height of stars and gas which is in addition to the global corrugation of the mid-plane of the disc. For the first time, we report observational evidence for this pinching on the \HI vertical structures in the Galactic region ($20^{\circ}< l<40^{\circ}$), also called W41--W44 region. The \HI vertical distribution is modelled by a double Gaussian profile that physically represents a narrow dense gas distribution confined to the mid-plane embedded in a wider diffuse \HI. We find that the estimate of the \HI scale height distribution of wider components, shows corrugated structures at the locations of molecular complexes, as theoretically predicted in literature. While the narrow component is less affected by the pinching, we found a hint of the disc being disrupted by the active dynamics in the local environment of the complex, e.g., supernova explosions. Molecular complexes of mass of several $10^6$ solar mass, associated with the mini-starburst region W43 and the supernova remnant W41 show the strongest evidence for the pinching; here a broad trough, with an average width of $\sim400$ pc and height $\sim300$ pc, in the disc thickness of the wider component is prominently visible. Searching for similar effect on the stars as well as in the location of other complexes in the Milky Way and other galaxies will be useful to establish this phenomenon more firmly.

astro-ph.GA↗

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↗

arXiv2024

Shock-driven synchrotron radio emission from the 2021 outburst of RS Ophiuchi

We present low-frequency radio observations of the Galactic symbiotic recurrent nova RS Ophiuchi during its 2021 outburst. The observations were carried out with the upgraded Giant Metrewave Radio Telescope (uGMRT) spanning a frequency range of 0.15$-$1.4 GHz during 23$-$287 days post the outburst. The average value of the optically thin spectral index is $α\sim$ $-$0.4 ($F_ν \propto ν^α$), indicating a non-thermal origin of the radio emission at the observed frequencies. The radio light curves are best represented by shock-driven synchrotron emission, initially absorbed by a clumpy ionized circumbinary medium. We estimate the mass-loss rate of the red giant companion star to be $\dot{M} \sim$ 7.5 $\times$ 10$^{-8}$ $M_{\odot}$ yr$^{-1}$ for an assumed stellar wind velocity of 20 km/s. The 0.15--1.4 GHz radio light curves of the 2021 outburst are systematically brighter than those of the 2006 outburst. Considering similar shock properties between the two outbursts, this is indicative of a relatively higher particle number density in the synchrotron emitting plasma in the current outburst.

astro-ph.SR↗

arXiv2023

Multiphase Neutral Interstellar Medium: Analyzing Simulation with H I 21cm Observational Data Analysis Techniques

Several different methods are regularly used to infer the properties of the neutral interstellar medium (ISM) using atomic hydrogen (H I) 21cm absorption and emission spectra. In this work, we study various techniques used for inferring ISM gas phase properties, namely the correlation between brightness temperature and optical depth $(T_B(v)$, $τ(v))$ at each channel velocity ($v$), and decomposition into Gaussian components, by creating mock spectra from a 3D magnetohydrodynamic simulation of a two-phase, turbulent ISM. We propose a physically motivated model to explain the $T_B(v)-τ(v)$ distribution and relate the model parameters to properties like warm gas spin temperature and cold cloud length scales. Two methods based on Gaussian decomposition -- using only absorption spectra and both absorption and emission spectra -- are used to infer the column density distribution as a function of temperature. In observations, such analysis reveals the puzzle of large amounts (significantly higher than in simulations) of gas with temperature in the thermally unstable range of $\sim$200 K to $\sim$2000 K and a lack of the expected bimodal (two-phase) temperature distribution. We show that, in simulation, both methods are able to recover the actual gas distribution in the simulation till temperatures $\lesssim2500$~K (and the two-phase distribution in general) reasonably well. We find our results to be robust to a range of effects such as noise, varying emission beam size, and simulation resolution. This shows that the observational inferences are unlikely to be artifacts, thus highlighting a tension between observations and simulations. We discuss possible reasons for this tension and ways to resolve it.

astro-ph.GA↗

arXiv2023

Towards $21$-cm intensity mapping at $z=2.28$ with uGMRT using the tapered gridded estimator III: Foreground removal

Neutral hydrogen (HI) $21$-cm intensity mapping (IM) is a promising probe of the large-scale structures in the Universe. However, a few orders of magnitude brighter foregrounds obscure the IM signal. Here we use the Tapered Gridded Estimator (TGE) to estimate the multi-frequency angular power spectrum (MAPS) $C_{\ell}(Δν)$ from a $24.4\,\rm{MHz}$ bandwidth uGMRT Band $3$ data at $432.8\,\rm{MHz}$. In $C_{\ell}(Δν)$ foregrounds remain correlated across the entire $Δν$ range, whereas the $21$-cm signal is localized within $Δν\le[Δν]$ (typically $0.5-1\,\rm{MHz}$). Assuming the range $Δν>[Δν]$ to have minimal $21$-cm signal, we use $C_{\ell}(Δν)$ in this range to model the foregrounds. This foreground model is extrapolated to $Δν\leq[Δν]$, and subtracted from the measured $C_{\ell}(Δν)$. The residual $[C_{\ell}(Δν)]_{\rm res}$ in the range $Δν\le[Δν]$ is used to constrain the $21$-cm signal, compensating for the signal loss from foreground subtraction. $[C_{\ell}(Δν)]_{\rm{res}}$ is found to be noise-dominated without any trace of foregrounds. Using $[C_{\ell}(Δν)]_{\rm res}$ we constrain the $21$-cm brightness temperature fluctuations $Δ^2(k)$, and obtain the $2σ$ upper limit $Δ_{\rm UL}^2(k)\leq(18.07)^2\,\rm{mK^2}$ at $k=0.247\,\rm{Mpc}^{-1}$. We further obtain the $2σ$ upper limit $ [Ω_{\rm HI}b_{\rm HI}]_{\rm UL}\leq0.022$ where $Ω_{\rm HI}$ and $b_{\rm HI}$ are the comoving HI density and bias parameters respectively. Although the upper limit is nearly $10$ times larger than the expected $21$-cm signal, it is $3$ times tighter over previous works using foreground avoidance on the same data.

astro-ph.CO↗

arXiv2023

Fast Localization and Tracking in City-Scale UWB Networks

Localization of networked nodes is an essential problem in emerging applications, including first-responder navigation, automated manufacturing lines, vehicular and drone navigation, asset navigation and tracking, Internet of Things and 5G communication networks. In this paper, we present Locate3D, a novel system for peer-to-peer node localization and orientation estimation in large networks. Unlike traditional range-only methods, Locate3D introduces angle-of-arrival (AoA) data as an added network topology constraint. The system solves three key challenges: it uses angles to reduce the number of measurements required by 4x and jointly use range and angle data for location estimation. We develop a spanning-tree approach for fast location updates, and to ensure the output graphs are rigid and uniquely realizable, even in occluded or weakly connected areas. Locate3D cuts down latency by up to 75% without compromising accuracy, surpassing standard range-only solutions. It has a 10.2 meters median localization error for large-scale networks (30,000 nodes, 15 anchors spread across 14km square) and 0.5 meters for small-scale networks (10 nodes).

cs.NI↗

arXiv2023

Simultaneously Learning Speaker's Direction and Head Orientation from Binaural Recordings

Estimation of a speaker's direction and head orientation with binaural recordings can be a critical piece of information in many real-world applications with emerging `earable' devices, including smart headphones and AR/VR headsets. However, it requires predicting the mutual head orientations of both the speaker and the listener, which is challenging in practice. This paper presents a system for jointly predicting speaker-listener head orientations by leveraging inherent human voice directivity and listener's head-related transfer function (HRTF) as perceived by the ear-mounted microphones on the listener. We propose a convolution neural network model that, given binaural speech recording, can predict the orientation of both speaker and listener with respect to the line joining the two. The system builds on the core observation that the recordings from the left and right ears are differentially affected by the voice directivity as well as the HRTF. We also incorporate the fact that voice is more directional at higher frequencies compared to lower frequencies.

eess.AS↗

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↗

arXiv2023

The GMRT archive atomic gas survey -- II. Mass modelling and dark matter halo properties across late-type spirals

Studying the kinematics and mass modelling of galaxies from HI 21 cm data provides valuable insights into the properties of both the baryonic components and the dark matter halo in nearby galaxies. Despite many observational studies, mass modelling of galaxies remains challenging due to different limitations. For example, most of the previous studies involving mass modelling are based on rotation curves derived from two-dimensional velocity fields from HI or H$α$ spectroscopic observation which are often affected by beam smearing and projection effect. However, kinematic modelling done by fitting the "Tilted ring model" to three-dimensional data cube is not affected by these issues. In this study, we present and compare 3D kinematic modelling of a pilot sample of eleven galaxies from the GMRT archive atomic gas survey (GARCIA) using two different publicly available pipelines. We model the observed HI rotation curve using 3.6 $μ$m infrared data and SDSS r-band data for stellar contribution, HI surface density profile for gas, and Navarro-Frenk-White (NFW) profile for dark matter halo; and employ the Markov Chain Monte Carlo (MCMC) optimization method for parameter estimation. Further, to validate our analysis, we revisit important scaling relations, e.g., the M$_{gas}$-M$_{star}$ relation, M$_{star}$-M$_{halo}$ relation, M$_{gas}$-M$_{halo}$ relation and Baryonic Tully-Fisher relation (BTFR). The scaling relations from our analysis are broadly consistent with that reported in the literature. A larger sample of galaxies from GARCIA in the near future will allow studying these scaling relations in greater details.

astro-ph.GA↗

arXiv2023

The magnetic fields around the cometary globules, L328, L323 and L331

This work presents the magnetic field geometry in a complex of three cometary (with head-tail morphology) globules, namely LDN 323, LDN 328, and LDN 331, using R-band polarization measurements of background stars. These observations were combined with a Planck sky survey to study the large-scale morphology of the magnetic fields in the region. The distances of the target stars were adopted from the Gaia catalog. The variation of degree of polarization and polarization position angle with distances of stars is analyzed. The field geometry is mostly found to follow the cometary shape of the cloud, with some randomness at certain locations. For studying the correlation between cloud morphology and magnetic field orientations, a modified version of the Histogram of Relative Orientation analysis was employed.

astro-ph.GA↗

arXiv2023

Lepto-hadronic interpretation of 2021 RS Ophiuchi nova outburst

Very high energy (VHE; 100 GeV $<$ E $\leq$ 100 TeV) and high energy (HE; 100 MeV $<$ E $\leq$ 100 GeV) gamma-rays were observed from the symbiotic recurrent nova RS Ophiuchi (RS Oph) during its outburst in August 2021, by various observatories such as High Energy Stereoscopic System (H.E.S.S.), Major Atmospheric Gamma Imaging Cherenkov (MAGIC), and {\it Fermi}-Large Area Telescope (LAT). The models explored so far tend to favor a hadronic scenario of particle acceleration over an alternative leptonic scenario. This paper explores a time-dependent lepto-hadronic scenario to explain the emission from the RS Oph source region. We have used simultaneous low frequency radio data observed by various observatories, along with the data provided by H.E.S.S., MAGIC, and \textit{Fermi}-LAT, to explain the multi-wavelength (MWL) spectral energy distributions (SEDs) corresponding to 4 days after the outburst. Our results show that a lepto-hadronic interpretation of the source not only explains the observed HE-VHE gamma-ray data but the corresponding model synchrotron component is also consistent with the first 4 days of low radio frequency data, indicating the presence of non-thermal radio emission at the initial stage of nova outburst. We have also calculated the expected neutrino flux from the source region and discussed the possibility of detecting neutrinos.

astro-ph.HE↗

arXiv2023

WhisperWand: Simultaneous Voice and Gesture Tracking Interface

This paper presents the design and implementation of WhisperWand, a comprehensive voice and motion tracking interface for voice assistants. Distinct from prior works, WhisperWand is a precise tracking interface that can co-exist with the voice interface on low sampling rate voice assistants. Taking handwriting as a specific application, it can also capture natural strokes and the individualized style of writing while occupying only a single frequency. The core technique includes an accurate acoustic ranging method called Cross Frequency Continuous Wave (CFCW) sonar, enabling voice assistants to use ultrasound as a ranging signal while using the regular microphone system of voice assistants as a receiver. We also design a new optimization algorithm that only requires a single frequency for time difference of arrival. WhisperWand prototype achieves 73 um of median error for 1D ranging and 1.4 mm of median error in 3D tracking of an acoustic beacon using the microphone array used in voice assistants. Our implementation of an in-air handwriting interface achieves 94.1% accuracy with automatic handwriting-to-text software, similar to writing on paper (96.6%). At the same time, the error rate of voice-based user authentication only increases from 6.26% to 8.28%.

cs.HC↗

arXiv2023

Towards $21$-cm intensity mapping at $z=2.28$ with uGMRT using the tapered gridded estimator II: Cross-polarization power spectrum

Neutral hydrogen ($\rm{HI}$) $21$-cm intensity mapping (IM) offers an efficient technique for mapping the large-scale structures in the universe. We introduce the 'Cross' Tapered Gridded Estimator (Cross TGE), which cross-correlates two cross-polarizations (RR and LL) to estimate the multi-frequency angular power spectrum (MAPS) $C_{\ell}(Δν)$. We expect this to mitigate several effects like noise bias, calibration errors etc., which affect the 'Total' TGE which combines the two polarizations. Here we apply the Cross TGE on a $24.4 \,\rm{MHz}$ bandwidth uGMRT Band $3$ data centred at $432.8 \,\rm{MHz}$ aiming $\rm{HI}$ IM at $z=2.28$. The measured $C_{\ell}(Δν)$ is modelled to yield maximum likelihood estimates of the foregrounds and the spherical power spectrum $P(k)$ in several $k$ bins. Considering the mean squared brightness temperature fluctuations, we report a $2σ$ upper limit $Δ_{UL}^{2}(k) \le (58.67)^{2} \, {\rm mK}^{2}$ at $k=0.804 \, {\rm Mpc}^{-1}$ which is a factor of $5.2$ improvement on our previous estimate based on the Total TGE. Assuming that the $\rm{HI}$ traces the underlying matter distribution, we have modelled $C_{\ell}(Δν)$ to simultaneously estimate the foregrounds and $[Ω_{\rm{HI}} b_{\rm{HI}}] $ where $Ω_{\rm{HI}}$ and $b_{\rm{HI}}$ are the $\rm{HI}$ density and linear bias parameters respectively. We obtain a best fit value of $[Ω_{\rm{HI}}b_{\rm{HI}}]^2 = 7.51\times 10^{-4} \pm 1.47\times 10^{-3}$ which is consistent with noise. Although the $2σ$ upper limit $[Ω_{\rm{HI}}b_{\rm{HI}}]_{UL} \leq 0.061$ is $\sim 50$ times larger than the expected value, this is a considerable improvement over earlier works at this redshift.

astro-ph.CO↗

arXiv2023

Detection of HI 21 cm emission from a strongly lensed galaxy at z~1.3

We report the first $5σ$ detection of HI 21 cm emission from a star-forming galaxy at redshift $z\sim1.3$ (nearly 9 billion years ago) using upgraded Giant Metrewave Radio Telescope (uGMRT). This is the highest redshift HI detection in emission from an individual galaxy to date. The emission is strongly boosted by the gravitational lens, an early type elliptical galaxy, at redshift $z \sim 0.13$. The measured HI mass of the galaxy is $\rm M_{HI} = (0.90 \pm 0.14 \pm 0.05) \times 10^{10}M_{\odot}$, which is almost twice the inferred stellar mass of the galaxy, indicating an extended structure of the HI gas inside the galaxy. By fitting two-dimensional Gaussian to the HI signal at the peak of the spectral line, we find the source to be marginally resolved with the position angle consistent with the emission being tangential to the critical curve of the lens mass distribution. This indicates that the solid angle of the approaching HI line flux comes very close to the inner lens caustic and results in very high magnification. These results, for the first time, demonstrate the feasibility of observing high redshift HI in a lensed system with a modest amount of telescope time and open up exciting new possibilities for probing the cosmic evolution of neutral gas with existing and upcoming low-frequency radio telescopes in the near future.

astro-ph.GA↗

arXiv2022

Properties of atomic hydrogen gas in the Galactic plane from THOR 21-cm absorption spectra: a comparison with the high latitude gas

The neutral hydrogen 21 cm line is an excellent tracer of the atomic interstellar medium in the cold and the warm phases. Combined 21 cm emission and absorption observations are very useful to study the properties of the gas over a wide range of density and temperature. In this work, we have used 21 cm absorption spectra from recent interferometric surveys, along with the corresponding emission spectra from earlier single dish surveys to study the properties of the atomic gas in the Milky Way. In particular, we focus on a comparison of properties between lines of sight through the gas disk in the Galactic plane and high Galactic latitude lines of sight through more diffuse gas. As expected, the analysis shows a lower average temperature for the gas in the Galactic plane compared to that along the high latitude lines of sight. The gas in the plane also has a higher molecular fraction, showing a sharp transition and flattening in the dust - gas correlation. On the other hand, the observed correlation between 21 cm brightness temperature and optical depth indicates some intrinsic difference in spin temperature distribution and a fraction of gas in the Galactic plane having intermediate optical depth (for 0.02 < $τ$ < 0.2) but higher spin temperature, compared to that of the diffuse gas at high latitude with the same optical depth. This may be due to a small fraction of cold gas with slightly higher temperature and lower density present on the Galactic plane.

astro-ph.GA↗