Line Flux Ratios Research Articles

The diffuse ionized gas (DIG) in star-forming galaxies: the influence of aperture effects on local H ii regions

ABSTRACT The diffuse ionized gas (DIG) contributes to the nebular emission of galaxies, resulting in emission line flux ratios that can be significantly different from those produced by H ii regions. Comparing the emission of [SII]λ6717,31 between pointed observations of H ii regions in nearby galaxies and integrated spectra of more distant galaxies, it has been recently claimed that the DIG can also deeply affect the emission of bright, star-forming galaxies, and that a large correction must be applied to observed line ratios to recover the genuine contribution from H ii regions. Here, we show instead that the e?ect of DIG on the integrated spectra of star-forming galaxies is lower than assumed in previous work. Here we show that, in contrast, aperture effects on the spectroscopy of nearby H ii regions are largely responsible for the observed difference: When spectra of local H ii regions are extracted using large enough apertures while still avoiding the DIG, the observed line ratios are the same as in more distant galaxies. This result is highly relevant for the use of strong-line methods to measure metallicity.

Strong Correlation between Fe ii/Mg ii Ratio and Eddington Ratio of Type 1 Active Galactic Nuclei

Abstract The Fe ii/Mg ii line flux ratio has been used as an indicator of the Fe/Mg abundance ratio in the broad-line region (BLR) of active galactic nuclei (AGNs). On the basis of archival rest-frame UV spectra obtained via the Hubble Space Telescope and the Sloan Digital Sky Survey, we investigate the Fe ii/Mg ii ratios of type 1 AGNs at z < 2. Over wide dynamic ranges of AGN properties (i.e., black hole mass, AGN luminosity, and Eddington ratio), we confirm that the Fe ii/Mg ii ratio strongly correlates with Eddington ratio but not with black hole mass, AGN luminosity, or redshift. Our results suggest that the metallicity in the BLR are physically related to the accretion activity of AGNs, but not to the global properties of galaxies (i.e., galaxy mass and luminosity). With regard to the relation between the BLR metallicity and the accretion rate of AGNs, we discuss that metal cooling may play an important role in enhancing the gas inflow into the central region of host galaxies, resulting in the high accretion rate of AGNs.

An empirical calibration of the helium abundance in H ii regions based in literature and CALIFA survey data

ABSTRACT Helium is the second most common chemical species in the Universe. The study of helium abundance has the potential to unravel the chemical evolution of and within galaxies. In this study, we provide an empirical calibration for the singly ionized helium abundance: 12 + log10(He+/H+), based on the emission line flux ratio Heλ5876/H α from Galactic and extragalactic H ii regions compiled from the literature. Based on this calibrator, we explore for the first time the helium abundance in a large sample of H ii regions located in galaxies representative of the nearby Universe from the CALIFA survey. Furthermore, this calibrator allows us to explore the variations of the helium abundance with respect to the oxygen abundance. The observed trends are in agreement with a change in the chemical enrichment with mass/oxygen abundance similar to the one observed due to the inside-out model in a MW-galaxy (highlighting the connection between resolved and global trends in galaxies). Our calibrator provides an empirical proxy to estimate the helium abundance at kpc scales as well as to constrain chemical evolutionary models.

High-ionization emission-line ratios from quasar broad-line regions: metallicity or density?

ABSTRACT The flux ratios of high-ionization lines are commonly assumed to indicate the metallicity of the broad emission-line region in luminous quasars. When accounting for the variation in their kinematic profiles, we show that the N v/C iv, (Si iv + O iv])/C iv, and N v/Ly α line ratios do not vary as a function of the quasar continuum luminosity, black hole mass, or accretion rate. Using photoionization models from cloudy, we further show that the observed changes in these line ratios can be explained by emission from gas with solar abundances, if the physical conditions of the emitting gas are allowed to vary over a broad range of densities and ionizing fluxes. The diversity of broad-line emission in quasar spectra can be explained by a model with emission from two kinematically distinct regions, where the line ratios suggest that these regions have either very different metallicity or density. Both simplicity and current galaxy evolution models suggest that near-solar abundances, with parts of the spectrum forming in high-density clouds, are more likely. Within this paradigm, objects with stronger outflow signatures show stronger emission from gas that is denser and located closer to the ionizing source, at radii consistent with simulations of line-driven disc-winds. Studies using broad-line ratios to infer chemical enrichment histories should consider changes in density and ionizing flux before estimating metallicities.

Hints for Icy Pebble Migration Feeding an Oxygen-rich Chemistry in the Inner Planet-forming Region of Disks

Abstract We present a synergic study of protoplanetary disks to investigate links between inner-disk gas molecules and the large-scale migration of solid pebbles. The sample includes 63 disks where two types of measurements are available: (1) spatially resolved disk images revealing the radial distribution of disk pebbles (millimeter to centimeter dust grains), from millimeter observations with the Atacama Large Millimeter/Submillimeter Array or the Submillimeter Array, and (2) infrared molecular emission spectra as observed with Spitzer. The line flux ratios of H2O with HCN, C2H2, and CO2 all anticorrelate with the dust disk radius R dust, expanding previous results found by Najita et al. for HCN/H2O and the dust disk mass. By normalization with the dependence on accretion luminosity common to all molecules, only the H2O luminosity maintains a detectable anticorrelation with disk radius, suggesting that the strongest underlying relation is between H2O and R dust. If R dust is set by large-scale pebble drift, and if molecular luminosities trace the elemental budgets of inner-disk warm gas, these results can be naturally explained with scenarios where the inner disk chemistry is fed by sublimation of oxygen-rich icy pebbles migrating inward from the outer disk. Anticorrelations are also detected between all molecular luminosities and the infrared index n 13–30, which is sensitive to the presence and size of an inner-disk dust cavity. Overall, these relations suggest a physical interconnection between dust and gas evolution, both locally and across disk scales. We discuss fundamental predictions to test this interpretation and study the interplay between pebble drift, inner disk depletion, and the chemistry of planet-forming material.

A multiwavelength study of the dual nuclei in Mrk 212

ABSTRACT We present radio observations of the galaxy merger remnant Mrk 212 with the Karl G. Jansky Very Large Array (VLA) and the upgraded Giant Meter Radio Telescope (uGMRT). Mrk 212 has two previously known radio sources associated with the two optical nuclei, S1 and S2, with a projected separation of ∼6 kpc, making it a dual active galactic nuclei (AGN) candidate. Our new 15-GHz VLA observations reveal that S1 is a double radio source centred around the optical nucleus; its total extent is ∼750 pc its average 1.4−8.5 GHz spectral index is −0.81 ± 0.06. S1 therefore resembles a compact symmetric object. The 15-GHz VLA image identifies the radio source at S2 to be a compact core. Our radio observations therefore strongly support the presence of a dual AGN in Mrk 212. The optical emission line flux ratios obtained from the Himalayan Chandra Telescope (HCT) observations however, show that S1 and S2 both fall in the AGN + star formation (SF) region of the Baldwin, Philips and Terlevich (BPT) diagram. Weak AGN lying in the SF or AGN + SF intermediate regions in the BPT diagram have indeed been reported in the literature; our sources clearly fall in the same category. We find an extended radio structure in our newly reduced 8.5-GHz-VLA data, that is offset by ∼1 arcsec from the optical nucleus S2. New deep FUV and NUV observations with the Ultraviolet Imaging Telescope aboard AstroSat reveal SF knots around S2 as well as kpc-scale tidal tails; the SF knots around S2 coincide with the extended radio structure detected at 8.5 GHz. The radio spectral indices are consistent with SF. Any possible association with the AGN in S2 is unclear at this stage.

A Catalog of 406 AGNs in MaNGA: A Connection between Radio-mode AGNs and Star Formation Quenching

Abstract Accurate active galactic nucleus (AGN) identifications and spatially resolved host galaxy properties are a powerful combination for studies of the role of AGNs and AGN feedback in the coevolution of galaxies and their central supermassive black holes. Here, we present robust identifications of 406 AGNs in the first 6261 galaxies observed by the integral field spectroscopy survey Mapping Nearby Galaxies at Apache Point Observatory. Instead of using optical line flux ratios, which can be difficult to interpret in light of the effects of shocks and metallicity, we identify the AGNs via mid-infrared Wide-field Infrared Survey Explorer colors, Swift/BAT ultrahard X-ray detections, NVSS and FIRST radio observations, and broad emission lines in SDSS spectra. We subdivide the AGNs into radio-quiet and radio-mode AGNs, and examine the correlations of the AGN classes with host galaxy star formation rates and stellar populations. When compared to the radio-quiet AGN host galaxies, we find that the radio-mode AGN host galaxies are preferentially elliptical, lie further beneath the star-forming main sequence (with lower star formation rates at fixed galaxy mass), have older stellar populations, and have more negative stellar age gradients with galactocentric distance (indicating inside-out quenching of star formation). These results establish a connection between radio-mode AGNs and the suppression of star formation.

The cooling-down central star of the planetary nebula SwSt 1: a late thermal pulse in a massive post-AGB star?

ABSTRACT SwSt 1 (PN G001.5-06.7) is a bright and compact planetary nebula containing a late [WC]-type central star. Previous studies suggested that the nebular and stellar lines are slowly changing with time. We studied new and archival optical and ultraviolet spectra of the object. The [O iii] 4959 and 5007 Å to H β line flux ratios decreased between about 1976 and 1997/2015. The stellar spectrum also shows changes between these epochs. We modelled the stellar and nebular spectra observed at different epochs. The analyses indicate a drop of the stellar temperature from about 42 kK to 40.5 kK between 1976 and 1993. We do not detect significant changes between 1993 and 2015. The observations show that the star performed a loop in the H–R diagram. This is possible when a shell source is activated during its post-AGB evolution. We infer that a late thermal pulse (LTP) experienced by a massive post-AGB star can explain the evolution of the central star. Such a star does not expand significantly as the result of the LTP and does not became a born-again red giant. However, the released energy can remove the tiny H envelope of the star.

Measuring elemental abundance ratios in protoplanetary disks at millimeter wavelengths

Over million years of evolution, gas dust and ice in protoplanetary disks can be chemically reprocessed. There is evidence that the gas-phase carbon and oxygen abundances are subsolar in disks belonging to nearby star forming regions. These findings have a major impact on the composition of the primary atmosphere of giant planets (but it may also be valid for super-Earths and sub-Neptunes) as they accrete their gaseous envelopes from the surrounding material in the disk. In this study, we performed a thermochemical modeling analysis with the aim of testing how reliable and robust are the estimates of elemental abundance ratios based on (sub)millimeter observations of molecular lines. We created a grid of disk models for the following different elemental abundance ratios: C/O, N/O, and S/O, and we computed the line flux of a set of carbon-nitrogen and sulphur-bearing species, namely CN, HCN, NO, C2H, c–C3H2, H2CO, HC3N, CH3CN, CS, SO, H2S, and H2CS, which have been detected with present (sub)millimeter facilities such as ALMA and NOEMA. We find that the line fluxes, once normalized to the flux of the 13CO J = 2−1 line, are sensitive to the elemental abundance ratios. On the other hand, the stellar and disk physical parameters have only a minor effect on the line flux ratios. Our results demonstrate that a simultaneous analysis of multiple molecular transitions is a valid approach to constrain the elemental abundance ratio in protoplanetary disks.

The Multi-object Spectroscopy Observation of Seven Interacting Galaxy Pairs: Metallicity Gradients and Star Formation Distributions

We use the Multi-Object Spectroscopy system of Xinglong 2.16 m telescope to study the gradients of gas-phase oxygen abundance and the distributions of star formation activities of seven nearby interacting galaxy pairs. On the basis of emission line luminosities and flux ratios, we estimate the gas-phase metallicities (log(O/H)), excitation mechanisms, and star formation rates (SFRs) of different regions in each galaxy. The average radial slope of log(O/H) from linear fitting of galaxy pairs in our sample is significantly flatter than that of isolated disk galaxies, also the central metallicity is much lower for interacting galaxies. This supports the predictions and results of previous theoretical numerical simulations and observational works which show the cold gas inflows induced by galaxy interactions can dilute the central metallicity. From the analysis of the radial distribution of SFR surface densities (ΣSFR), we find that, in our sample, galaxies in Spiral–Spiral (S+S) pairs have peak ΣSFR at the center region and lower SFRs at the outer parts than that at the center. On the other hand, for two spirals in Spiral-Elliptical (S+E) pairs (Arp142 and J0338+2120), they both have relatively higher (ΣSFR) in their off-nuclei regions. This may hint a dependence of star formation distributions for paired galaxies on their companion morphologies.

AGN-Driven Outflows in Dwarf Galaxies

Abstract We present spatially resolved kinematic measurements of active galactic nucleus (AGN)-driven outflows in dwarf galaxies in the stellar mass range ∼6 × 108–9 × 109 M ⊙, selected from SDSS DR7 and DR8 and followed-up with Keck/LRIS spectroscopy. We find spatially extended (∼1 half-light radius), high-velocity ionized gas outflows (W 80 up to ∼2000 km s−1) in 13/50 dwarf galaxies with and without an AGN. Outflow velocities in all 13 galaxies exceed the escape velocities of their halos. Nine of these 13 galaxies are classified as AGNs according to their narrow line flux ratios. Of these, six have outflow components with emission-line ratios consistent with AGN ionization. Although black holes (BHs) have been known to populate the centers of at least a few dwarf galaxies and indirect evidence of AGN quenching of star formation in dwarfs has begun to surface, our measurements constitute the first direct detection and measurement of AGN impact on the large-scale kinematics and gas content in dwarf galaxies. Furthermore, we find evidence suggestive of ongoing star formation suppression, possibly regulated by the AGN. Galaxy formation models must therefore be able to account not only for the formation and growth of BHs at the centers of dwarf galaxies, but should also be revised to include AGNs as important—and perhaps dominant—sources of feedback in low-mass galaxies.

The Quadratic Programming Method for Extracting Emission Line Maps from Line-blended Narrowband Images

Abstract We present a method to separate blended line emission from a set of narrowband (NB) images whose distinct throughput profiles overlap with each other. This method, based on the principle of quadratic programming (QP), treats line blending as a linearly constrained quadratic optimization problem and finds the solution by imposing a condition that the desired individual line intensities are always positive while allowing measurement uncertainties. We demonstrate the method for the quintessential example of line blending in the Hα complex by the Hα and [N ii] lines using the archived Hubble Space Telescope WFPC2 and WFC3 NB images of the planetary nebula NGC 6720, in comparison with its genuine line maps extracted from a spectral cube obtained by our own long-slit scan mapping observations with the Gemini Multi-Object Spectrograph at Gemini-North. Such line-blended NB images typically permit only qualitative line diagnostics, which can lead to a factor of a few difference in the line flux ratios. This novel QP method, however, can extract individual line emission maps as long as the number of line-blended but well-calibrated NB images of differing throughputs is equal to or grater than the number of emission line maps to be extracted. The proposed QP method provides a viable alternative to the existing line emission calibration methods: a simple but innovative way to enable quantitative fully 2D plasma diagnostics at the pixel scale of input NB images available from various instruments, telescopes, and archives.

Massive White Dwarfs in the Galactic Center: A Chandra X-Ray Spectroscopy of Cataclysmic Variables

Abstract Previous X-ray observations toward the nuclear star cluster (NSC) at the Galactic center have discovered thousands of point sources, most of which were believed to be cataclysmic variables (CVs), i.e., a white dwarf (WD) accreting from a low-mass companion. However, the population properties of these CVs remain unclear, which otherwise would provide important information about the evolutionary history of the NSC. In this work we utilize ultra-deep archival Chandra observations to study the spectral properties of the NSC CVs, in close comparison with those in the solar vicinity. We find that the NSC CVs have strong Fe xxv and Fe xxvi lines (both of which show equivalent widths ∼200–300 eV), indicating metal-rich companions. Moreover, their Fe xxvi to Fe xxv line flux ratio is used to diagnose the characteristic WD mass (M WD) of NSC CVs. The results show that the CVs with erg s−1 have a mean M WD of ∼0.6/1.0 M ⊙ if they are magnetic/nonmagnetic CVs; while those with between 1 and 6 × 1031 erg s−1 have a mean M WD of ∼0.8/1.2 M ⊙ if they are magnetic/nonmagnetic CVs. All these Chandra detected CVs collectively contribute ∼30%–50% of the unresolved 20–40 keV X-ray emission from the NSC. The CV population with massive (i.e., M WD ∼ 1.2 M ⊙) WDs have not been observed in the solar vicinity or the Galactic bulge, and they might have been formed via dynamical encounters in the NSC.

V1082-Sgr: A magnetic cataclysmic variable with a lobe-filling companion star

ABSTRACT V1082 Sgr is a cataclysmic variable with accretion luminosity above 1034 erg s−1, indicating a mass-transfer rate above $10^{-9}\, \mathrm{M}_{\odot }$ yr−1. However, its K-type companion was suggested to be underfilling its Roche lobe (RL), making the high mass-transfer rate a mystery. In this work we propose a possible model to explain this discrepancy. The system is proposed to be an intermediate polar, with its K-type companion filling its RL. The mass of the white dwarf star is evaluated to be $0.77\pm 0.11\, \mathrm{M}_{\odot }$ from both X-ray continuum fitting and Fe line flux ratio measurements. We make numerical simulations to search for the possible progenitors of the system. The results show that a binary with an initial 1.5–2.5$\, \mathrm{M}_{\odot }$ companion in a one to two day orbit (or an initial 1.0–1.4$\, \mathrm{M}_{\odot }$ companion in a 3.2–4.1 d orbit) may naturally evolve to a cataclysmic variable with a $\sim 0.55 \pm 0.11\, \mathrm{M}_{\odot }$, Roche-lobe-filling companion in a 0.86 d orbit. The effective temperature of the donor star, the mass-transfer rate, and the derived V-band magnitude are all consistent with previous observations.

The Nuclear Source of the Galactic Wind in NGC 253

Abstract We present Brγ emission line kinematics of the nuclear region of NGC 253, recently known to host a strong galactic wind that limits the global star formation of the galaxy. We obtained high-resolution long-slit spectroscopic data with PHOENIX at Gemini South, positioning the slit on the nucleus infrared core (IRC), close to the nuclear disk major axis. The spatial resolution was 0.″35 (∼6 pc) and the slit length was 14″ (∼240 pc). The spectral resolution was ∼74,000, unprecedentedly high for galactic nuclei observations at ∼2.1 μm. The line profiles appear highly complex, with blue asymmetry up to 3.″5 away of the IRC, and red asymmetries further away to northeast. Several Gaussian components are necessary to fit the profile, nevertheless a narrow and a wide one predominate. The IRC presents kinematic widths above 700 km s−1 (FWZI), and broad component FWHM ∼ 400 km s−1, the highest detected in a nearby galaxy. At the IRC, the blueshifted broad component displays a 90 km s−1 bump in radial velocity distribution, a feature we previously detected in molecular gas kinematics. The narrow component velocity dispersion (∼32 km s−1) is within the expected for normal galaxies and luminous infrared galaxies (LIRGs). Intermediate components (FWHM ∼ 150 km s−1, redshifted to the northeast, blueshifted to the southwest) appear at some positions, as well as weaker blue (−215 km s−1) and red line wings (+300 km s−1). The IRC depicts a large broad-versus-narrow line flux ratio (F(B)/F(N) ∼ 1.35), and the broad component seems only comparable with those observed at very high star-forming rate galaxies. The results indicate that the IRC would be the main source of the galactic winds originated in the central region of NGC 253.

The Fe Line Flux Ratio as a Diagnostic of the Maximum Temperature and the White Dwarf Mass of Cataclysmic Variables

Abstract The flux ratio of Fe xxvi–Lyα to Fe xxv–Heα lines (I 7.0/I 6.7) is a sensitive indicator of the maximum temperature (T max), and therefore the mass of white dwarf stars (M WD) in cataclysmic variables (CVs). To examine and calibrate the theoretical I 7.0/I 6.7–T max–M WD relations, reliable measurements of T max and I 7.0/I 6.7 are necessary. In this work, we conduct a thorough investigation of 3–50 keV X-ray spectra of 25 solar neighborhood magnetic and nonmagnetic CVs based on archival NuSTAR and Suzaku observations. The measured T max are compared to the I 7.0/I 6.7 and M WD. The results show that the sampled CVs closely follow the theoretical I 7.0/I 6.7–T max relation. Moreover, all the M WD estimated from I 7.0/I 6.7 are consistent with the dynamically measured ones. We conclude that I 7.0/I 6.7 can be used as a good diagnostic for T max and M WD in both magnetic and nonmagnetic CVs.

A Spectroscopic Study of a Rich Cluster at z = 1.52 with Subaru and LBT: The Environmental Impacts on the Mass–Metallicity Relation

We present the results of our near-infrared (NIR) spectroscopic observations of a rich cluster candidate around a radio galaxy at $z=1.52$ (4C65.22) with Subaru/MOIRCS and LBT/LUCI. We observed 71 galaxies mostly on the star-forming main sequence selected by our previous broad-band (photo-$z$) and narrow-band H$\alpha$ imaging observation in this cluster environment. We successfully confirmed the redshifts of 39 galaxies, and conclude that this is a gravitationally bound, real cluster at $z=1.517$. Our spectroscopic data also suggest a hint of large-scale filaments or sheet-like three-dimensional structures crossing at the highest-density cluster core. By stacking the spectra to derive their average interstellar medium (ISM) gas-phase metallicity based on the [NII]/H$\alpha$ emission line flux ratio, we find that the mass-metallicity relation (MZR) in the 4C65.22 cluster environment is consistent with that of H$\alpha$-selected field galaxies at similar redshifts. Our results suggest that the environmental impacts on the MZR is small at high redshifts, but a larger sample of high-$z$ clusters and their member galaxies is still required to fully address the effect of environment as well as its cluster-cluster variation.

Searching for the 380 GHz H2O emission from the high-z lensed quasi-stellar object MG J0414+0534

Abstract We report the results of our search for the 380 GHz H2O line emissions from the quadruply lensed quasi-stellar object (QSO) MG J0104+0534 at z = 2.639 with the Atacama Large Millimeter/submillimeter Array. Our observation shows a tentative detection of the 380 GHz line from the lensed QSO, and the line spectrum shows a broad spectral distribution that has a velocity width of ∼290 km s−1 and a peak flux of ∼0.8 mJy. The integrated intensity map of the H2O line shows lensed emissions at the A1 and A2 component of the QSO, with the A2 component slightly resolved. The integrated line flux ratio between the A1 and A2 components shows an unexpected difference from the continuum flux ratio. Based on our gravitational lens modeling, assuming our tentative detection is real, this flux ratio anomaly would suggest that the 380 GHz line emissions come from two or three spatially displaced locations in the QSO, with the dominant one located at the position of the continuum emission from the QSO, and the other one(s) displaced from the continuum by ∼1.5 kpc on the source plane.

The Fe ii/Mg ii Flux Ratio of Low-luminosity Quasars at z ∼ 3

Abstract The Fe ii/Mg ii line flux ratio has been used to investigate the chemical evolution of high-redshift active galactic nuclei (AGNs). No strong evolution has been found out to z ∼ 6, implying that the SN 1a activity already occurred in the early universe. However, the trend of no evolution can be caused by the sample selection bias because previous studies have utilized mostly very luminous AGNs, which may be already chemically matured at the observed redshift. As motivated by the previously reported correlation between AGN luminosity and metallicity, we investigate the Fe ii/Mg ii flux ratio over a large dynamic range of luminosity, by adding a new sample of 12 quasars at z ∼ 3, of which the lower-luminosity limit is more than 1 dex smaller than that of the previously studied high-z quasars. Based on the Gemini/GNIRS observations, we find that the seven low-luminosity quasars with a mean bolometric luminosity log L bol ∼ 46.5 ± 0.2 has an average Fe ii/Mg ii ratio of 0.68 ± 0.11 dex. This ratio is comparable to that of high-luminosity quasars (log L bol ∼ 47.3 ± 0.3) in our sample (i.e., Fe ii/Mg ii ratio of 0.59 ± 0.15 dex) and that of the previously studied high-luminosity quasars at higher redshifts. One possible scenario is that the low-luminosity quasars in our sample are still relatively luminous and already chemically matured. To search for chemically young AGNs, and to fully understand the chemical evolution based on the Fe ii/Mg ii/flux ratio, investigations of much lower-luminosity AGNs are required.

Spectroscopic modelling of four neutron star low-mass X-ray binaries usingcloudy

Low-mass X-ray binaries (LMXBs) have a wide range of X-ray properties which can be utilised to reveal many physical conditions of the associated accretion discs. We use the spectral synthesis code CLOUDY to perform a detailed modelling of neutron star LMXBs GX 13+1, MXB 1659--298, 4U 1323--62 and XB 1916--053; and characterise the underlying physical conditions, such as density, radiation field, metallicity, wind velocity, etc. For this purpose we model highly ionised spectra of Fe, Ca, S, Si, Mg, Al in the soft X-ray band, and compare the predicted line flux ratios with the observed values. We also find that the strength and profile of these spectral lines get modified in the presence of magnetic field in the accretion disc. Using this, we estimate an upper limit of the existing magnetic field to be about a few hundred to a few thousand G in the accretion discs of these four LMXBs.