arXiv subjects

Nirupam Roy

Publications and source records attributed to Nirupam Roy.

123 records · Page 7Linked to original sources

arXiv2011

An HI column density threshold for cold gas formation in the Galaxy

We report the discovery of a threshold in the HI column density of Galactic gas clouds below which the formation of the cold phase of HI is inhibited. This threshold is at $N_{HI} = 2 \times 10^{20}$ per cm$^{2}$; sightlines with lower HI column densities have high spin temperatures (median $T_s \sim 1800$ K), indicating low fractions of the cold neutral medium (CNM), while sightlines with $N_{HI} \ge 2 \times 10^{20}$ per cm$^{2}$ have low spin temperatures (median $T_s \sim 240$ K), implying high CNM fractions. The threshold for CNM formation is likely to arise due to inefficient self-shielding against ultraviolet photons at lower HI column densities. The threshold is similar to the defining column density of a damped Lyman-$α$ absorber; this indicates a physical difference between damped and sub-damped Lyman-$α$ systems, with the latter class of absorbers containing predominantly warm gas.

astro-ph.GA↗

arXiv2011

EVLA Nova Project Observations of the Classical Nova V1723 Aquilae

We present radio light curves and spectra of the classical nova V1723 Aql obtained with the Expanded Very Large Array (EVLA). This is the first paper to showcase results from the EVLA Nova Project, which comprises a team of observers and theorists utilizing the greatly enhanced sensitivity and frequency coverage of EVLA radio observations, along with observations at other wavelengths, to reach a deeper understanding of the energetics, morphology, and temporal characteristics of nova explosions. Our observations of V1723 Aql span 1-37 GHz in frequency, and we report on data from 14-175 days following the time of the nova explosion. The broad frequency coverage and frequent monitoring show that the radio behavior of V1723 Aql does not follow the classic Hubble-flow model of homologous spherically expanding thermal ejecta. The spectra are always at least partially optically thin, and the flux rises on faster timescales than can be reproduced with linear expansion. Therefore, any description of the underlying physical processes must go beyond this simple picture. The unusual spectral properties and light curve evolution might be explained by multiple emitting regions or shocked material. Indeed, X-ray observations from Swift reveal that shocks are likely present.

astro-ph.HE↗

arXiv2011

Imaging of the CCS 22.3 GHz emission in the Taurus Molecular Cloud complex

Thioxoethenylidene (CCS) is an abundant interstellar molecule, and a good tracer of high density and evolutionary stage of dense molecular clouds. It is also a suitable candidate for Zeeman splitting observations for its high splitting factor and narrow thermal linewidths. We report here EVLA 22.3 GHz observations of three dense molecular cores TMC-1, TMC-1C and L1521B in the Taurus Molecular Cloud complex to image the CCS 2_1-1_0 transition. For all three sources, the clumpy CCS emission is most likely tracing the starless cores. However, these compact structures account for only ~ 1-13% of the integrated emission detected in single-dish observations, indicating the presence of significant large scale diffuse emission in favorable conditions for producing CCS.

astro-ph.GA↗

arXiv2011

Constraining the MOdified Newtonian Dynamics from spherically symmetrical hydrodynamic accretion

The MOdified Newtonian Dynamics (MOND) is an alternative to the dark matter assumption that can explain the observed flat rotation curve of galaxies. Here hydrodynamic accretion is considered to critically check the consistency and to constrain the physical interpretation of this theory. It is found that, in case of spherically symmetrical hydrodynamic accretion, the modified Euler's equation has real solution if the interpretation is assumed to be a modification of the law of dynamics. There is no real solution if it is assumed to be an acceleration scale dependent modification of the gravitational law. With the modified Euler's equation, the steady state mass accretion rate is found to change up to ~ 15%. The astrophysical and cosmological implications of these results are also discussed.

astro-ph.GA↗

arXiv2010

Effect of dark matter halo substructures on galaxy rotation curves

The effect of halo substructures on galaxy rotation curves is investigated in this paper using a simple model of dark matter clustering. A dark matter halo density profile is developed based only on the scale free nature of clustering that leads to a statistically self-similar distribution of the substructures at galactic scale. Semi-analytical method is used to derive rotation curves for such a clumpy dark matter density profile. It is found that the halo substructures significantly affect the galaxy velocity field. Based on the fractal geometry of the halo, this self-consistent model predicts an NFW-like rotation curve and a scale free power spectrum of the rotation velocity fluctuations.

astro-ph.GA↗

arXiv2010

HI 21 cm opacity fluctuations power spectra towards Cassiopeia A

The angular power spectrum of HI 21 cm opacity fluctuations is a useful statistic for quantifying the observed opacity fluctuations as well as for comparing these with theoretical models. We present here the HI 21 cm opacity fluctuation power spectrum towards the supernova remnant Cas A from interferometric data with spacial resolution of 5" and spectral resolution of 0.4 km/s. The power spectrum has been estimated using a simple but robust visibility based technique. We find that the power spectrum is well fit by a power law P_tau(U) = U^{alpha} with a power law index of alpha ~ -2.86 +/- 0.10 (3 sigma error) over the scales of 0.07 - 2.3 pc for the gas in the Perseus spiral arm and 0.002 - 0.07 pc (480 - 15730 au) for that in the Local arm. This estimated power law index is consistent with earlier observational results based on both HI emission over larger scales and absorption studies over a similar range of scales. We do not detect any statistically significant change in the power law index with the velocity width of the frequency channels. This constrains the power law index of the velocity structure function to be beta = 0.2 +/- 0.6 (3 sigma error).

astro-ph.GA↗

arXiv2010

The curious case of J113924.74+164144.0: a possible new group of galaxies at z = 0.0693

J113924.74+164144.0 is an interesting galaxy at z = 0.0693, i.e. D_L ~ 305 Mpc, with tidal-tail-like extended optical features on both sides. There are two neighbouring galaxies, a spiral galaxy J113922.85+164136.3 which has a strikingly similar 'tidal' morphology, and a faint galaxy J113923.58+164129.9. We report HI 21 cm observations of this field to search for signatures of possible interaction. Narrow HI emission is detected from J113924.74+164144.0, but J113922.85+164136.3 shows no detectable emission. The total HI mass detected in J113924.74+164144.0 is 7.7 x 10^9 M_solar. The HI emission from the galaxy is found to be extended and significantly offset from the optical position of the galaxy. We interpret this as signature of possible interaction with the neighbouring spiral galaxy. There is also a possible detection of HI emission from another nearby galaxy J113952.31+164531.8 at z = 0.0680 at a projected distance of 600 kpc, and with a total HI mass of 5.3 x 10^9 M_solar, suggesting that all these galaxies form a loose group at z ~ 0.069.

astro-ph.CO↗

arXiv2009

Fractal features in accretion discs

Fractal concepts have been introduced in the accretion disc as a new feature. Due to the fractal nature of the flow, its continuity condition undergoes modifications. The conserved stationary fractal flow admits only saddle points and centre-type points in its phase portrait. Completely analytical solutions of the equilibrium point conditions indicate that the fractal properties enable the flow to behave like an effective continuum of lesser density, and facilitates the generation of transonicity. However, strongly fractal flows inhibit multitransonicity from developing. The mass accretion rate exhibits a fractal scaling behaviour, and the entire fractal accretion disc is stable under linearised dynamic perturbations.

astro-ph.HE↗

arXiv2009

Magnetohydrodynamic turbulence in supernova remnants

We present estimates of the angular power spectra of the synchrotron radiation intensity fluctuations at 6 and 20 cm for the shell type supernova remnant Cas A and the filled-centre Crab supernova remnant. We find that the intensity fluctuations of both sources have a power law power spectrum with index -3.24 +/- 0.03. This power law power spectrum is consistent with the magnetohydrodynamic turbulence in the synchrotron emitting plasma. For Cas A, there is a break in the power spectrum and the power law index changes from -3.2 to -2.2 at large angular scale. This transition occurs at an angular scale that corresponds to the shell thickness of Cas A. We interpret this as a transition from three dimensional turbulence to two dimensional turbulence on scales that are respectively smaller and larger than the shell thickness.

astro-ph↗

arXiv2008

HI aperture synthesis and optical observations of the pair of galaxies NGC 6907 and 6908

NGC 6908, a S0 galaxy situated in direction of NGC 6907, was only recently recognized as a distinct galaxy, instead of only a part of NGC 6907. We present 21 cm radio synthesis observations obtained with the GMRT and optical images and spectroscopy obtained with the Gemini North telescope of this pair of interacting galaxies. From the radio observations we obtained the velocity field and the HI column density map of the whole region containing the NGC 6907/8 pair, and by means of the Gemini multi-object spectroscopy we obtained high quality photometric images and $5 Å$ resolution spectra sampling the two galaxies. By comparing the rotation curve of NGC 6907 obtained from the two opposite sides around the main kinematic axis, we were able to distinguish the normal rotational velocity field from the velocity components produced by the interaction between the two galaxies. Taking into account the rotational velocity of NGC 6907 and the velocity derived from the absorption lines for NGC 6908, we verified that the relative velocity between these systems is lower than 60 km s$^{-1}$. The emission lines observed in the direction of NGC 6908, not typical of S0 galaxies, have the same velocity expected for the NGC 6907 rotation curve. Some of them, superimposed on the absorption profiles, which reinforces the idea that they were not formed in NGC 6908. Finally, the HI profile exhibits details of the interaction, showing three components: one for NGC 6908, another for the excited gas in the NGC 6907 disk and a last one for the gas with higher relative velocities left behind NGC 6908 by dynamical friction, used to estimate the time when the interaction started in $(3.4 \pm 0.6)\times10^7$ years ago.

astro-ph↗

arXiv2008

Turbulence measurements from HI absorption spectra

We use the millennium Arecibo 21 cm absorption-line survey measurements to examine the issue of the non-thermal contribution to the observed Galactic HI line widths. If we assume a simple, constant pressure model for the HI in the Galaxy, we find that the non-thermal contribution to the line width, v_{nt} scales as v^2_{nt} \propto l^α, for v_{nt} larger than \sim 0.7 km s^{-1}. Here l is a derived length scale and α\sim 0.7 \pm 0.1. This is consistent with what one would expect from a turbulent medium with a Kolmogorov scaling. Such a scaling is also predicted by theoretical models and numerical simulations of turbulence in a magnetized medium. For non-thermal line widths narrower than \sim 0.7 km s^{-1}, this scaling breaks down, and we find that the likely reason is ambiguities arising from Gaussian decomposition of intrinsically narrow, blended lines. We use the above estimate of the non-thermal contribution to the line width to determine corrected HI kinetic temperature. The new limits that we obtain imply that a significantly smaller (\sim 40% as opposed to 60%) fraction of the atomic interstellar medium in our Galaxy is in the warm neutral medium phase.

astro-ph↗

arXiv2007

Occultation observation to probe the turbulence scale size in the plasma tail of comet Schwassmann-Wachmann 3-B

We present the occultation observation of compact radio source B0019-000 through the plasma tail of comet Schwassmann-Wachmann 3-B. The observation was made with the Ooty Radio Telescope at 326.5 MHz on 2006 May 26, when the plasma tail of the comet was in front of this source. The scintillation was found to be increased significantly for the target source compared to that of a control source. The intensity fluctuation power spectra show both steepening at high spatial scales and excess power at low spatial scales. This observation can be attributed to the turbulence in the comet plasma tail. A two-regime plasma turbulence can explain the time-evolution of the power spectrum during the occultation observation.

astro-ph↗

arXiv2007

Critical properties of spherically symmetric accretion in a fractal medium

Spherically symmetric transonic accretion of a fractal medium has been studied in both the stationary and the dynamic regimes. The stationary transonic solution is greatly sensitive to infinitesimal deviations in the outer boundary condition, but the flow becomes transonic and stable, when its evolution is followed through time. The evolution towards transonicity is more pronounced for a fractal medium than what is it for a continuum. The dynamic approach also shows that there is a remarkable closeness between an equation of motion for a perturbation in the flow, and the metric of an analogue acoustic black hole. The stationary inflow solutions of a fractal medium are as much stable under the influence of linearised perturbations, as they are for the fluid continuum.

astro-ph↗

arXiv2007

On spherically symmetrical accretion in fractal media

We use fractional integrals to generalize the description of hydrodynamic accretion in fractal media. The fractional continuous medium model allows the generalization of the equations of balance of mass density and momentum density. These make it possible to consider the general case of spherical hydrodynamic accretion onto a gravitating mass embedded in a fractal medium. The general nature of the solution is similar to the "Bondi solution", but the accretion rate may vary substantially and the dependence on central mass may change significantly depending on dimensionality of the fractal medium. The theory shows consistency with the observational data and numerical simulation results for the particular case of accretion onto pre-main-sequence stars.

astro-ph↗

arXiv2005

A multiwavelength investigation of the temperature of the cold neutral medium

We present measurements of the H I spin temperatures (T_s) of the Cold Neutral Medium (CNM) towards radio sources that are closely aligned with stars for which published H_2 ortho-para temperatures (T_{01}) are available from UV observations. Our sample consists of 18 radio sources close to 16 nearby stars. The transverse separation of the lines of sight of corresponding the UV and radio observations varies from 0.1 to 12.0 pc at the distance of the star. The ultraviolet (UV) measurements do not have velocity information, so we use the velocities of low ionization species (e.g Na I, K I, C I) observed towards these same stars to make a plausible identification of the CNM corresponding to the H_2 absorption. We then find that T_{01} and T_s match within observational uncertainties for lines-of-sight with H_2 column density above 10^{15.8} cm^{-2}, but deviate from each other below this threshold. This is consistent with the expectation that in the CNM T_s tracks the kinetic temperature due to collisions and that T_{01} is driven towards the kinetic temperature by proton exchange reactions.

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