Null Geodesics and Strong Field Gravitational Lensing in a String Cloud Background

<div>Radius of photon sphere (<svg height="11.2341pt" id="M52" style="vertical-align:-5.23405pt" version="1.1" viewbox="-0.0498162 -6.00005 16.0957 11.2341" width="16.0957pt" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><g transform="matrix(.013,0,0,-0.013,0,0)"><path d="M545 403C545 425 527 451 498 451C448 451 400 404 312 286L290 341C262 412 246 451 219 451C182 451 138 404 92 345L112 323C152 368 169 378 181 378C192 378 201 359 216 321L257 214C184 115 142 69 113 69C102 69 93 73 88 79S78 89 68 89C47 89 24 61 24 40C24 8 49 -12 75 -12C125 -12 175 31 271 175L312 64C330 15 357 -12 382 -12C421 -12 475 35 515 98L496 121C466 88 439 62 420 62C402 62 384 92 363 147L325 247C349 280 374 309 397 333C422 361 446 381 462 381C471 381 481 374 488 366C493 360 499 357 505 357C521 357 545 380 545 403Z" id="g50-121"></path><glyph.data ascent="3443" descent="-2856" horiz-adv-x="569" vert-adv-y="569"></glyph.data></g><g transform="matrix(.0091,0,0,-0.0091,7.332,3.001)"><path d="M169 380V459C122 440 66 423 24 416V392C86 384 90 382 90 317V-135C90 -201 81 -207 17 -213V-240H253V-213C176 -207 169 -201 169 -125V6C182 -1 208 -11 238 -12C368 12 487 109 487 260C487 358 421 449 310 449C298 449 279 444 261 433L169 380ZM169 346C196 367 237 389 269 389C341 389 403 329 403 221C403 109 347 37 263 37C228 37 191 53 169 76V346Z" id="g190-113"></path><glyph.data ascent="989" descent="-360" horiz-adv-x="524" vert-adv-y="524"></glyph.data></g><g transform="matrix(.0091,0,0,-0.0091,12.1,3.001)"><path d="M319 325C317 349 306 409 297 431C277 440 250 449 209 449C117 449 57 389 57 319C57 243 122 209 182 182C232 159 261 135 261 91C261 48 227 21 190 21C130 21 85 79 68 145L41 140C41 104 51 36 58 22C75 7 121 -12 172 -12C252 -12 337 35 337 126C337 195 286 231 210 262C166 281 126 304 126 348C126 388 152 417 191 417C240 417 274 378 294 318L319 325Z" id="g190-116"></path><glyph.data ascent="989" descent="-360" horiz-adv-x="367" vert-adv-y="367"></glyph.data></g></svg>) for the string cloud versus string parameter <svg height="6.20645pt" id="M53" style="vertical-align:-0.2063999pt" version="1.1" viewbox="-0.0498162 -6.00005 8.55521 6.20645" width="8.55521pt" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><g transform="matrix(.013,0,0,-0.013,0,0)"><path d="M624 292C624 400 569 451 537 451C520 451 497 442 490 436L489 427C521 404 550 351 550 280C550 170 513 51 423 51C383 51 361 87 361 150C361 197 384 301 397 339L391 343L310 328C307 297 301 249 293 212C270 102 223 56 182 56C144 56 114 93 114 172C114 278 161 369 243 430L223 457C99 392 24 290 24 156C24 43 81 -12 141 -12C187 -12 247 32 286 88C295 31 325 -12 380 -12C431 -12 475 16 511 49C581 113 624 195 624 292Z" id="g50-246"></path><glyph.data ascent="3443" descent="-2856" horiz-adv-x="648" vert-adv-y="648"></glyph.data></g></svg>. Red curve corresponds to <svg height="11.2341pt" id="M54" style="vertical-align:-5.23405pt" version="1.1" viewbox="-0.0498162 -6.00005 16.0957 11.2341" width="16.0957pt" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><g transform="matrix(.013,0,0,-0.013,0,0)"><path d="M545 403C545 425 527 451 498 451C448 451 400 404 312 286L290 341C262 412 246 451 219 451C182 451 138 404 92 345L112 323C152 368 169 378 181 378C192 378 201 359 216 321L257 214C184 115 142 69 113 69C102 69 93 73 88 79S78 89 68 89C47 89 24 61 24 40C24 8 49 -12 75 -12C125 -12 175 31 271 175L312 64C330 15 357 -12 382 -12C421 -12 475 35 515 98L496 121C466 88 439 62 420 62C402 62 384 92 363 147L325 247C349 280 374 309 397 333C422 361 446 381 462 381C471 381 481 374 488 366C493 360 499 357 505 357C521 357 545 380 545 403Z" id="g50-121"></path><glyph.data ascent="3443" descent="-2856" horiz-adv-x="569" vert-adv-y="569"></glyph.data></g><g transform="matrix(.0091,0,0,-0.0091,7.332,3.001)"><path d="M169 380V459C122 440 66 423 24 416V392C86 384 90 382 90 317V-135C90 -201 81 -207 17 -213V-240H253V-213C176 -207 169 -201 169 -125V6C182 -1 208 -11 238 -12C368 12 487 109 487 260C487 358 421 449 310 449C298 449 279 444 261 433L169 380ZM169 346C196 367 237 389 269 389C341 389 403 329 403 221C403 109 347 37 263 37C228 37 191 53 169 76V346Z" id="g190-113"></path><glyph.data ascent="989" descent="-360" horiz-adv-x="524" vert-adv-y="524"></glyph.data></g><g transform="matrix(.0091,0,0,-0.0091,12.1,3.001)"><path d="M319 325C317 349 306 409 297 431C277 440 250 449 209 449C117 449 57 389 57 319C57 243 122 209 182 182C232 159 261 135 261 91C261 48 227 21 190 21C130 21 85 79 68 145L41 140C41 104 51 36 58 22C75 7 121 -12 172 -12C252 -12 337 35 337 126C337 195 286 231 210 262C166 281 126 304 126 348C126 388 152 417 191 417C240 417 274 378 294 318L319 325Z" id="g190-116"></path><glyph.data ascent="989" descent="-360" horiz-adv-x="367" vert-adv-y="367"></glyph.data></g></svg> whereas the green line (dashed) indicates Schwarzschild limit.</div>

Advances in High Energy Physics

fig2

Figure 2

Figure 2: Null Geodesics and Strong Field Gravitational Lensing in a String Cloud Background 